Editing Electron transport chain (section)

==Prokaryotic electron transport chains==
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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:

                      '''Donor'''            '''Donor'''                    '''Donor'''
                        ↓                ↓                        ↓
                  '''dehydrogenase'''   →   '''quinone'''   →  ''' ''bc{{sub|1}}'' '''  →   '''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]] 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.

=== Electron acceptors and terminal oxidase/reductase ===
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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.