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Glycolysis is the
metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. ...
that converts
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
C6H12O6, into
pyruvic acid Pyruvic acid (CH3COCOOH) is the simplest of the keto acids, alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways through ...

pyruvic acid
, CH3COCOOH. The free energy released in this process is used to form the high-energy molecules
adenosine triphosphate Adenosine triphosphate (ATP) is an and that provides energy to drive many processes in living , such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of life, ATP is ofte ...

adenosine triphosphate
(ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by
enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules known as . Almost all in the need in order to occu ...

enzyme
s. Glycolysis is a metabolic pathway that does not require oxygen. The wide occurrence of glycolysis in other species indicates that it is an ancient metabolic pathway. Indeed, the reactions that make up glycolysis and its parallel pathway, the
pentose phosphate pathwayIn chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atom, atoms. The chemical formula of all pentoses is , and their molecular weight is 150.13 g/mol.
, occur in the oxygen-free conditions of the
Archean The Archean Eon ( , also spelled Archaean or Archæan) is one of the four geologic Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient ...

Archean
oceans, also in the absence of enzymes, catalyzed by metal. In most organisms, glycolysis occurs in the liquid part of cells, the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a sm ...
. The most common type of glycolysis is the ''Embden–Meyerhof–Parnas (EMP) pathway'', which was discovered by
Gustav EmbdenGustav Georg Embden (10 November 1874 – 25 July 1933) was a German physiological chemist. Background Gustav Embden was a son of the Hamburg lawyer and politician George Heinrich Embden. His grandmother Charlotte Heine was a well-known Salon (g ...
, Otto Meyerhof, and
Jakub Karol Parnas Jakub Karol Parnas, also known as Yakov Oskarovich Parnas (russian: Яков Оскарович Парнас) (January 16, 1884 – January 29, 1949) was a prominent Jewish-Poland, Polish–Soviet Union, Soviet biochemist who contributed to th ...
. Glycolysis also refers to other pathways, such as the ''
Entner–Doudoroff pathway 200px, Diagram of the Entner-Doudoroff pathway (KDPG: 2-keto-3-deoxy-6-phosphogluconate) The Entner-Doudoroff Pathway (ED Pathway) is a metabolic pathway that is most notably in Gram-negative bacteria, certain Gram-positive bacteria and archaea. G ...

Entner–Doudoroff pathway
'' and various heterofermentative and homofermentative pathways. However, the discussion here will be limited to the Embden–Meyerhof–Parnas pathway. The glycolysis pathway can be separated into two phases:Glycolysis – Animation and Notes
/ref> # Investment phase – wherein ATP is consumed # Yield phase – wherein more ATP is produced than originally consumed


Overview

The overall reaction of glycolysis is:
The use of symbols in this equation makes it appear unbalanced with respect to oxygen atoms, hydrogen atoms, and charges. Atom balance is maintained by the two phosphate (Pi) groups: * Each exists in the form of a hydrogen phosphate anion (HPO42−), dissociating to contribute 2 H+ overall * Each liberates an oxygen atom when it binds to an
adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, ele ...

adenosine diphosphate
(ADP) molecule, contributing 2O overall Charges are balanced by the difference between ADP and ATP. In the cellular environment, all three hydroxyl groups of ADP dissociate into −O and H+, giving ADP3−, and this ion tends to exist in an ionic bond with Mg2+, giving ADPMg. ATP behaves identically except that it has four hydroxyl groups, giving ATPMg2−. When these differences along with the true charges on the two phosphate groups are considered together, the net charges of −4 on each side are balanced. For simple fermentations, the metabolism of one molecule of glucose to two molecules of pyruvate has a net yield of two molecules of ATP. Most cells will then carry out further reactions to "repay" the used NAD+ and produce a final product of
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an . It is a simple with the C2H6O. Its formula can be also written as −− or (an linked to a group), and is often as EtOH. Ethanol is a , , ...

ethanol
or
lactic acid Lactic acid is an organic acid. It has a molecular formula CH3CH(OH)COOH. It is white in the solid state and it is miscibility, miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial s ...

lactic acid
. Many bacteria use inorganic compounds as hydrogen acceptors to regenerate the NAD+. Cells performing
aerobic respiration Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
synthesize much more ATP, but not as part of glycolysis. These further aerobic reactions use
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is a key intermediate in several metabolic pathways throughout the c ...

pyruvate
, and NADH + H+ from glycolysis. Eukaryotic aerobic respiration produces approximately 34 additional molecules of ATP for each glucose molecule, however most of these are produced by a mechanism vastly different from the
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group attached ...
in glycolysis. The lower-energy production, per glucose, of anaerobic respiration relative to aerobic respiration, results in greater flux through the pathway under hypoxic (low-oxygen) conditions, unless alternative sources of anaerobically oxidizable substrates, such as fatty acids, are found.


History

The pathway of glycolysis as it is known today took almost 100 years to fully elucidate. The combined results of many smaller experiments were required in order to understand the pathway as a whole. The first steps in understanding glycolysis began in the nineteenth century with the wine industry. For economic reasons, the French wine industry sought to investigate why wine sometimes turned distasteful, instead of fermenting into alcohol. French scientist
Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conducts S ...

Louis Pasteur
researched this issue during the 1850s, and the results of his experiments began the long road to elucidating the pathway of glycolysis. His experiments showed that fermentation occurs by the action of living
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms ar ...
s, yeasts, and that yeast's glucose consumption decreased under aerobic conditions of fermentation, in comparison to anaerobic conditions (the
Pasteur effect The Pasteur effect is an inhibiting effect of oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Ch ...
). Insight into the component steps of glycolysis were provided by the non-cellular fermentation experiments of
Eduard Buchner Eduard Buchner (20 May 1860 – 13 August 1917) was a German chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conducts Scientific meth ...
during the 1890s. Buchner demonstrated that the conversion of glucose to ethanol was possible using a non-living extract of yeast, due to the action of
enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules known as . Almost all in the need in order to occu ...

enzyme
s in the extract. This experiment not only revolutionized biochemistry, but also allowed later scientists to analyze this pathway in a more controlled laboratory setting. In a series of experiments (1905-1911), scientists
Arthur Harden Sir Arthur Harden, FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resourc ...
and William Young discovered more pieces of glycolysis. They discovered the regulatory effects of ATP on glucose consumption during alcohol fermentation. They also shed light on the role of one compound as a glycolysis intermediate: fructose 1,6-bisphosphate. The elucidation of fructose 1,6-bisphosphate was accomplished by measuring CO2 levels when yeast juice was incubated with glucose. CO2 production increased rapidly then slowed down. Harden and Young noted that this process would restart if an inorganic phosphate (Pi) was added to the mixture. Harden and Young deduced that this process produced organic phosphate esters, and further experiments allowed them to extract fructose diphosphate (F-1,6-DP).
Arthur Harden Sir Arthur Harden, FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family Resourc ...
and William Young along with Nick Sheppard determined, in a second experiment, that a heat-sensitive high-molecular-weight subcellular fraction (the enzymes) and a heat-insensitive low-molecular-weight cytoplasm fraction (ADP, ATP and NAD+ and other cofactors) are required together for fermentation to proceed. This experiment begun by observing that dialyzed (purified) yeast juice could not ferment or even create a sugar phosphate. This mixture was rescued with the addition of undialyzed yeast extract that had been boiled. Boiling the yeast extract renders all proteins inactive (as it denatures them). The ability of boiled extract plus dialyzed juice to complete fermentation suggests that the cofactors were non-protein in character. In the 1920s was able to link together some of the many individual pieces of glycolysis discovered by Buchner, Harden, and Young. Meyerhof and his team were able to extract different glycolytic enzymes from
muscle tissue Muscle tissues are soft tissue of a tendon A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton i ...

muscle tissue
, and combine them to artificially create the pathway from glycogen to lactic acid. In one paper, Meyerhof and scientist Renate Junowicz-Kockolaty investigated the reaction that splits fructose 1,6-diphosphate into the two triose phosphates. Previous work proposed that the split occurred via 1,3-diphosphoglyceraldehyde plus an oxidizing enzyme and cozymase. Meyerhoff and Junowicz found that the equilibrium constant for the isomerase and aldoses reaction were not affected by inorganic phosphates or any other cozymase or oxidizing enzymes. They further removed diphosphoglyceraldehyde as a possible intermediate in glycolysis. With all of these pieces available by the 1930s,
Gustav EmbdenGustav Georg Embden (10 November 1874 – 25 July 1933) was a German physiological chemist. Background Gustav Embden was a son of the Hamburg lawyer and politician George Heinrich Embden. His grandmother Charlotte Heine was a well-known Salon (g ...
proposed a detailed, step-by-step outline of that pathway we now know as glycolysis. The biggest difficulties in determining the intricacies of the pathway were due to the very short lifetime and low steady-state concentrations of the intermediates of the fast glycolytic reactions. By the 1940s, Meyerhof, Embden and many other biochemists had finally completed the puzzle of glycolysis. The understanding of the isolated pathway has been expanded in the subsequent decades, to include further details of its regulation and integration with other metabolic pathways.


Sequence of reactions


Summary of reactions


Preparatory phase

The first five steps of Glycolysis are regarded as the preparatory (or investment) phase, since they consume energy to convert the glucose into two three-carbon sugar phosphates (). The first step is phosphorylation of glucose by a family of enzymes called
hexokinase A hexokinase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the e ...
s to form glucose 6-phosphate (G6P). This reaction consumes ATP, but it acts to keep the glucose concentration low, promoting continuous transport of glucose into the cell through the plasma membrane transporters. In addition, it blocks the glucose from leaking out – the cell lacks transporters for G6P, and free diffusion out of the cell is prevented due to the charged nature of G6P. Glucose may alternatively be formed from the
phosphorolysisPhosphorolysis is the cleavage of a compound in which inorganic phosphate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: ...
or
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord for ...

hydrolysis
of intracellular starch or glycogen. In
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s, an
isozymeIn biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. diff ...
of hexokinase called
glucokinase Glucokinase () is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzy ...

glucokinase
is also used in the liver, which has a much lower affinity for glucose (Km in the vicinity of normal glycemia), and differs in regulatory properties. The different substrate affinity and alternate regulation of this enzyme are a reflection of the role of the liver in maintaining blood sugar levels. ''Cofactors:'' Mg2+ G6P is then rearranged into
fructose 6-phosphate Fructose 6-phosphate (sometimes called the Neuberg ester) is a derivative of fructose, which has been phosphorylated at the 6-hydroxy group. It is one of several possible fructosephosphates. The β-D-form of this compound is very common in cell (bi ...

fructose 6-phosphate
(F6P) by glucose phosphate isomerase.
Fructose Fructose, or fruit sugar, is a ketonic simple sugar Monosaccharides (from Greek language, Greek ''wikt:μόνος, monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar and the most basic units (monomers ...

Fructose
can also enter the glycolytic pathway by phosphorylation at this point. The change in structure is an isomerization, in which the G6P has been converted to F6P. The reaction requires an enzyme, phosphoglucose isomerase, to proceed. This reaction is freely reversible under normal cell conditions. However, it is often driven forward because of a low concentration of F6P, which is constantly consumed during the next step of glycolysis. Under conditions of high F6P concentration, this reaction readily runs in reverse. This phenomenon can be explained through
Le Chatelier's Principle Le Chatelier's principle (pronounced or ), also called Chatelier's principle, is a principle of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mole ...
. Isomerization to a keto sugar is necessary for carbanion stabilization in the fourth reaction step (below). The energy expenditure of another ATP in this step is justified in 2 ways: The glycolytic process (up to this step) becomes irreversible, and the energy supplied destabilizes the molecule. Because the reaction catalyzed by
phosphofructokinase 1 Phosphofructokinase-1 (PFK-1) is one of the most important regulatory enzymes () of glycolysis. It is an allosteric enzyme made of 4 subunits and controlled by many Enzyme activator, activators and Enzyme inhibitor, inhibitors. PFK-1 catalyzes the ...
(PFK-1) is coupled to the hydrolysis of ATP (an energetically favorable step) it is, in essence, irreversible, and a different pathway must be used to do the reverse conversion during
gluconeogenesis Gluconeogenesis (GNG) is a that results in the generation of from certain non- carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly i ...

gluconeogenesis
. This makes the reaction a key regulatory point (see below). This is also the rate-limiting step. Furthermore, the second phosphorylation event is necessary to allow the formation of two charged groups (rather than only one) in the subsequent step of glycolysis, ensuring the prevention of free diffusion of substrates out of the cell. The same reaction can also be catalyzed by pyrophosphate-dependent phosphofructokinase (PFP or PPi-PFK), which is found in most plants, some bacteria, archea, and protists, but not in animals. This enzyme uses pyrophosphate (PPi) as a phosphate donor instead of ATP. It is a reversible reaction, increasing the flexibility of glycolytic metabolism. A rarer ADP-dependent PFK enzyme variant has been identified in archaean species. ''Cofactors:'' Mg2+ Destabilizing the molecule in the previous reaction allows the hexose ring to be split by
aldolase Fructose-bisphosphate aldolase (), often just aldolase, is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called subst ...
into two triose sugars:
dihydroxyacetone phosphate Dihydroxyacetone phosphate (DHAP, also glycerone phosphate in older texts) is the anion with the formula HOCH2C(O)CH2OPO32-. This anion is involved in many metabolic pathways, including the Calvin cycle in plants and glycolysis.Nelson, D. L.; Cox ...
(a ketose), and
glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is the metabolite In biochemistry Biochemistry or biological chemistry, is the study of chemical ...

glyceraldehyde 3-phosphate
(an aldose). There are two classes of aldolases: class I aldolases, present in animals and plants, and class II aldolases, present in fungi and bacteria; the two classes use different mechanisms in cleaving the ketose ring. Electrons delocalized in the carbon-carbon bond cleavage associate with the alcohol group. The resulting carbanion is stabilized by the structure of the carbanion itself via resonance charge distribution and by the presence of a charged ion prosthetic group.
Triosephosphate isomerase Triose-phosphate isomerase (TPI or TIM) is an enzyme () that catalyst, catalyzes the reversible interconversion of the triose phosphate isomers dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate. TPI plays an important role in glycoly ...

Triosephosphate isomerase
rapidly interconverts dihydroxyacetone phosphate with
glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is the metabolite In biochemistry Biochemistry or biological chemistry, is the study of chemical ...

glyceraldehyde 3-phosphate
(GADP) that proceeds further into glycolysis. This is advantageous, as it directs dihydroxyacetone phosphate down the same pathway as glyceraldehyde 3-phosphate, simplifying regulation.


Pay-off phase

The second half of glycolysis is known as the pay-off phase, characterised by a net gain of the energy-rich molecules ATP and NADH. Since glucose leads to two triose sugars in the preparatory phase, each reaction in the pay-off phase occurs twice per glucose molecule. This yields 2 NADH molecules and 4 ATP molecules, leading to a net gain of 2 NADH molecules and 2 ATP molecules from the glycolytic pathway per glucose. The aldehyde groups of the triose sugars are
oxidised (mild reducing agent) are added to powdered potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4 and composed of potassium ion, K+ and permanganate, . It is a purplish-black crystalline salt, th ...
, and
inorganic phosphate In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo durin ...

inorganic phosphate
is added to them, forming . The hydrogen is used to reduce two molecules of , a hydrogen carrier, to give NADH + H+ for each triose. Hydrogen atom balance and charge balance are both maintained because the phosphate (Pi) group actually exists in the form of a hydrogen phosphate anion (HPO42−), which dissociates to contribute the extra H+ ion and gives a net charge of -3 on both sides. Here,
arsenate The arsenate ion is . An arsenate (compound) is any compound that contains this ion. Arsenates are salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), ...

arsenate
(AsO43−), an anion akin to inorganic phosphate may replace phosphate as a substrate to form 1-arseno-3-phosphoglycerate. This, however, is unstable and readily hydrolyzes to form , the intermediate in the next step of the pathway. As a consequence of bypassing this step, the molecule of ATP generated from in the next reaction will not be made, even though the reaction proceeds. As a result, arsenate is an uncoupler of glycolysis. This step is the enzymatic transfer of a phosphate group from to ADP by
phosphoglycerate kinase Phosphoglycerate kinase () (PGK 1) is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), sub ...

phosphoglycerate kinase
, forming ATP and . At this step, glycolysis has reached the break-even point: 2 molecules of ATP were consumed, and 2 new molecules have now been synthesized. This step, one of the two
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group attached ...
steps, requires ADP; thus, when the cell has plenty of ATP (and little ADP), this reaction does not occur. Because ATP decays relatively quickly when it is not metabolized, this is an important regulatory point in the glycolytic pathway. ADP actually exists as ADPMg, and ATP as ATPMg2−, balancing the charges at −5 both sides. ''Cofactors:'' Mg2+
Phosphoglycerate mutase :''This enzyme is not to be confused with Bisphosphoglycerate mutase which catalyzes the conversion of 1,3-bisphosphoglycerate to 2,3-bisphosphoglycerate.'' Phosphoglycerate mutase (PGM) is any enzyme that catalyzes step 8 of glycolysis. The ...
isomerises into .
Enolase Enolase, also known as phosphopyruvate hydratase, is a metalloenzyme responsible for the catalysis of the conversion of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP), the ninth and penultimate step of glycolysis. The chemical reactio ...
next converts to
phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the ester An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl (alkoxy) group, as in the substi ...
. This reaction is an elimination reaction involving an
E1cB The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where the hydrogen to be removed is relatively acidic, while the leaving group (such as -OH or -OR) is a relatively poor one. Usually a moderate to ...
mechanism. ''Cofactors:'' 2 Mg2+, one "conformational" ion to coordinate with the carboxylate group of the substrate, and one "catalytic" ion that participates in the dehydration. A final
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group attached ...
now forms a molecule of
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is a key intermediate in several metabolic pathways throughout the c ...

pyruvate
and a molecule of ATP by means of the enzyme
pyruvate kinase Pyruvate kinase is the enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enz ...

pyruvate kinase
. This serves as an additional regulatory step, similar to the phosphoglycerate kinase step. ''Cofactors:'' Mg2+


Biochemical logic

The existence of more than one point of regulation indicates that intermediates between those points enter and leave the glycolysis pathway by other processes. For example, in the first regulated step,
hexokinase A hexokinase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the e ...
converts glucose into glucose-6-phosphate. Instead of continuing through the glycolysis pathway, this intermediate can be converted into glucose storage molecules, such as
glycogen Glycogen is a multibranched polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () usi ...

glycogen
or
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
. The reverse reaction, breaking down, e.g., glycogen, produces mainly glucose-6-phosphate; very little free glucose is formed in the reaction. The glucose-6-phosphate so produced can enter glycolysis ''after'' the first control point. In the second regulated step (the third step of glycolysis),
phosphofructokinase Phosphofructokinase (PFK) is a kinase In biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . O ...
converts fructose-6-phosphate into fructose-1,6-bisphosphate, which then is converted into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. The dihydroxyacetone phosphate can be removed from glycolysis by conversion into glycerol-3-phosphate, which can be used to form triglycerides. Conversely,
triglyceride A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an derived from and three (from ' and '). Triglycerides are the main constituents of in humans and other vertebrates, as well as . They are also present in the blood to enab ...

triglyceride
s can be broken down into fatty acids and glycerol; the latter, in turn, can be
converted Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ''Cyberman'' * Conversion (Stargate Atlantis), "Conversion" (''Stargate Atlantis ...

converted
into dihydroxyacetone phosphate, which can enter glycolysis ''after'' the second control point.


Free energy changes

The change in free energy, Δ''G'', for each step in the glycolysis pathway can be calculated using Δ''G'' = Δ''G''°' + ''RT''ln ''Q'', where ''Q'' is the
reaction quotientIn chemical thermodynamics Chemical thermodynamics is the study of the interrelation of heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter ...
. This requires knowing the concentrations of the
metabolites In biochemistry, a metabolite is an intermediate or end product of metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabo ...
. All of these values are available for
erythrocytes Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...

erythrocytes
, with the exception of the concentrations of NAD+ and NADH. The ratio of NAD+ to NADH in the cytoplasm is approximately 1000, which makes the oxidation of glyceraldehyde-3-phosphate (step 6) more favourable. Using the measured concentrations of each step, and the standard free energy changes, the actual free energy change can be calculated. (Neglecting this is very common - the delta G of ATP hydrolysis in cells is not the standard free energy change of ATP hydrolysis quoted in textbooks). From measuring the physiological concentrations of metabolites in an erythrocyte it seems that about seven of the steps in glycolysis are in equilibrium for that cell type. Three of the steps — the ones with large negative free energy changes — are not in equilibrium and are referred to as ''irreversible''; such steps are often subject to regulation. Step 5 in the figure is shown behind the other steps, because that step is a side-reaction that can decrease or increase the concentration of the intermediate glyceraldehyde-3-phosphate. That compound is converted to dihydroxyacetone phosphate by the enzyme triose phosphate isomerase, which is a catalytically perfect enzyme; its rate is so fast that the reaction can be assumed to be in equilibrium. The fact that Δ''G'' is not zero indicates that the actual concentrations in the erythrocyte are not accurately known.


Regulation

Enzymes are the main components which drive the metabolic pathway and hence, exploring the regulatory mechanisms on these enzymes will give us insights to the regulatory processes affecting glycolysis. There are in total 9 primary steps in glycolysis which is driven by 14 different enzymes. Enzymes can be modified or are affected using 5 main regulatory processes including post-translational modification (PTM) and localization.


Biological mechanisms by which enzymes are regulated

1. Gene Expression
2. Allostery
3. Protein-protein interaction (PPI)
4. Post translational modification (PTM)
5. Localization


Regulation by insulin in animals

In animals, regulation of blood glucose levels by the pancreas in conjunction with the liver is a vital part of
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
. The
beta cells Beta cells (β cells) are a type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse li ...
in the
pancreatic islets The pancreatic islets or islets of Langerhans are the regions of the pancreas The pancreas is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and ...
are sensitive to the blood glucose concentration. A rise in the blood glucose concentration causes them to release
insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main Anabolism, anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and p ...

insulin
into the blood, which has an effect particularly on the liver, but also on
fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

fat
and
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
cells, causing these tissues to remove glucose from the blood. When the blood sugar falls the pancreatic beta cells cease insulin production, but, instead, stimulate the neighboring pancreatic to release
glucagon Glucagon is a peptide hormonePeptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on the endocrine ...

glucagon
into the blood. This, in turn, causes the liver to release glucose into the blood by breaking down stored
glycogen Glycogen is a multibranched polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () usi ...

glycogen
, and by means of gluconeogenesis. If the fall in the blood glucose level is particularly rapid or severe, other glucose sensors cause the release of
epinephrine Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a co ...

epinephrine
from the
adrenal glands The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex wh ...

adrenal glands
into the blood. This has the same action as glucagon on glucose metabolism, but its effect is more pronounced. In the liver glucagon and epinephrine cause the
phosphorylation In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...

phosphorylation
of the key, rate limiting enzymes of glycolysis,
fatty acid synthesis Fatty acid synthesis is the creation of fatty acid fatty acids have perfectly straight chain structure. Unsaturated ones are typically bent, unless they have a trans configuration. In chemistry Chemistry is the scientific discipline invo ...
, , gluconeogenesis, and glycogenolysis. Insulin has the opposite effect on these enzymes. The phosphorylation and dephosphorylation of these enzymes (ultimately in response to the glucose level in the blood) is the dominant manner by which these pathways are controlled in the liver, fat, and muscle cells. Thus the phosphorylation of
phosphofructokinase Phosphofructokinase (PFK) is a kinase In biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . O ...
inhibits glycolysis, whereas its dephosphorylation through the action of insulin stimulates glycolysis.


Regulation of the rate limiting enzymes

The four regulatory enzymes are
hexokinase A hexokinase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the e ...
(or
glucokinase Glucokinase () is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzy ...

glucokinase
in the liver),
phosphofructokinase Phosphofructokinase (PFK) is a kinase In biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . O ...
, and
pyruvate kinase Pyruvate kinase is the enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enz ...

pyruvate kinase
. The
flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
through the glycolytic pathway is adjusted in response to conditions both inside and outside the cell. The internal factors that regulate glycolysis do so primarily to provide
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
in adequate quantities for the cell's needs. The external factors act primarily on the
liver The liver is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's t ...

liver
,
fat tissue Adipose tissue, body fat, or simply fat is a loose connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In bi ...

fat tissue
, and
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s, which can remove large quantities of glucose from the blood after meals (thus preventing
hyperglycemia Hyperglycemia is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a blood sugar level higher than 11.1 Blood sugar#Units, mmol/l (200 Blood sugar#Units, mg/dl), but symptoms may not st ...

hyperglycemia
by storing the excess glucose as fat or glycogen, depending on the tissue type). The liver is also capable of releasing glucose into the blood between meals, during fasting, and exercise thus preventing
hypoglycemia Hypoglycemia, also known as low blood sugar, is a fall in blood sugar The blood sugar level, blood sugar concentration, or blood glucose level is the concentration of glucose Glucose is a simple with the . Glucose is the most abundant , a ...

hypoglycemia
by means of
glycogenolysis Glycogenolysis is the breakdown of glycogen (n) to glucose-1-phosphate Glucose 1-phosphate (also called cori ester) is a glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abu ...
and
gluconeogenesis Gluconeogenesis (GNG) is a that results in the generation of from certain non- carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly i ...

gluconeogenesis
. These latter reactions coincide with the halting of glycolysis in the liver. In addition hexokinase and
glucokinase Glucokinase () is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzy ...

glucokinase
act independently of the hormonal effects as controls at the entry points of glucose into the cells of different tissues. Hexokinase responds to the
glucose-6-phosphate Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Gluco ...
(G6P) level in the cell, or, in the case of glucokinase, to the blood sugar level in the blood to impart entirely intracellular controls of the glycolytic pathway in different tissues (see
below Below may refer to: *Earth *Ground (disambiguation) *Soil *Floor *Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Fred Below (1926–1988), American blues drummer *Fritz von Below (1853 ...
). When glucose has been converted into G6P by hexokinase or glucokinase, it can either be converted to glucose-1-phosphate (G1P) for conversion to
glycogen Glycogen is a multibranched polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () usi ...

glycogen
, or it is alternatively converted by glycolysis to Pyruvic acid, pyruvate, which enters the mitochondrion where it is converted into acetyl-CoA and then into citrate. Excess citrate is exported from the mitochondrion back into the cytosol, where ATP citrate lyase regenerates acetyl-CoA and oxaloacetic acid, oxaloacetate (OAA). The acetyl-CoA is then used for fatty acid synthesis and , two important ways of utilizing excess glucose when its concentration is high in blood. The rate limiting enzymes catalyzing these reactions perform these functions when they have been dephosphorylated through the action of insulin on the liver cells. Between meals, during fasting, Physical exercise, exercise or hypoglycemia, glucagon and epinephrine are released into the blood. This causes liver glycogen to be converted back to G6P, and then converted to glucose by the liver-specific enzyme glucose 6-phosphatase and released into the blood. Glucagon and epinephrine also stimulate gluconeogenesis, which coverts non-carbohydrate substrates into G6P, which joins the G6P derived from glycogen, or substitutes for it when the liver glycogen store have been depleted. This is critical for brain function, since the brain utilizes glucose as an energy source under most conditions. The simultaneously phosphorylation of, particularly,
phosphofructokinase Phosphofructokinase (PFK) is a kinase In biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . O ...
, but also, to a certain extent pyruvate kinase, prevents glycolysis occurring at the same time as gluconeogenesis and glycogenolysis.


Hexokinase and glucokinase

All cells contain the enzyme
hexokinase A hexokinase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the e ...
, which catalyzes the conversion of glucose that has entered the cell into
glucose-6-phosphate Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Gluco ...
(G6P). Since the cell membrane is impervious to G6P, hexokinase essentially acts to transport glucose into the cells from which it can then no longer escape. Hexokinase is inhibited by high levels of G6P in the cell. Thus the rate of entry of glucose into cells partially depends on how fast G6P can be disposed of by glycolysis, and by Glycogenesis, glycogen synthesis (in the cells which store glycogen, namely liver and muscles). Glucokinase, unlike
hexokinase A hexokinase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the e ...
, is not inhibited by G6P. It occurs in liver cells, and will only phosphorylate the glucose entering the cell to form
glucose-6-phosphate Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Gluco ...
(G6P), when the glucose in the blood is abundant. This being the first step in the glycolytic pathway in the liver, it therefore imparts an additional layer of control of the glycolytic pathway in this organ.


Phosphofructokinase

Phosphofructokinase 1, Phosphofructokinase is an important control point in the glycolytic pathway, since it is one of the irreversible steps and has key allosteric effectors, Adenosine monophosphate, AMP and fructose 2,6-bisphosphate (F2,6BP). Fructose 2,6-bisphosphate (F2,6BP) is a very potent activator of phosphofructokinase (PFK-1) that is synthesized when F6P is phosphorylated by a second phosphofructokinase (PFK2). In the liver, when blood sugar is low and
glucagon Glucagon is a peptide hormonePeptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on the endocrine ...

glucagon
elevates cAMP, PFK2 is phosphorylated by protein kinase A. The phosphorylation inactivates PFK2, and another domain on this protein becomes active as fructose bisphosphatase-2, which converts F2,6BP back to F6P. Both
glucagon Glucagon is a peptide hormonePeptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on the endocrine ...

glucagon
and
epinephrine Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a co ...

epinephrine
cause high levels of cAMP in the liver. The result of lower levels of liver fructose-2,6-bisphosphate is a decrease in activity of
phosphofructokinase Phosphofructokinase (PFK) is a kinase In biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . O ...
and an increase in activity of fructose 1,6-bisphosphatase, so that gluconeogenesis (in essence, "glycolysis in reverse") is favored. This is consistent with the role of the liver in such situations, since the response of the liver to these hormones is to release glucose to the blood. Adenosine triphosphate, ATP competes with Adenosine monophosphate, AMP for the allosteric effector site on the PFK enzyme. ATP concentrations in cells are much higher than those of AMP, typically 100-fold higher, but the concentration of ATP does not change more than about 10% under physiological conditions, whereas a 10% drop in ATP results in a 6-fold increase in AMP. Thus, the relevance of ATP as an allosteric effector is questionable. An increase in AMP is a consequence of a decrease in energy charge in the cell. Citrate inhibits phosphofructokinase when tested ''in vitro'' by enhancing the inhibitory effect of ATP. However, it is doubtful that this is a meaningful effect ''in vivo'', because citrate in the cytosol is utilized mainly for conversion to acetyl-CoA for fatty acid and cholesterol synthesis. TP53-inducible glycolysis and apoptosis regulator, TIGAR, a p53 induced enzyme, is responsible for the regulation of
phosphofructokinase Phosphofructokinase (PFK) is a kinase In biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided into three fields: , and . O ...
and acts to protect against oxidative stress. TIGAR is a single enzyme with dual function that regulates F2,6BP. It can behave as a phosphatase (fructuose-2,6-bisphosphatase) which cleaves the phosphate at carbon-2 producing F6P. It can also behave as a kinase (PFK2) adding a phosphate onto carbon-2 of F6P which produces F2,6BP. In humans, the TIGAR protein is encoded by ''C12orf5'' gene. The TIGAR enzyme will hinder the forward progression of glycolysis, by creating a build up of fructose-6-phosphate (F6P) which is isomerized into glucose-6-phosphate (G6P). The accumulation of G6P will shunt carbons into the pentose phosphate pathway.


Pyruvate kinase

Pyruvate kinase enzyme catalyst, catalyzes the last step of glycolysis, in which pyruvate and ATP are formed. Pyruvate kinase catalyzes the transfer of a phosphate group from
phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the ester An ester is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH hydroxyl group is replaced by an –O– alkyl (alkoxy) group, as in the substi ...
(PEP) to adenosine diphosphate, ADP, yielding one molecule of
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is a key intermediate in several metabolic pathways throughout the c ...

pyruvate
and one molecule of
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
. Liver pyruvate kinase is indirectly regulated by
epinephrine Adrenaline, also known as epinephrine, is a hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a co ...

epinephrine
and
glucagon Glucagon is a peptide hormonePeptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acid chain lengths than the former. These hormones have an effect on the endocrine ...

glucagon
, through protein kinase A. This protein kinase phosphorylates liver pyruvate kinase to deactivate it. Muscle pyruvate kinase is not inhibited by epinephrine activation of protein kinase A. Glucagon signals fasting (no glucose available). Thus, glycolysis is inhibited in the liver but unaffected in muscle when fasting. An increase in blood sugar leads to secretion of
insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main Anabolism, anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and p ...

insulin
, which activates phosphoprotein phosphatase I, leading to dephosphorylation and activation of pyruvate kinase. These controls prevent pyruvate kinase from being active at the same time as the enzymes that catalyze the reverse reaction (pyruvate carboxylase and phosphoenolpyruvate carboxykinase), preventing a futile cycle.


Post-glycolysis processes

The overall process of glycolysis is: :Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 H+ + 2 ATP If glycolysis were to continue indefinitely, all of the NAD+ would be used up, and glycolysis would stop. To allow glycolysis to continue, organisms must be able to oxidize NADH back to NAD+. How this is performed depends on which external electron acceptor is available.


Anoxic regeneration of NAD+

One method of doing this is to simply have the pyruvate do the oxidation; in this process, pyruvate is converted to lactic acid, lactate (the conjugate base of lactic acid) in a process called lactic acid fermentation: :Pyruvate + NADH + H+ → lactate + NAD+ This process occurs in the bacterium, bacteria involved in making yogurt (the lactic acid causes the milk to curdle). This process also occurs in animals under hypoxic (or partially anaerobic) conditions, found, for example, in overworked muscles that are starved of oxygen. In many tissues, this is a cellular last resort for energy; most animal tissue cannot tolerate anaerobic conditions for an extended period of time. Some organisms, such as yeast, convert NADH back to NAD+ in a process called ethanol fermentation. In this process, the pyruvate is converted first to acetaldehyde and carbon dioxide, and then to ethanol. Lactic acid fermentation and ethanol fermentation can occur in the absence of oxygen. This anaerobic fermentation allows many single-cell organisms to use glycolysis as their only energy source. Anoxic regeneration of NAD+ is only an effective means of energy production during short, intense exercise in vertebrates, for a period ranging from 10 seconds to 2 minutes during a maximal effort in humans. (At lower exercise intensities it can sustain muscle activity in Mammalian diving reflex, diving animals, such as seals, whales and other aquatic vertebrates, for very much longer periods of time.) Under these conditions NAD+ is replenished by NADH donating its electrons to pyruvate to form lactate. This produces 2 ATP molecules per glucose molecule, or about 5% of glucose's energy potential (38 ATP molecules in bacteria). But the speed at which ATP is produced in this manner is about 100 times that of oxidative phosphorylation. The pH in the cytoplasm quickly drops when hydrogen ions accumulate in the muscle, eventually inhibiting the enzymes involved in glycolysis. The burning sensation in muscles during hard exercise can be attributed to the release of hydrogen ions during the shift to glucose fermentation from glucose oxidation to carbon dioxide and water, when aerobic metabolism can no longer keep pace with the energy demands of the muscles. These hydrogen ions form a part of lactic acid. The body falls back on this less efficient but faster method of producing ATP under low oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen reached high concentrations in the atmosphere between 2000 and 2500 million years ago, and thus would represent a more ancient form of energy production than the aerobic replenishment of NAD+ in cells. The liver in mammals gets rid of this excess lactate by transforming it back into pyruvate under aerobic conditions; see Cori cycle. Fermentation of pyruvate to lactate is sometimes also called "anaerobic glycolysis", however, glycolysis ends with the production of pyruvate regardless of the presence or absence of oxygen. In the above two examples of fermentation, NADH is oxidized by transferring two electrons to pyruvate. However, anaerobic bacteria use a wide variety of compounds as the terminal electron acceptors in cellular respiration: nitrogenous compounds, such as nitrates and nitrites; sulfur compounds, such as sulfates, sulfites, sulfur dioxide, and elemental sulfur; carbon dioxide; iron compounds; manganese compounds; cobalt compounds; and uranium compounds.


Aerobic regeneration of NAD+, and disposal of pyruvate

In aerobic organisms, a complex mechanism has been developed to use the oxygen in air as the final electron acceptor. * Firstly, the Nicotinamide adenine dinucleotide, NADH + H+ generated by glycolysis has to be transferred to the mitochondrion to be oxidized, and thus to regenerate the NAD+ necessary for glycolysis to continue. However the inner mitochondrial membrane is impermeable to NADH and NAD+. Use is therefore made of two “shuttles” to transport the electrons from NADH across the mitochondrial membrane. They are the malate-aspartate shuttle and the glycerol phosphate shuttle. In the former the electrons from NADH are transferred to cytosolic Oxaloacetic acid, oxaloacetate to form Malic acid, malate. The malate then traverses the inner mitochondrial membrane into the mitochondrial matrix, where it is reoxidized by NAD+ forming intra-mitochondrial oxaloacetate and NADH. The oxaloacetate is then re-cycled to the cytosol via its conversion to aspartate which is readily transported out of the mitochondrion. In the glycerol phosphate shuttle electrons from cytosolic NADH are transferred to dihydroxyacetone to form glycerol-3-phosphate which readily traverses the outer mitochondrial membrane. Glycerol-3-phosphate is then reoxidized to dihydroxyacetone, donating its electrons to Flavin adenine dinucleotide, FAD instead of NAD+. This reaction takes place on the inner mitochondrial membrane, allowing FADH2 to donate its electrons directly to coenzyme Q (ubiquinone) which is part of the electron transport chain which ultimately transfers electrons to molecular oxygen (O2), with the formation of water, and the release of energy eventually captured in the form of Adenosine triphosphate, ATP. * The glycolytic end-product, pyruvate (plus NAD+) is converted to acetyl-CoA, CO2 and NADH + H+ within the mitochondria in a process called pyruvate decarboxylation. * The resulting acetyl-CoA enters the citric acid cycle (or Krebs Cycle), where the acetyl group of the acetyl-CoA is converted into carbon dioxide by two decarboxylation reactions with the formation of yet more intra-mitochondrial NADH + H+. * The intra-mitochondrial NADH + H+ is oxidized to NAD+ by the electron transport chain, using oxygen as the final electron acceptor to form water. The energy released during this process is used to create a hydrogen ion (or proton) gradient across the inner membrane of the mitochondrion. * Finally, the proton gradient is used to produce about 2.5 Adenosine triphosphate, ATP for every NADH + H+ oxidized in a process called oxidative phosphorylation.


Conversion of carbohydrates into fatty acids and cholesterol

The pyruvate produced by glycolysis is an important intermediary in the conversion of carbohydrates into fatty acids and cholesterol. This occurs via the conversion of pyruvate into acetyl-CoA in the mitochondrion. However, this acetyl CoA needs to be transported into cytosol where the synthesis of fatty acids and cholesterol occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, Citric acid, citrate (produced by the condensation of acetyl CoA with Oxaloacetic acid, oxaloacetate) is removed from the citric acid cycle and carried across the inner mitochondrial membrane into the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a sm ...
. There it is cleaved by ATP citrate lyase into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to mitochondrion as malate (and then back into oxaloacetate to transfer more acetyl-CoA out of the mitochondrion). The cytosolic acetyl-CoA can be carboxylated by acetyl-CoA carboxylase into Malonyl-CoA, malonyl CoA, the first committed step in the Fatty acid synthesis, synthesis of fatty acids, or it can be combined with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) which is the rate limiting step controlling the Mevalonate pathway, synthesis of cholesterol. Cholesterol can be used as is, as a structural component of cellular membranes, or it can be used to synthesize the Steroid#Steroidogenesis, steroid hormones, Bile acids, bile salts, and vitamin D.


Conversion of pyruvate into oxaloacetate for the citric acid cycle

Pyruvate molecules produced by glycolysis are active transport, actively transported across the inner Mitochondrion, mitochondrial membrane, and into the matrix where they can either be Redox, oxidized and combined with coenzyme A to form CO2, acetyl-CoA, and NADH, or they can be carboxylated (by pyruvate carboxylase) to form oxaloacetate. This latter reaction "fills up" the amount of oxaloacetate in the citric acid cycle, and is therefore an anaplerotic reaction (from the Greek meaning to "fill up"), increasing the cycle's capacity to metabolize acetyl-CoA when the tissue's energy needs (e.g. in Cardiac muscle, heart and Skeletal striated muscle, skeletal muscle) are suddenly increased by activity. In the citric acid cycle all the intermediates (e.g. citrate, iso-citrate, alpha-ketoglutarate, succinate, fumarate, malate and oxaloacetate) are regenerated during each turn of the cycle. Adding more of any of these intermediates to the mitochondrion therefore means that that additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other. Hence the addition of oxaloacetate greatly increases the amounts of all the citric acid intermediates, thereby increasing the cycle's capacity to metabolize acetyl CoA, converting its acetate component into CO2 and water, with the release of enough energy to form 11 Adenosine triphosphate, ATP and 1 Guanosine triphosphate, GTP molecule for each additional molecule of acetyl CoA that combines with oxaloacetate in the cycle. To cataplerotically remove oxaloacetate from the citric cycle, malate can be transported from the mitochondrion into the cytoplasm, decreasing the amount of oxaloacetate that can be regenerated. Furthermore, citric acid intermediates are Citric acid cycle#Citric acid cycle intermediates serve as substrates for biosynthetic processes, constantly used to form a variety of substances such as the purines, pyrimidines and porphyrins.


Intermediates for other pathways

This article concentrates on the catabolic role of glycolysis with regard to converting potential chemical energy to usable chemical energy during the oxidation of glucose to pyruvate. Many of the metabolites in the glycolytic pathway are also used by anabolic pathways, and, as a consequence, flux through the pathway is critical to maintain a supply of carbon skeletons for biosynthesis. The following metabolic pathways are all strongly reliant on glycolysis as a source of metabolites: and many more. * Pentose phosphate pathway, which begins with the dehydrogenation of
glucose-6-phosphate Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Gluco ...
, the first intermediate to be produced by glycolysis, produces various pentose sugars, and Nicotinamide adenine dinucleotide phosphate, NADPH for the synthesis of fatty acids and cholesterol. * Glycogenesis, Glycogen synthesis also starts with glucose-6-phosphate at the beginning of the glycolytic pathway. * Glycerol, for the formation of
triglyceride A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an derived from and three (from ' and '). Triglycerides are the main constituents of in humans and other vertebrates, as well as . They are also present in the blood to enab ...

triglyceride
s and phospholipids, is produced from the glycolytic intermediate glyceraldehyde-3-phosphate. * Various post-glycolytic pathways: :* Fatty acid metabolism#Fatty acid Synthesis, Fatty acid synthesis :* Cholesterol#Biosynthesis, Cholesterol synthesis :* The citric acid cycle which in turn leads to: ::*Amino acid synthesis ::*Nucleotide#Synthesis, Nucleotide synthesis ::*Porphyrin#Biosynthesis, Tetrapyrrole synthesis Although
gluconeogenesis Gluconeogenesis (GNG) is a that results in the generation of from certain non- carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly i ...

gluconeogenesis
and glycolysis share many intermediates the one is not functionally a branch or tributary of the other. There are two regulatory steps in both pathways which, when active in the one pathway, are automatically inactive in the other. The two processes can therefore not be simultaneously active. Indeed, if both sets of reactions were highly active at the same time the net result would be the hydrolysis of four high energy phosphate bonds (two ATP and two GTP) per reaction cycle. Nicotinamide adenine dinucleotide, NAD+ is the oxidizing agent in glycolysis, as it is in most other energy yielding metabolic reactions (e.g. beta-oxidation of fatty acids, and during the citric acid cycle). The NADH thus produced is primarily used to ultimately transfer electrons to O2 to produce water, or, when O2 is not available, to produced compounds such as Lactic acid, lactate or
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an . It is a simple with the C2H6O. Its formula can be also written as −− or (an linked to a group), and is often as EtOH. Ethanol is a , , ...

ethanol
(see ''Anoxic regeneration of NAD+'' above). NADH is rarely used for synthetic processes, the notable exception being
gluconeogenesis Gluconeogenesis (GNG) is a that results in the generation of from certain non- carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly i ...

gluconeogenesis
. During Fatty acid metabolism#Fatty acid Synthesis, fatty acid and Cholesterol#Biosyntesis, cholesterol synthesis the reducing agent is Nicotinamide adenine dinucleotide phosphate, NADPH. This difference exemplifies a general principle that NADPH is consumed during biosynthetic reactions, whereas NADH is generated in energy-yielding reactions. The source of the NADPH is two-fold. When Malic acid, malate is oxidatively decarboxylated by “NADP+-linked malic enzyme" Pyruvic acid, pyruvate, CO2 and NADPH are formed. NADPH is also formed by the
pentose phosphate pathwayIn chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atom, atoms. The chemical formula of all pentoses is , and their molecular weight is 150.13 g/mol.
which converts glucose into ribose, which can be used in synthesis of nucleotides and nucleic acids, or it can be catabolized to pyruvate.


Glycolysis in disease


Diabetes

Cellular uptake of glucose occurs in respgjgnals, and glucose is subsequently broken down through glycolysis, lowering blood sugar levels. However, the low insulin levels seen in diabetes result in hyperglycemia, where glucose levels in the blood rise and glucose is not properly taken up by cells. Hepatocytes further contribute to this hyperglycemia through
gluconeogenesis Gluconeogenesis (GNG) is a that results in the generation of from certain non- carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly i ...

gluconeogenesis
. Glycolysis in hepatocytes controls hepatic glucose production, and when glucose is overproduced by the liver without having a means of being broken down by the body, hyperglycemia results.


Genetic diseases

Glycolytic mutations are generally rare due to importance of the metabolic pathway, this means that the majority of occurring mutations result in an inability for the cell to respire, and therefore cause the death of the cell at an early stage. However, some mutations are seen with one notable example being Pyruvate kinase deficiency, leading to chronic hemolytic anemia.


Cancer

Malignant tumor cells perform glycolysis at a rate that is ten times faster than their noncancerous tissue counterparts. During their genesis, limited capillary support often results in hypoxia (decreased O2 supply) within the tumor cells. Thus, these cells rely on anaerobic metabolic processes such as glycolysis for ATP (adenosine triphosphate). Some tumor cells overexpress specific glycolytic enzymes which result in higher rates of glycolysis. Often these enzymes are Isoenzymes, of traditional glycolysis enzymes, that vary in their susceptibility to traditional feedback inhibition. The increase in glycolytic activity ultimately counteracts the effects of hypoxia by generating sufficient ATP from this anaerobic pathway. This phenomenon was first described in 1930 by Otto Heinrich Warburg, Otto Warburg and is referred to as the Warburg effect (oncology), Warburg effect. The Warburg hypothesis claims that cancer is primarily caused by dysfunctionality in mitochondrial metabolism, rather than because of the uncontrolled growth of cells. A number of theories have been advanced to explain the Warburg effect. One such theory suggests that the increased glycolysis is a normal protective process of the body and that malignant change could be primarily caused by energy metabolism. This high glycolysis rate has important medical applications, as high aerobic glycolysis by malignant tumors is utilized clinically to diagnose and monitor treatment responses of cancers by Chemical imaging, imaging uptake of Fluorodeoxyglucose, 2-18F-2-deoxyglucose (FDG) (a radioactive modified hexokinase substrate (biochemistry), substrate) with positron emission tomography (PET). There is ongoing research to affect mitochondrial metabolism and treat cancer by reducing glycolysis and thus starving cancerous cells in various new ways, including a ketogenic diet.


Interactive pathway map

The diagram below shows human protein names. Names in other organisms may be different and the number of
isozymeIn biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. diff ...
s (such as HK1, HK2, ...) is likely to be different too.


Alternative nomenclature

Some of the metabolites in glycolysis have alternative names and nomenclature. In part, this is because some of them are common to other pathways, such as the Calvin cycle.


Structure of glycolysis components in Fischer projections and polygonal model

The intermediates of glycolysis depicted in Fischer projections show the chemical changing step by step. Such image can be compared to polygonal model representation.Bonafe, C. F. S.; Bispo, J. A. C.; de Jesus, M. B. (2018). The Polygonal Model: A Simple Representation of Biomolecules as a Tool for Teaching Metabolism. Biochemistry and Molecular Biology Education. 46: 66-75. DOI - 10.1002/bmb.21093. Another comparation of Fischer projections and Poligonal Model in glycolysis is shown in a video. Video animations in the same channel in YouTube can be seen for another metabolic pathway (Krebs Cycle) and the representation and applying of Polygonal Model in Organic Chemistry


See also

* Carbohydrate catabolism * Citric acid cycle * Cori cycle * Fermentation (biochemistry) * Gluconeogenesis * Glycolytic oscillation * Pentose phosphate pathway * Pyruvate decarboxylation * Triose kinase


References


External links


A Detailed Glycolysis Animation provided
by IUBMB
Adobe Flash
Required)

at RCSB PDB
Glycolytic cycle with animations
at wdv.com
Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry, Molecular Biology and Cell Biology


at ufp.pt

at ExPASy *
''metpath'': Interactive representation of glycolysis
{{Authority control Glycolysis, Biochemistry Carbohydrates Cellular respiration Metabolic pathways