electron transport chain

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An electron transport chain (ETC) is a series of
protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple active site, catalytic domains are found in a single polypeptide chain ...
es and other molecules that transfer
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...
s from electron donors to
electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself redox, reduced in the process. Electron acceptors are sometimes ...
s via
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s (H+ ions) across a
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
. The electrons that transferred from NADH and FADH2 to the ETC involves 4 multi-subunit large enzymes complexes and 2 mobile electron carriers. Many of the
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s in the electron transport chain are membrane-bound. The flow of electrons through the electron transport chain is an
exergonic process An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an Endergonic reaction, endergonic process. Constant pressure, constant temperature reactions are exergonic if and ...
. The energy from the redox reactions creates an electrochemical proton gradient that drives the synthesis of
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all ...
(ATP). In
aerobic 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 ...
, the flow of electrons terminates with molecular
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 Chemical reaction, reactive nonmetal, and an oxidizing a ...
as the final electron acceptor. In anaerobic respiration, other electron acceptors are used, such as
sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
. In an electron transport chain, the redox reactions are driven by the difference in 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 (physics), work that may be performed by a closed system, thermodynamically closed system a ...
of reactants and products. The free energy released when a higher-energy electron donor and acceptor convert to lower-energy products, while electrons are transferred from a lower to a higher
redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
, is used by the complexes in the electron transport chain to create an electrochemical gradient of ions. It is this electrochemical gradient that drives the synthesis of ATP via coupling with
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order t ...
with ATP synthase. In eukaryotic organisms the electron transport chain, and site of oxidative phosphorylation, is found on the
inner mitochondrial membrane The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space. Structure The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. Th ...
. The energy released by reactions of oxygen and reduced compounds such as cytochrome ''c'' and (indirectly)
NADH Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphat ...
and FADH2 is used by the electron transport chain to pump protons into the intermembrane space, generating the electrochemical gradient over the inner mitochondrial membrane. In photosynthetic eukaryotes, the electron transport chain is found on the thylakoid membrane. Here, light energy drives electron transport through a proton pump and the resulting proton gradient causes subsequent synthesis of ATP. In
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, the electron transport chain can vary between species but it always constitutes a set of redox reactions that are coupled to the synthesis of ATP through the generation of an electrochemical gradient and oxidative phosphorylation through ATP synthase.

# Mitochondrial electron transport chains

Most
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
cells have
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
, which produce ATP from reactions of oxygen with products of the
citric acid 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 Redox, oxidation of acetyl-CoA derived from carbohydrates, fats, and p ...
,
fatty acid metabolism Fatty acid metabolism consists of various metabolic processes involving or closely related to fatty acids, a family of molecules classified within the lipid macronutrient category. These processes can mainly be divided into (1) catabolic proces ...
, and amino acid metabolism. At the
inner mitochondrial membrane The inner mitochondrial membrane (IMM) is the mitochondrial membrane which separates the mitochondrial matrix from the intermembrane space. Structure The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. Th ...
, electrons from
NADH Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphat ...
and FADH pass through the electron transport chain to oxygen, which provides the energy driving the process as it is reduced to water. The electron transport chain comprises an enzymatic series of electron donors and acceptors. Each electron donor will pass electrons to an acceptor of higher redox potential, which in turn donates these electrons to another acceptor, a process that continues down the series until electrons are passed to oxygen, the terminal electron acceptor in the chain. Each reaction releases energy because a higher-energy donor and acceptor convert to lower-energy products. Via the transferred electrons, this energy is used to generate a proton gradient across the mitochondrial membrane by "pumping" protons into the intermembrane space, producing a state of higher free energy that has the potential to do work. This entire process is called
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order t ...
since ADP is phosphorylated to ATP by using the electrochemical gradient that the redox reactions of the electron transport chain have established driven by energy-releasing reactions of oxygen.

## Mitochondrial redox carriers

Energy associated with the transfer of electrons down the electron transport chain is used to pump protons from the
mitochondrial matrix In the mitochondrion, the matrix is the space within the Inner mitochondrial membrane, inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contain ...
into the intermembrane space, creating an electrochemical proton gradient ( ΔpH) across the inner mitochondrial membrane. This proton gradient is largely but not exclusively responsible for the mitochondrial
membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological Cell (biology), cell. That is, there is a difference in the energy required for e ...
(ΔΨ). It allows ATP synthase to use the flow of H+ through the enzyme back into the matrix to generate ATP from
adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells (biology), cells. ADP consists of three important structural components: ...
(ADP) and inorganic phosphate. Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q (ubiquinone; labeled Q), which also receives electrons from Complex II (
succinate dehydrogenase Succinate dehydrogenase (SDH) or succinate-coenzyme Q reductase (SQR) or respiratory complex II is an enzyme complex, found in many bacterial Cell (biology), cells and in the inner mitochondrial membrane of eukaryotes. It is the only enzyme that ...
; labeled II). Q passes electrons to Complex III ( cytochrome bc1 complex; labeled III), which passes them to cytochrome ''c'' (cyt ''c''). Cyt ''c'' passes electrons to Complex IV ( cytochrome ''c'' oxidase; labeled IV). Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are proton pumps. The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain can be summarized as follows: NADH+H → ''Complex I'' → Q ↑ ''Complex II'' ↑ ''Succinate'' → ''Complex III'' → cytochrome ''c'' → ''Complex IV'' → HO ↑ ''Complex II'' ↑ ''Succinate''

### Complex I

In Complex I (NADH ubiquinone oxidoreductase, Type I NADH dehydrogenase, or mitochondrial complex I; ), two electrons are removed from NADH and transferred to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH), freely diffuses within the membrane, and Complex I translocates four protons (H) across the membrane, thus producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide.Lauren, Biochemistry, Johnson/Cole, 2010, pp 598-611 The pathway of electrons is as follows:
NADH Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphat ...
is oxidized to NAD, by reducing
flavin mononucleotide Flavin mononucleotide (FMN), or riboflavin-5′-phosphate, is a biomolecule produced from riboflavin (vitamin B2) by the enzyme riboflavin kinase and functions as the prosthetic group of various oxidoreductases, including NADH dehydrogenase, as we ...
to FMNH in one two-electron step. FMNH is then oxidized in two one-electron steps, through a semiquinone intermediate. Each electron thus transfers from the FMNH to an Fe–S cluster, from the Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical ( semiquinone) form of Q, and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH. During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space.Garrett & Grisham, Biochemistry, Brooks/Cole, 2010, pp 598-611 As the electrons move through the complex an electron current is produced along the 180 Angstrom width of the complex within the membrane. This current powers the active transport of four protons to the intermembrane space per two electrons from NADH.

### Complex II

In Complex II (
succinate dehydrogenase Succinate dehydrogenase (SDH) or succinate-coenzyme Q reductase (SQR) or respiratory complex II is an enzyme complex, found in many bacterial Cell (biology), cells and in the inner mitochondrial membrane of eukaryotes. It is the only enzyme that ...
or succinate-CoQ reductase; ) additional electrons are delivered into the
quinone The quinones are a class of organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains ...
pool (Q) originating from succinate and transferred (via flavin adenine dinucleotide (FAD)) to Q. Complex II consists of four protein subunits: succinate dehydrogenase (SDHA); succinate dehydrogenase biquinoneiron–sulfur subunit mitochondrial (SDHB); succinate dehydrogenase complex subunit C (SDHC) and succinate dehydrogenase complex subunit D (SDHD). Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD). Complex II is a parallel electron transport pathway to complex 1, but unlike Complex I, no protons are transported to the intermembrane space in this pathway. Therefore, the pathway through Complex II contributes less energy to the overall electron transport chain process.

### Complex III

In Complex III ( cytochrome ''bc1'' complex or CoQH-cytochrome ''c'' reductase; ), the Q-cycle contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH at the QO site and sequentially transferred to two molecules of cytochrome ''c'', a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Qi site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol (2H+2e-) oxidations at the Qo site to form one quinone (2H+2e-) at the Qi site. (In total, four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules.) : QH2 + 2$\textc$(Fe^) + 2 H$^+_\text$ -> Q + 2$\textc$(Fe^) + 4 H$^+_\text$ When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation. This complex is inhibited by dimercaprol (British Antilewisite, BAL), Napthoquinone and Antimycin.

### Complex IV

In
Complex IV The enzyme cytochrome c oxidase or Complex IV, (was , now reclassified as a translocasEC 7.1.1.9 is a large transmembrane protein complex found in bacteria, archaea, and mitochondria of eukaryotes. It is the last enzyme in the Cellular respirati ...
( cytochrome ''c'' oxidase; ), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome ''c'' and transferred to molecular oxygen (O) and four protons, producing two molecules of water. The complex contains coordinated copper ions and several heme groups. At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient. The exact details of proton pumping in Complex IV are still under study. Cyanide is an inhibitor of Complex IV.

## Coupling with oxidative phosphorylation

According to the chemiosmotic coupling hypothesis, proposed by
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
winner Peter D. Mitchell, the electron transport chain and
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order t ...
are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates 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 Concentration, solute concentration across a me ...
(proton gradient). This gradient is used by the FF ATP synthase complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes described as ''Complex V'' of the electron transport chain. The F component of ATP synthase acts as an
ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...
that provides for a proton flux back into the mitochondrial matrix. It is composed of a, b and c subunits. Protons in the inter-membrane space of mitochondria first enter the ATP synthase complex through an ''a'' subunit channel. Then protons move to the c subunits. The number of c subunits determines how many protons are required to make the F turn one full revolution. For example, in humans, there are 8 c subunits, thus 8 protons are required. After ''c'' subunits, protons finally enter the matrix through an ''a'' subunit channel that opens into the mitochondrial matrix. This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD and Q) with energy provided by O. The free energy is used to drive ATP synthesis, catalyzed by the F component of the complex.
Coupling with oxidative phosphorylation is a key step for ATP production. However, in specific cases, uncoupling the two processes may be biologically useful. The uncoupling protein, thermogenin—present in the inner mitochondrial membrane of
brown adipose tissue Brown adipose tissue (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals. Classification of brown fat refers to two distinct cell populations with si ...
—provides for an alternative flow of protons back to the inner mitochondrial matrix. Thyroxine is also a natural uncoupler. This alternative flow results in
thermogenesis Thermogenesis is the process of heat production in organisms. It occurs in all warm-blooded animals, and also in a few species of thermogenic plants such as the Eastern skunk cabbage, the Voodoo lily (''Sauromatum venosum''), and the giant wate ...
rather than ATP production.

## Reverse electron flow

Reverse electron flow is the transfer of electrons through the electron transport chain through the reverse redox reactions. Usually requiring a significant amount of energy to be used, this can reduce the oxidized forms of electron donors. For example, NAD+ can be reduced to NADH by Complex I. There are several factors that have been shown to induce reverse electron flow. However, more work needs to be done to confirm this. One example is blockage of ATP synthase, resulting in a build-up of protons and therefore a higher
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, adenosine triphosphate (ATP) by the movement of hydrogen ion ...
, inducing reverse electron flow.

# Bacterial electron transport chains

In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is NADH → ''Complex I'' → Q → ''Complex III'' → cytochrome ''c'' → ''Complex IV'' → O where ''Complexes I, III'' and'' IV'' are proton pumps, while Q and cytochrome ''c'' are mobile electron carriers. The electron acceptor for this process is molecular oxygen. In
prokaryotes A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek language, Greek wikt:πρό#Ancient Greek, πρό (, 'before') a ...
(
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
and
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
) the situation is more complicated, because there are several different electron donors and several different electron acceptors. The generalized electron transport chain in bacteria is: Donor Donor Donor ↓ ↓ ↓ dehydrogenase → quinone → ''bc'' → cytochrome ↓ ↓ oxidase(reductase) oxidase(reductase) ↓ ↓ Acceptor Acceptor Electrons can enter the chain at three levels: at the level of a dehydrogenase, at the level of the quinone pool, or at the level of a mobile
cytochrome Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a Cofactor (biochemistry), cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of h ...
electron carrier. These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor. In other words, they correspond to successively smaller Gibbs free energy changes for the overall redox reaction. Individual bacteria use multiple electron transport chains, often simultaneously. Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. For example, ''E. coli'' (when growing aerobically using glucose and oxygen as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously. A common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane. Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain two or at least one.

## Electron donors

In the current biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an electron source are called '' organotrophs''. Chemoorganotrophs (animals, fungi, protists) and '' photolithotrophs'' (plants and algae) constitute the vast majority of all familiar life forms. Some prokaryotes can use inorganic matter as an electron source. Such an organism is called a '' (chemo)lithotroph'' ("rock-eater"). Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, manganese oxide, and ferrous iron. Lithotrophs have been found growing in rock formations thousands of meters below the surface of Earth. Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere. The use of inorganic electron donors such as hydrogen as an energy source is of particular interest in the study of evolution. This type of metabolism must logically have preceded the use of organic molecules and oxygen as an energy source.

## Complexes I and II

Bacteria can use several different electron donors. When organic matter is the electron source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar to ''Complex I'' in mitochondria) or succinate dehydrogenase (similar to ''Complex II''). Other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, H dehydrogenase ( hydrogenase), electron transport chain. Some dehydrogenases are also proton pumps, while others funnel electrons into the quinone pool. Most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs triggered by the environment in which the cells grow. In the case of lactate dehydrogenase in ''E. coli'', the enzyme is used aerobically and in combination with other dehydrogenases. It is inducible and is expressed when the concentration of DL-lactate in the cell is high.

## Quinone carriers

Quinone The quinones are a class of organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains ...
s are mobile, lipid-soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in the membrane. Bacteria use ubiquinone (Coenzyme Q, the same quinone that mitochondria use) and related quinones such as menaquinone (Vitamin K). Archaea in the genus '' Sulfolobus'' use caldariellaquinone. The use of different quinones is due to slight changes in redox potentials caused by changes in structure. The change in redox potentials of these quinones may be suited to changes in the electron acceptors or variations of redox potentials in bacterial complexes.

## Proton pumps

A '' proton pump'' is any process that creates a proton gradient across a membrane. Protons can be physically moved across a membrane, as seen in mitochondrial ''Complexes I'' and ''IV''. The same effect can be produced by moving electrons in the opposite direction. The result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm. Mitochondrial ''Complex III'' uses this second type of proton pump, which is mediated by a quinone (the Q cycle). Some dehydrogenases are proton pumps, while others are not. Most oxidases and reductases are proton pumps, but some are not. Cytochrome ''bc1'' is a proton pump found in many, but not all, bacteria (not in ''E. coli''). As the name implies, bacterial ''bc1'' is similar to mitochondrial ''bc1'' (''Complex III'').

## Cytochrome electron carriers

Cytochromes are proteins that contain iron. They are found in two very different environments. Some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile macromolecular structures imbedded in the membrane. The mobile cytochrome electron carrier in mitochondria is cytochrome ''c''. Bacteria use a number of different mobile cytochrome electron carriers. Other cytochromes are found within macromolecules such as ''Complex III'' and ''Complex IV''. They also function as electron carriers, but in a very different, intramolecular, solid-state environment. Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier. For example, electrons from inorganic electron donors (nitrite, ferrous iron, electron transport chain) enter the electron transport chain at the cytochrome level. When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule.

## Terminal oxidases and reductases

When bacteria grow in
aerobic 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 ...
environments, the terminal electron acceptor (O) is reduced to water by an enzyme called an ''oxidase''. When bacteria grow in
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: *Adhesive#Anaerobic, Anaerobic ad ...
environments, the terminal electron acceptor is reduced by an enzyme called a reductase. In mitochondria the terminal membrane complex (''Complex IV'') is cytochrome oxidase.
Aerobic 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 ...
bacteria use a number of different terminal oxidases. For example, ''E. coli'' (a facultative anaerobe) does not have a cytochrome oxidase or a ''bc1'' complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water. Bacterial Complex IV can be split into classes according to the molecules act as terminal electron acceptors. Class I oxidases are cytochrome oxidases and use oxygen as the terminal electron acceptor. Class II oxidases are quinol oxidases and can use a variety of terminal electron acceptors. Both of these classes can be subdivided into categories based on what redox-active components they contain. E.g. Heme aa3 Class 1 terminal oxidases are much more efficient than Class 2 terminal oxidases Anaerobic bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor. For example, ''E. coli'' can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine-N-oxide reductase, depending on the availability of these acceptors in the environment. Most terminal oxidases and reductases are ''inducible''. They are synthesized by the organism as needed, in response to specific environmental conditions.

## Electron acceptors

Just as there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are a number of different electron acceptors, both organic and inorganic. If oxygen is available, it is most often used as the terminal electron acceptor in aerobic bacteria and facultative anaerobes. Mostly in anaerobic environments different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate.

# Photosynthetic

In
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order t ...
, electrons are transferred from an electron donor such as NADH to an acceptor such as O through an electron transport chain, releasing energy. In
photophosphorylation In the process of photosynthesis Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism' ...
, the energy of sunlight is used to ''create'' a high-energy electron donor which can subsequently reduce oxidized components and couple to ATP synthesis via proton translocation by the electron transport chain. Photosynthetic electron transport chains, like the mitochondrial chain, can be considered as a special case of the bacterial systems. They use mobile, lipid-soluble quinone carriers (
phylloquinone Phytomenadione, also known as vitamin K1 or phylloquinone, is a vitamin found in food and used as a dietary supplement. It is on the WHO Model List of Essential Medicines, World Health Organization's List of Essential Medicines. As a supplemen ...
and plastoquinone) and mobile, water-soluble carriers (
cytochrome Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a Cofactor (biochemistry), cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of h ...
s). They also contain a proton pump. The proton pump in ''all'' photosynthetic chains resembles mitochondrial ''Complex III''. The commonly-held theory of
symbiogenesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly ...
proposes that both organelles descended from bacteria.

*
Charge-transfer complex In chemistry, a charge-transfer (CT) complex or electron-donor-acceptor complex describes a type of supramolecular assembly of two or more molecules or ions. The assembly consists of two molecules that self-attract through Electrostatics, elect ...
*
CoRR hypothesis The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("redox") state of its gene products. CoRR is short for "co-location for redox regulation ...
*Electron equivalent *Hydrogen hypothesis *Respirasome