View source for Electron transport chain

{{Short description|Energy-producing metabolic pathway}}
An '''electron transport chain''' ('''ETC'''<ref>{{cite book |doi=10.1016/B978-0-443-10281-3.00013-0 |chapter=Biochemistry |title=Basic Science in Obstetrics and Gynaecology |year=2010 |last1=Lyall |first1=Fiona |pages=143–171 |isbn=978-0-443-10281-3 }}</ref>) is a series of [[protein complex]]es and other molecules which [[electron transfer|transfer]] [[electron]]s from [[electron donor]]s to [[electron acceptor]]s via [[redox]] reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of [[proton]]s (H<sup>+</sup> ions) across a [[biological membrane|membrane]]. Many of the [[enzyme]]s in the electron transport chain are embedded within the [[membrane]].

The flow of electrons through the electron transport chain is an [[exergonic process]]. The energy from the redox reactions creates an [[electrochemical gradient|electrochemical proton gradient]] that drives the synthesis of [[adenosine triphosphate]] (ATP). In [[Cellular respiration#Aerobic respiration|aerobic respiration]], the flow of electrons terminates with molecular [[oxygen]] as the final electron acceptor. In [[anaerobic respiration]], other electron acceptors are used, such as [[sulfate]].

In an electron transport chain, the redox reactions are driven by the difference in the [[Gibbs free energy]] 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]], 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]] with [[ATP synthase]].<ref name="Anraku 101–132">{{cite journal | vauthors = Anraku Y | title = Bacterial electron transport chains | journal = Annual Review of Biochemistry | volume = 57 | issue = 1 | pages = 101–32 | date = June 1988 | pmid = 3052268 | doi = 10.1146/annurev.bi.57.070188.000533 }}</ref>

In [[Eukaryotes|eukaryotic organisms]], the electron transport chain, and site of [[oxidative phosphorylation]], is found on the [[inner mitochondrial membrane]]. The energy released by reactions of oxygen and reduced compounds such as [[cytochrome]] ''c'' and (indirectly) [[Nicotinamide adenine dinucleotide|NADH]] and [[Flavin adenine dinucleotide|FADH{{sub|2}}]] is used by the electron transport chain to pump protons into the [[intermembrane space]], generating the [[electrochemical gradient]] over the inner [[mitochondrial membrane]]. In [[Photosynthesis|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]], 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.<ref>{{cite journal | vauthors = Kracke F, Vassilev I, Krömer JO | title = Microbial electron transport and energy conservation - the foundation for optimizing bioelectrochemical systems | language = en | journal = Frontiers in Microbiology | volume = 6 | page = 575 | date = 2015 | pmid = 26124754 | pmc = 4463002 | doi = 10.3389/fmicb.2015.00575 | doi-access = free }} – This source shows four ETCs (''Geobacter'', ''Shewanella'', ''Moorella '', ''Acetobacterium'') in figures 1 and 2.</ref>

==Mitochondrial electron transport chains==
[[Image:Mitochondrial electron transport chain—Etc4.svg|thumb|300px|The electron transport chain in the [[mitochondrion]] is the site of [[oxidative phosphorylation]] in [[eukaryote]]s. It mediates the reaction between [[NADH]] or [[Succinic acid|succinate]] generated in the [[citric acid cycle]] and oxygen to power [[ATP synthase]].]]
Most [[eukaryotic]] cells have [[mitochondria]], which produce ATP from reactions of oxygen with products of the [[citric acid cycle]], [[fatty acid metabolism]], and [[amino acid metabolism]]. At the [[inner mitochondrial membrane]], electrons from [[Nicotinamide adenine dinucleotide|NADH]] and [[Flavin adenine dinucleotide|FADH{{sub|2}}]] pass through the electron transport chain to oxygen, which provides the energy driving the process as it is reduced to water.<ref>{{Cite journal| vauthors = Waldenström JG |date=2009-04-24|title=Biochemistry. By Lubert Stryer|journal=Acta Medica Scandinavica|volume=198|issue=1–6|page=436|doi=10.1111/j.0954-6820.1975.tb19571.x|issn=0001-6101}}</ref> The electron transport chain comprises an [[enzymatic]] series of electron donors and acceptors. Each [[electron donor]] will pass electrons to an [[Electron acceptor|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 [[proton pump|"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]] 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.{{cn|date=January 2025}}

===Mitochondrial redox carriers===<!-- This section is linked from [[Mitochondrion]] -->
Energy associated with the transfer of electrons down the electron transport chain is used to pump protons from the [[mitochondrial matrix]] into the intermembrane space, creating an [[Electrochemical gradient|electrochemical proton gradient]] ([[Oxidative phosphorylation#Chemiosmosis|ΔpH]]) across the inner mitochondrial membrane. This proton gradient is largely but not exclusively responsible for the mitochondrial [[membrane potential]] (ΔΨ{{sub|M}}).<ref name="Zorova">{{cite journal | vauthors = Zorova LD, Popkov VA, Plotnikov EY, Silachev DN, Pevzner IB, Jankauskas SS, Babenko VA, Zorov SD, Balakireva AV, Juhaszova M, Sollott SJ, Zorov DB | display-authors = 6 | title = Mitochondrial membrane potential | journal = Analytical Biochemistry | volume = 552 | pages = 50–59 | date = July 2018 | pmid = 28711444 | pmc = 5792320 | doi = 10.1016/j.ab.2017.07.009 }}</ref> It allows [[ATP synthase]] to use the flow of H<sup>+</sup> through the enzyme back into the matrix to generate ATP from [[adenosine diphosphate]] (ADP) and [[inorganic phosphate]]. Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the [[Krebs cycle]] electron carrier [[Nicotinamide adenine dinucleotide|nicotinamide adenine dinucleotide (NADH)]], and passes them to [[coenzyme Q]] ([[ubiquinone]]; labeled Q), which also receives electrons from Complex II ([[succinate dehydrogenase]]; labeled II). Q passes electrons to Complex III ([[cytochrome bc1 complex|cytochrome bc<sub>1</sub> complex]]; labeled III), which passes them to [[cytochrome c|cytochrome ''c'']] (cyt ''c''). Cyt ''c'' passes electrons to Complex IV ([[cytochrome c oxidase|cytochrome ''c'' oxidase]]; labeled IV).{{cn|date=January 2025}}

Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex [[Transmembrane protein|transmembrane]] structure that is embedded in the inner membrane. Three of them are [[proton pump]]s. 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:

[[File: Mitochondrial ETC.svg|420px|alt=Flowchart of the respiratory electron transport chain of a mitochondrion. It shows two different entry points – one from the top left corner and another from the bottom – and one "O2" exit point. The long or central chain has a pathway starting at the top left corner with NADH, leading downwards to NADH:ubiquinone oxidoreductase, then leftwards to Ubiquinone, then again to Cytochrome bc1, then to Cytochrome C, then downwards to Cytochrome C oxidase, and then to O2. The second pathway starts at the bottom with Succinate, leading upwards to Succinate dehydrogenase, and then it converges with the central chain at Ubiquinone.]]

====Complex I====
{{Further|Respiratory complex I}}
In [[Respiratory complex I|Complex I]] (NADH ubiquinone oxidoreductase, Type I NADH dehydrogenase, or mitochondrial complex I; {{EC number|1.6.5.3}}), two electrons are removed from NADH and transferred to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH{{sub|2}}), freely diffuses within the membrane, and Complex I translocates four protons (H{{sup|+}}) 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]].<ref name = "Lauren">Lauren, Biochemistry, Johnson/Cole, 2010, pp 598-611</ref>

The pathway of electrons is as follows:

[[NADH]] is oxidized to NAD{{sup|+}}, by reducing [[flavin mononucleotide]] to FMNH{{sub|2}} in one two-electron step. FMNH{{sub|2}} is then oxidized in two one-electron steps, through a [[Ubiquinone#Chemical properties|semiquinone]] intermediate. Each electron thus transfers from the FMNH{{sub|2}} to an [[iron–sulfur cluster|Fe–S cluster]], from the Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical ([[Ubiquinone#Chemical properties|semiquinone]]) form of Q, and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH{{sub|2}}. During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space.<ref name = "Garrett">Garrett & Grisham, Biochemistry, Brooks/Cole, 2010, pp 598-611</ref> 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.<ref>{{Cite book|title=biochemistry| vauthors = Garrett R, Grisham CM |year=2016|isbn=978-1-305-57720-6| location = Boston | publisher = Cengage |page=687}}</ref>

====Complex II====
In [[Respiratory complex II|Complex II]] ([[succinate dehydrogenase]] or succinate-CoQ reductase; {{EC number|1.3.5.1}}) additional electrons are delivered into the [[quinone]] pool (Q) originating from succinate and transferred (via [[Flavin adenine dinucleotide|flavin adenine dinucleotide (FAD)]]) to Q. Complex II consists of four protein subunits: succinate dehydrogenase (SDHA); succinate dehydrogenase [ubiquinone] iron–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 I, 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.{{cn|date=January 2025}}

====Complex III====
In [[Complex III]] ([[cytochrome bc1 complex|cytochrome ''bc<sub>1</sub>'' complex]] or CoQH{{sub|2}}-cytochrome ''c'' reductase; {{EC number|1.10.2.2}}), the [[Q cycle|Q-cycle]] contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH{{sub|2}} at the Q<sub>O</sub> site and sequentially transferred to two molecules of [[cytochrome c|cytochrome ''c'']], a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Q<sub>i</sub> site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol (<chem>2H+2e-</chem>) oxidations at the Q<sub>o</sub> site to form one quinone (<chem>2H+2e-</chem>) at the Q<sub>i</sub> site. (In total, four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules.){{cn|date=January 2025}}
: <chem>  QH2 + 2</chem><math> \text{ cytochrome }c</math><chem>(Fe^{III}) + 2 H</chem><math>^+_\text{in}</math><chem> -> Q + 2</chem><math> \text{ cytochrome }c</math><chem>(Fe^{II}) + 4 H</chem><math>^+_\text{out}</math>

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 Anti-Lewisite, BAL), [[naphthoquinone]] and antimycin.

====Complex IV====
In [[Complex IV]] ([[cytochrome c oxidase|cytochrome ''c'' oxidase]]; {{EC number|1.9.3.1}}), sometimes called cytochrome AA3, four electrons are removed from four molecules of [[cytochrome c|cytochrome ''c'']] and transferred to molecular oxygen (O{{sub|2}}) 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.<ref name=":1">{{Cite book|last=Stryer.|title=Biochemistry|publisher=toppan|oclc=785100491}}</ref> [[Cyanide]] is an inhibitor of Complex IV.{{cn|date=January 2025}}

===Coupling with oxidative phosphorylation===
[[File:ATP-Synthase.svg|thumb|240px|Depiction of [[ATP synthase]], the site of oxidative phosphorylation to generate ATP.]]
According to the [[chemiosmosis|chemiosmotic coupling hypothesis]], proposed by [[Nobel Prize in Chemistry]] winner [[Peter D. Mitchell]], the electron transport chain and [[oxidative phosphorylation]] are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates an [[electrochemical gradient]] (proton gradient). This gradient is used by the F{{sub|O}}F{{sub|1}} [[ATP synthase]] complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes described as ''Complex V'' of the electron transport chain.<ref>{{cite journal | vauthors = Jonckheere AI, Smeitink JA, Rodenburg RJ | title = Mitochondrial ATP synthase: architecture, function and pathology | journal = Journal of Inherited Metabolic Disease | volume = 35 | issue = 2 | pages = 211–25 | date = March 2012 | pmid = 21874297 | pmc = 3278611 | doi = 10.1007/s10545-011-9382-9 }}</ref> The F{{sub|O}} component of [[ATP synthase]] acts as an [[ion channel]] 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.<ref name=":0">{{Cite book|title=Biochemistry| vauthors = Garrett RH, Grisham CM |publisher=Cengage learning|year=2012|isbn=978-1-133-10629-6|edition=5th|page=664}}</ref> The number of c subunits determines how many protons are required to make the F{{sub|O}} turn one full revolution. For example, in humans, there are 8 c subunits, thus 8 protons are required.<ref>{{cite journal | vauthors = Fillingame RH, Angevine CM, Dmitriev OY | title = Mechanics of coupling proton movements to c-ring rotation in ATP synthase | journal = FEBS Letters | volume = 555 | issue = 1 | pages = 29–34 | date = November 2003 | pmid = 14630314 | doi = 10.1016/S0014-5793(03)01101-3 | s2cid = 38896804 | doi-access = free }}</ref> After ''c'' subunits, protons finally enter the matrix through an ''a'' subunit channel that opens into the mitochondrial matrix.<ref name=":0" /> This reflux releases [[Gibb's free energy|free energy]] produced during the generation of the oxidized forms of the electron carriers (NAD{{sup|+}} and Q) with energy provided by O{{sub|2}}. The free energy is used to drive ATP synthesis, catalyzed by the F{{sub|1}} component of the complex.<ref>{{Cite journal|last1=Berg|first1=Jeremy M.|last2=Tymoczko|first2=John L.|last3=Stryer|first3=Lubert | name-list-style = vanc |date=2002-01-01|title=A Proton Gradient Powers the Synthesis of ATP|url=https://www.ncbi.nlm.nih.gov/books/NBK22388/|archive-url=https://web.archive.org/web/20150104034733/http://www.ncbi.nlm.nih.gov/books/NBK22388/|archive-date=January 4, 2015|language=en}}</ref><br />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]]—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]] rather than ATP production.<ref>{{cite journal | vauthors = Cannon B, Nedergaard J | title = Brown adipose tissue: function and physiological significance | journal = Physiological Reviews | volume = 84 | issue = 1 | pages = 277–359 | date = January 2004 | pmid = 14715917 | doi = 10.1152/physrev.00015.2003 | url = http://physrev.physiology.org/content/84/1/277 | url-access = subscription | archive-date = 2017-12-03 | access-date = 2017-03-10 | archive-url = https://web.archive.org/web/20171203133600/http://physrev.physiology.org/content/84/1/277 }}</ref>

=== 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<sup>+</sup> can be reduced to NADH by Complex I.<ref>{{cite book |last1=Kim|first1=Byung Hong |last2=Gadd|first2=Geoffrey Michael | name-list-style = vanc |chapter=Introduction to bacterial physiology and metabolism| title =Bacterial Physiology and Metabolism|pages=1–6|publisher=Cambridge University Press|isbn=978-0-511-79046-1|year=2008|doi=10.1017/cbo9780511790461.002}}</ref> 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]], inducing [[reverse electron flow]].<ref>{{cite journal | vauthors = Mills EL, Kelly B, Logan A, Costa AS, Varma M, Bryant CE, Tourlomousis P, Däbritz JH, Gottlieb E, Latorre I, Corr SC, McManus G, Ryan D, Jacobs HT, Szibor M, Xavier RJ, Braun T, Frezza C, Murphy MP, O'Neill LA | display-authors = 6 | title = Succinate Dehydrogenase Supports Metabolic Repurposing of Mitochondria to Drive Inflammatory Macrophages | journal = Cell | volume = 167 | issue = 2 | pages = 457–470.e13 | date = October 2016 | pmid = 27667687 | doi = 10.1016/j.cell.2016.08.064 | pmc = 5863951 | doi-access = free }}</ref>

==Prokaryotic electron transport chains==
{{Citations needed|section|date=December 2023}}
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{{sub|2}}''' 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]] ([[bacteria]] and [[archaea]]) 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:

[[File:Prokaryote generalized modular ETC.svg|420px|alt=Flowchart combining various respiratory electron transport chains of prokaryotes into one generalized pathway. It shows three "Donor" entry points at the top, and two "Acceptor" exit points at the bottom. The long or central chain has a pathway starting with the top left corner Donor, downwards to Dehydrogenase, then rightwards to Quinone, then to Cytochrome bc complex, then to Cytochrome C, then downwards to Oxidoreductase, and then again to the bottom right corner Acceptor. The top middle Donor converges with the central chain at Quinone, and the top right corner at Cytochrome C; Quinone is also a diverging point, leading downwards to Oxidoreductase, and then again to the bottom middle 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]] 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 ''[[organotroph]]s''. Chemoorganotrophs (animals, fungi, protists) and ''[[phototrophs|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 ''[[lithotroph|(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 [[organotroph]]s and [[phototroph]]s in our [[biosphere]].

The use of inorganic electron donors such as [[Methanogenesis|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.

==== Dehydrogenases: equivalents to 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{{sub|2}} 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.{{citation needed|date=August 2020}}

=== Quinone carriers ===
[[Quinone]]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{{sub|2}}). Archaea in the genus ''[[Sulfolobus]]'' use caldariellaquinone.<ref>{{EC number|1.3.5.1}}</ref> 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.<ref>{{cite journal | vauthors = Ingledew WJ, Poole RK | title = The respiratory chains of Escherichia coli | journal = Microbiological Reviews | volume = 48 | issue = 3 | pages = 222–71 | date = September 1984 | pmid = 6387427 | pmc = 373010 | doi = 10.1128/mmbr.48.3.222-271.1984 | doi-access = free }}</ref>

=== 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'' is 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 ''bc<sub>1</sub>'' is a proton pump found in many, but not all, bacteria (not in ''E. coli''). As the name implies, bacterial ''bc<sub>1</sub>'' is similar to mitochondrial ''bc<sub>1</sub>'' (''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.

It has been observed that inter-protein electron transport between cytochromes ''c'' and ''c''<sub>1</sub> (''complex III'') depends on pH and the presence of oxygen, suggesting that [[Proton|protons]] and [[superoxide]] may act as [[Redox|redox mediators]] in the long-distance electron transport process through the aqueous solution.<ref>{{Cite journal |last=Lagunas |first=Anna |last2=Gomila |first2=Alexandre M. J. |last3=Nin-Hill |first3=Alba |last4=Guerra-Castellano |first4=Alejandra |last5=Pérez-Mejías |first5=Gonzalo |last6=Samitier |first6=Josep |last7=Rovira |first7=Carme |last8=la Rosa |first8=Miguel A. De |last9=Díaz-Moreno |first9=Irene |last10=Gorostiza |first10=Pau |title=Long-Distance Charge Transport between Cytochrome c and Complex III is Mediated by Protons and Reactive Oxygen Species |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202501286 |journal=Small |language=en |volume=n/a |issue=n/a |article-number=e01286 |doi=10.1002/smll.202501286 |issn=1613-6829|url-access=subscription |pmc=12548003 }}</ref>

=== Electron acceptors and terminal oxidase/reductase ===
{{cleanup section|reason=We talk as if oxidases are not also reductases, and as if reductases are not also oxidizing something. That's messed up.|date=December 2023}}
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. As with other steps of the ETC, an enzyme is required to help with the process.

If oxygen is available, it is most often used as the terminal electron acceptor in aerobic bacteria and facultative anaerobes. An [[oxidase]] reduces the O{{sub|2}} to water while oxidizing something else. In mitochondria, the terminal membrane complex (''Complex IV'') is cytochrome oxidase, which oxidizes the cytochrome. [[Cellular respiration#Aerobic respiration|Aerobic]] bacteria use a number of different terminal oxidases. For example, ''E. coli'' (a [[Facultative anaerobe|facultative]] anaerobe) does not have a cytochrome oxidase or a ''bc<sub>1</sub>'' complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water.

Bacterial terminal oxidases 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.<ref name="Anraku 101–132"/>

Mostly in anaerobic environments different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate. When bacteria grow in [[Hypoxia (environmental)|anaerobic]] environments, the terminal electron acceptor is reduced by an enzyme called a reductase. ''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.

==Photosynthetic==
{{Further|Light-dependent reaction|Photosynthetic reaction center}}
[[File:Innerworkings of a thylakoid.png|thumb|Photosynthetic electron transport chain of the [[thylakoid membrane]].]]
In [[oxidative phosphorylation]], electrons are transferred from an electron donor such as NADH to an acceptor such as O{{sub|2}} through an electron transport chain, releasing energy. In [[photophosphorylation]], 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.<ref name=":1" />

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]] and [[plastoquinone]]) and mobile, water-soluble carriers ([[cytochrome]]s). They also contain a [[proton pump]]. The proton pump in ''all'' photosynthetic chains resembles mitochondrial ''Complex III''. The commonly held theory of [[symbiogenesis]] proposes that both organelles descended from bacteria.

== See also ==
* [[Charge-transfer complex]]
* [[CoRR hypothesis]]
* [[Electron equivalent]]
* [[Hydrogen hypothesis]]
* [[Respirasome]]
* [[Electric bacteria]]

== References ==
{{Reflist|30em}}

== Further reading ==
{{refbegin|30em}}
* {{cite book| vauthors = Fenchel T, King GM, Blackburn TH |title=Bacterial Biogeochemistry: The Ecophysiology of Mineral Cycling|edition=2nd|publisher=Elsevier|date=September 2006|isbn=978-0-12-103455-9}}
* {{cite book|isbn=978-0-632-05357-5|author=Lengeler JW|editor1=Drews G|editor2=Schlegel HG |title=Biology of the Prokaryotes|publisher=Blackwell Science|date=January 1999}}
* {{cite book| vauthors = Nelson DL, Cox MM|title=Lehninger Principles of Biochemistry|edition=4th|publisher=W. H. Freeman|date=April 2005|isbn=978-0-7167-4339-2|url=https://archive.org/details/lehningerprincip00lehn_0}}
* {{cite book| vauthors = Nicholls DG, Ferguson SJ|title=Bioenergetics 3|publisher=Academic Press|date=July 2002|isbn=978-0-12-518121-1}}
* {{cite book|isbn=978-0-471-51185-4|author=Stumm W|author2=Morgan JJ|title=Aquatic Chemistry|edition=3rd|publisher=[[John Wiley & Sons]]|year=1996}}
* {{cite journal | vauthors = Thauer RK, Jungermann K, Decker K | title = Energy conservation in chemotrophic anaerobic bacteria | journal = Bacteriological Reviews | volume = 41 | issue = 1 | pages = 100–80 | date = March 1977 | pmid = 860983 | pmc = 413997 | doi = 10.1128/MMBR.41.1.100-180.1977 }}
* {{cite book| vauthors = White D|title=The Physiology and Biochemistry of Prokaryotes|edition=2nd|publisher=[[Oxford University Press]]|date=September 1999|isbn=978-0-19-512579-5|url=https://archive.org/details/physiologybioche00whit}}
* {{cite book| vauthors = Voet D, Voet JG |title=Biochemistry|journal=Biochemical Education|volume=28|issue=3|pages=[https://archive.org/details/biochemistry00voet_0/page/124 124]|publisher=[[John Wiley & Sons]]|date=March 2004|doi=10.1016/s0307-4412(00)00032-7|edition=3rd|isbn=978-0-471-58651-7|pmid=10878303|url=https://archive.org/details/biochemistry00voet_0/page/124}}
* {{cite journal | vauthors = Kim HS, Patel K, Muldoon-Jacobs K, Bisht KS, Aykin-Burns N, Pennington JD, van der Meer R, Nguyen P, Savage J, Owens KM, Vassilopoulos A, Ozden O, Park SH, Singh KK, Abdulkadir SA, Spitz DR, Deng CX, Gius D | display-authors = 6 | title = SIRT3 is a mitochondria-localized tumor suppressor required for maintenance of mitochondrial integrity and metabolism during stress | journal = Cancer Cell | volume = 17 | issue = 1 | pages = 41–52 | date = January 2010 | pmid = 20129246 | pmc = 3711519 | doi = 10.1016/j.ccr.2009.11.023 }}
* {{cite journal | vauthors = Raimondi V, Ciccarese F, Ciminale V | title = Oncogenic pathways and the electron transport chain: a dangeROS liaison | journal = Br J Cancer | volume = 122 | issue = 2 | pages = 168–181 | date = January 2020 | pmid = 31819197 | pmc = 7052168 | doi = 10.1038/s41416-019-0651-y }}
* {{cite journal |last1=Reguera |first1=Gemma |title=Biological electron transport goes the extra mile |journal=Proceedings of the National Academy of Sciences |date=29 May 2018 |volume=115 |issue=22 |pages=5632–5634 |doi=10.1073/pnas.1806580115|doi-access=free |pmid=29769327 |bibcode=2018PNAS..115.5632R |pmc=5984551 }} – Editorial commentary mentioning two unusual ETCs: that of ''Geobacter sulfurreducens'' and that of [[cable bacteria]]. Also has schematic of ''E. coli'' ETC.
{{refend}}

== External links ==
* {{MeshName|Electron+Transport+Chain+Complex+Proteins}}
* [http://www.khanacademy.org/video/electron-transport-chain?playlist=Biology Khan Academy, video lecture]
* [[KEGG]] [https://www.genome.jp/pathway/pfo00190 pathway: Oxidative phosphorylation, overlaid with genes found in ''Pseudomonas fluorescens'' Pf0-1.] Click "help" for a how-to.

{{Cellular respiration}}
{{Electron transport chain}}

{{DEFAULTSORT:Electron Transport Chain}}
[[Category:Cellular respiration]]
[[Category:Integral membrane proteins]]