Editing Oxidative phosphorylation (section)

== Prokaryotic electron transport chains ==
{{further|Microbial metabolism}}
In contrast to the general similarity in structure and function of the electron transport chains in eukaryotes, [[bacteria]] and [[archaea]] possess a large variety of electron-transfer enzymes. These use an equally wide set of chemicals as substrates.<ref>{{cite journal | vauthors = Nealson KH | title = Post-Viking microbiology: new approaches, new data, new insights | journal = Origins of Life and Evolution of the Biosphere | volume = 29 | issue = 1 | pages = 73–93 | date = January 1999 | pmid = 11536899 | doi = 10.1023/A:1006515817767 | s2cid = 12289639 | bibcode = 1999OLEB...29...73N }}</ref> In common with eukaryotes, prokaryotic electron transport uses the energy released from the oxidation of a substrate to pump ions across a membrane and generate an electrochemical gradient. In the bacteria, oxidative phosphorylation in ''[[Escherichia coli]]'' is understood in most detail, while archaeal systems are at present poorly understood.<ref>{{cite journal | vauthors = Schäfer G, Engelhard M, Müller V | title = Bioenergetics of the Archaea | journal = Microbiology and Molecular Biology Reviews | volume = 63 | issue = 3 | pages = 570–620 | date = September 1999 | pmid = 10477309 | pmc = 103747 | doi = 10.1128/MMBR.63.3.570-620.1999 }}</ref>

The main difference between eukaryotic and prokaryotic oxidative phosphorylation is that bacteria and archaea use many different substances to donate or accept electrons. This allows prokaryotes to grow under a wide variety of environmental conditions.<ref name=Ingledew>{{cite journal | vauthors = Ingledew WJ, Poole RK | title = The respiratory chains of Escherichia coli | journal = Microbiological Reviews | volume = 48 | issue = 3 | pages = 222–271 | date = September 1984 | pmid = 6387427 | pmc = 373010 | doi = 10.1128/mmbr.48.3.222-271.1984 }}</ref> In ''E. coli'', for example, oxidative phosphorylation can be driven by a large number of pairs of reducing agents and oxidizing agents, which are listed below. The [[Standard electrode potential#Non-standard condition|midpoint potential]] of a chemical measures how much energy is released when it is oxidized or reduced, with reducing agents having negative potentials and oxidizing agents positive potentials.

{| class="wikitable" style="margin-left: auto; margin-right: auto;"
|+ Respiratory enzymes and substrates in ''E. coli''.<ref name=Unden>{{cite journal | vauthors = Unden G, Bongaerts J | title = Alternative respiratory pathways of Escherichia coli: energetics and transcriptional regulation in response to electron acceptors | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1320 | issue = 3 | pages = 217–234 | date = July 1997 | pmid = 9230919 | doi = 10.1016/S0005-2728(97)00034-0 | doi-access = free }}</ref>
|-
!Respiratory enzyme
![[redox|Redox pair]]
! [[Standard electrode potential#Non-standard condition|Midpoint potential]]&nbsp;
(Volts)
|-
| [[Formate dehydrogenase]]
| [[Bicarbonate]] / [[Formate]]
| align=center | −0.43
|-
| [[Hydrogenase]]
| [[Proton]] / [[Hydrogen]]
| align=center | −0.42
|-
| [[NADH dehydrogenase]]
| [[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>]] / [[Nicotinamide adenine dinucleotide|NADH]]
| align=center | −0.32
|-
| [[Glycerol-3-phosphate dehydrogenase]]
| [[dihydroxyacetone phosphate|DHAP]] / [[Glycerol 3-phosphate|Gly-3-P]]
| align=center | −0.19
|-
| [[Pyruvate dehydrogenase#Related enzymes|Pyruvate oxidase]]
|  [[acetic acid|Acetate]] + [[Carbon dioxide]] / [[pyruvic acid|Pyruvate]]
| align=center | ?
|-
| [[Lactate dehydrogenase]]
| [[pyruvic acid|Pyruvate]] / [[Lactic acid|Lactate]]
| align=center | −0.19
|-
| [[D-amino acid dehydrogenase|<small>D</small>-amino acid dehydrogenase]]
|  [[Oxoacid|2-oxoacid]] + [[ammonia]] / [[Amino acid|<small>D</small>-amino acid]]
| align=center | ?
|-
| [[Quinoprotein glucose dehydrogenase|Glucose dehydrogenase]]
| [[gluconic acid|Gluconate]] / [[Glucose]]
| align=center | −0.14
|-
| [[Succinate - coenzyme Q reductase|Succinate dehydrogenase]]
| [[Fumaric acid|Fumarate]] / [[Succinic acid|Succinate]]
| align=center | +0.03
|-
| [[Ubiquinol oxidase]]
| [[Oxygen]] / [[Water]]
| align=center | +0.82
|-
| [[Nitrate reductase]]
| [[Nitrate]] / [[Nitrite]]
| align=center | +0.42
|-
| [[Nitrite reductase]]
| [[Nitrite]] / [[Ammonia]]
| align=center | +0.36
|-
| [[DMSO reductase|Dimethyl sulfoxide reductase]]
| [[dimethyl sulfoxide|DMSO]] / [[Dimethyl sulfide|DMS]]
| align=center | +0.16
|-
| [[Trimethylamine N-oxide reductase|Trimethylamine ''N''-oxide reductase]]
| [[Trimethylamine N-oxide|TMAO]] / [[Trimethylamine|TMA]]
| align=center | +0.13
|-
| [[Fumarate reductase]]
| [[Fumaric acid|Fumarate]] / [[Succinic acid|Succinate]]
| align=center | +0.03
|}

As shown above, ''E. coli'' can grow with reducing agents such as formate, hydrogen, or lactate as electron donors, and nitrate, DMSO, or oxygen as acceptors.<ref name=Ingledew/> The larger the difference in midpoint potential between an oxidizing and reducing agent, the more energy is released when they react. Out of these compounds, the succinate/fumarate pair is unusual, as its midpoint potential is close to zero. Succinate can therefore be oxidized to fumarate if a strong oxidizing agent such as oxygen is available, or fumarate can be reduced to succinate using a strong reducing agent such as formate. These alternative reactions are catalyzed by [[Succinate - coenzyme Q reductase|succinate dehydrogenase]] and [[fumarate reductase]], respectively.<ref>{{cite journal | vauthors = Cecchini G, Schröder I, Gunsalus RP, Maklashina E | title = Succinate dehydrogenase and fumarate reductase from Escherichia coli | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1553 | issue = 1–2 | pages = 140–157 | date = January 2002 | pmid = 11803023 | doi = 10.1016/S0005-2728(01)00238-9 | doi-access = free }}</ref>

Some prokaryotes use redox pairs that have only a small difference in midpoint potential. For example, [[nitrification|nitrifying]] bacteria such as ''[[Nitrobacter]]'' oxidize nitrite to nitrate, donating the electrons to oxygen. The small amount of energy released in this reaction is enough to pump protons and generate ATP, but not enough to produce NADH or NADPH directly for use in [[anabolism]].<ref>{{cite journal | vauthors = Freitag A, Bock E |year=1990 |title=Energy conservation in Nitrobacter |journal=FEMS Microbiology Letters |volume=66 |issue=1–3 |pages=157–62 |doi=10.1111/j.1574-6968.1990.tb03989.x |doi-access=free }}</ref> This problem is solved by using a [[nitrite oxidoreductase]] to produce enough proton-motive force to run part of the electron transport chain in reverse, causing complex I to generate NADH.<ref>{{cite journal | vauthors = Starkenburg SR, Chain PS, Sayavedra-Soto LA, Hauser L, Land ML, Larimer FW, Malfatti SA, Klotz MG, Bottomley PJ, Arp DJ, Hickey WJ | title = Genome sequence of the chemolithoautotrophic nitrite-oxidizing bacterium Nitrobacter winogradskyi Nb-255 | journal = Applied and Environmental Microbiology | volume = 72 | issue = 3 | pages = 2050–2063 | date = March 2006 | pmid = 16517654 | pmc = 1393235 | doi = 10.1128/AEM.72.3.2050-2063.2006 | bibcode = 2006ApEnM..72.2050S }}</ref><ref>{{cite journal | vauthors = Yamanaka T, Fukumori Y | title = The nitrite oxidizing system of Nitrobacter winogradskyi | journal = FEMS Microbiology Reviews | volume = 54 | issue = 4 | pages = 259–270 | date = December 1988 | pmid = 2856189 | doi = 10.1111/j.1574-6968.1988.tb02746.x | doi-access = free }}</ref>

Prokaryotes control their use of these electron donors and acceptors by varying which enzymes are produced, in response to environmental conditions.<ref>{{cite journal | vauthors = Iuchi S, Lin EC | title = Adaptation of Escherichia coli to redox environments by gene expression | journal = Molecular Microbiology | volume = 9 | issue = 1 | pages = 9–15 | date = July 1993 | pmid = 8412675 | doi = 10.1111/j.1365-2958.1993.tb01664.x | s2cid = 39165641 }}</ref> This flexibility is possible because different oxidases and reductases use the same ubiquinone pool. This allows many combinations of enzymes to function together, linked by the common ubiquinol intermediate.<ref name=Unden/> These respiratory chains therefore have a [[modular design]], with easily interchangeable sets of enzyme systems.

In addition to this metabolic diversity, prokaryotes also possess a range of [[isozyme]]s&nbsp;– different enzymes that catalyze the same reaction. For example, in ''E. coli'', there are two different types of ubiquinol oxidase using oxygen as an electron acceptor. Under highly aerobic conditions, the cell uses an oxidase with a low affinity for oxygen that can transport two protons per electron. However, if levels of oxygen fall, they switch to an oxidase that transfers only one proton per electron, but has a high affinity for oxygen.<ref>{{cite journal | vauthors = Calhoun MW, Oden KL, Gennis RB, de Mattos MJ, Neijssel OM | title = Energetic efficiency of Escherichia coli: effects of mutations in components of the aerobic respiratory chain | journal = Journal of Bacteriology | volume = 175 | issue = 10 | pages = 3020–3025 | date = May 1993 | pmid = 8491720 | pmc = 204621 | doi = 10.1128/jb.175.10.3020-3025.1993 | url = http://jb.asm.org/cgi/reprint/175/10/3020.pdf | url-status = live | archive-url = https://web.archive.org/web/20070927135240/http://jb.asm.org/cgi/reprint/175/10/3020.pdf | archive-date = 2007-09-27 }}</ref>