Substrate-level phosphorylation generating ATP

Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

group from a substrate directly to ADP or

GDP Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced and sold (not resold) in a specific time period by countries. Due to its complex and subjective nature this measure is ofte ...

. Transferring from a higher energy (whether phosphate group attached or not) into a lower energy product. This process uses some of the released

chemical energy Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (2018). "How ...

, the

Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...

, to transfer a

phosphoryl {{unreferenced, date=May 2015 A phosphoryl group is the chemical ion or radical: P+O32−, containing phosphorus and oxygen. (The correct chemical name for this −PO32− group is phosphonato, and phosphono for −PO3H2; as ''phosphoryl'' in ch ...

(PO₃) group to ADP or GDP from another phosphorylated compound. Occurs in glycolysis and in the citric acid cycle. Unlike

oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing chemical energy in order to produce adenosine tri ...

, oxidation and phosphorylation are not coupled in the process of substrate-level phosphorylation, and reactive intermediates are most often gained in the course of

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

processes in catabolism. Most ATP is generated by oxidative phosphorylation in

aerobic Aerobic means "requiring 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 * Aerobic respiration, the aerobic process of cel ...

anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing re ...

while substrate-level phosphorylation provides a quicker, less efficient source of ATP, independent of external electron acceptors. This is the case in human

erythrocyte 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 ...

s, which have no

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

, and in oxygen-depleted muscle.

Overview

Adenosine triphosphate is a major "energy currency" of the cell. The high energy bonds between the phosphate groups can be broken to power a variety of reactions used in all aspects of cell function. Substrate-level phosphorylation occurs in the cytoplasm of cells during glycolysis and in mitochondria either during the

Krebs cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protein ...

or by MTHFD1L
EC 6.3.4.3
, an enzyme interconverting ADP + phosphate + 10-formyltetrahydrofolate to ATP + formate + tetrahydrofolate (reversibly), under both

and

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

conditions. In the pay-off phase of glycolysis, a net of 2 ATP are produced by substrate-level phosphorylation.

Glycolysis

The first substrate-level phosphorylation occurs after the conversion of 3-phosphoglyceraldehyde and Pi and NAD+ to 1,3-bisphosphoglycerate via

glyceraldehyde 3-phosphate dehydrogenase Glyceraldehyde 3-phosphate dehydrogenase (abbreviated GAPDH) () is an enzyme of about 37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules. In addition to this long establishe ...

. 1,3-bisphosphoglycerate is then dephosphorylated via

phosphoglycerate kinase Phosphoglycerate kinase () (PGK 1) is an enzyme that catalyzes the reversible transfer of a phosphate group from 1,3-bisphosphoglycerate (1,3-BPG) to ADP producing 3-phosphoglycerate (3-PG) and ATP : :1,3-bisphosphoglycerate + ADP glycerat ...

, producing 3-phosphoglycerate and ATP through a substrate-level phosphorylation. The second substrate-level phosphorylation occurs by dephosphorylating

phosphoenolpyruvate Phosphoenolpyruvate (2-phosphoenolpyruvate, PEP) is the ester derived from the enol of pyruvate and phosphate. It exists as an anion. PEP is an important intermediate in biochemistry. It has the highest-energy phosphate bond found (−61.9 kJ/ ...

, catalyzed by

pyruvate kinase Pyruvate kinase is the enzyme involved in the last step of glycolysis. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate (PEP) to adenosine diphosphate (ADP), yielding one molecule of pyruvate and one molecule of ATP. P ...

, producing pyruvate and ATP. During the preparatory phase, each 6-carbon glucose molecule is broken into two 3-carbon molecules. Thus, in glycolysis dephosphorylation results in the production of 4 ATP. However, the prior preparatory phase consumes 2 ATP, so the net yield in glycolysis is 2 ATP. 2 molecules of NADH are also produced and can be used in oxidative phosphorylation to generate more ATP.

Mitochondria

ATP can be generated by substrate-level phosphorylation in

in a pathway that is independent from the

proton motive force Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...

. In the

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

there are three reactions capable of substrate-level phosphorylation, utilizing either

phosphoenolpyruvate carboxykinase Phosphoenolpyruvate carboxykinase (, PEPCK) is an enzyme in the lyase family used in the metabolic pathway of gluconeogenesis. It converts oxaloacetate into phosphoenolpyruvate and carbon dioxide. It is found in two forms, cytosolic and mitoch ...

or succinate-CoA ligase, or monofunctional C1-tetrahydrofolate synthase.

Phosphoenolpyruvate carboxykinase

Mitochondrial phosphoenolpyruvate carboxykinase is thought to participate in the transfer of the phosphorylation potential from the matrix to the cytosol and vice versa. However, it is strongly favored towards GTP hydrolysis, thus it is not really considered as an important source of intra-mitochondrial substrate-level phosphorylation.

Succinate-CoA ligase

Succinate-CoA ligase is a heterodimer composed of an invariant α-subunit and a substrate-specific ß-subunit, encoded by either SUCLA2 or SUCLG2. This combination results in either an ADP-forming succinate-CoA ligase (A-SUCL, EC 6.2.1.5) or a GDP-forming succinate-CoA ligase (G-SUCL, EC 6.2.1.4). The ADP-forming succinate-CoA ligase is potentially the only matrix enzyme generating ATP in the absence of a proton motive force, capable of maintaining matrix ATP levels under energy-limited conditions, such as transient hypoxia.

Monofunctional C1-tetrahydrofolate synthase

This enzyme is encoded by MTHFD1L and reversibly interconverts ADP + phosphate + 10-formyltetrahydrofolate to ATP + formate + tetrahydrofolate.

Other mechanisms

In working skeletal muscles and the brain,

Phosphocreatine Phosphocreatine, also known as creatine phosphate (CP) or PCr (Pcr), is a phosphorylated form of creatine that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle, myocardium and the brain to recycle adenosine tr ...

is stored as a readily available high-energy phosphate supply, and the enzyme

creatine phosphokinase Creatine kinase (CK), also known as creatine phosphokinase (CPK) or phosphocreatine kinase, is an enzyme () expressed by various tissues and cell types. CK catalyses the conversion of creatine and uses adenosine triphosphate (ATP) to create pho ...

transfers a phosphate from phosphocreatine to ADP to produce ATP. Then the ATP releases giving chemical energy. This is sometimes erroneously considered to be substrate-level phosphorylation, although it is a

transphosphorylation Transphosphorylation is a chemical reaction in which a phosphate group or a phosphono group is transferred between a substrate and a receptor. There are various phosphate esters in living body including nucleic acid, and phosphorylation reaction ...

Importance of substrate-level phosphorylation in anoxia

During anoxia, provision of ATP by substrate-level phosphorylation in the matrix is important not only as a mere means of energy, but also to prevent mitochondria from straining glycolytic ATP reserves by maintaining the

adenine nucleotide translocator Adenine nucleotide translocator (ANT), also known as the ADP/ATP translocase (ANT), ADP/ATP carrier protein (AAC) or mitochondrial ADP/ATP carrier, exchanges free ATP with free ADP across the inner mitochondrial membrane. ANT is the most abund ...

in ‘forward mode’ carrying ATP towards the cytosol.

Oxidative phosphorylation

An alternative method used to create ATP is through

, which takes place during

cellular respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...

. This process utilizes the oxidation of

NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...

to NAD⁺, yielding 3 ATP, and of FADH₂ to FAD, yielding 2 ATP. The

potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...

stored as an

electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and th ...

of protons (H⁺) across the inner mitochondrial membrane is required to generate ATP from ADP and P_i (inorganic phosphate molecule), a key difference from substrate-level phosphorylation. This gradient is exploited by ATP synthase acting as a pore, allowing H⁺ from the mitochondrial intermembrane space to move down its electrochemical gradient into the matrix and coupling the release of free energy to ATP synthesis. Conversely, electron transfer provides the energy required to actively pump H⁺ out of the matrix.

References

{{reflist Metabolism Phosphorus