Editing Oxidative phosphorylation (section)

== Electron and proton transfer molecules ==
{{further|Coenzyme|Cofactor (biochemistry)}}
[[File:Ubiquinone–ubiquinol conversion.svg|thumb|250px|left|Reduction of [[coenzyme Q]] from its [[ubiquinone]] form (Q) to the reduced ubiquinol form (QH<sub>2</sub>).]]
The electron transport chain carries both protons and electrons, passing electrons from donors to acceptors, and transporting protons across a membrane. These processes use both soluble and protein-bound transfer molecules. In the mitochondria, electrons are transferred within the intermembrane space by the water-[[soluble]] electron transfer protein [[cytochrome c]].<ref>{{cite journal | vauthors = Mathews FS | title = The structure, function and evolution of cytochromes | journal = Progress in Biophysics and Molecular Biology | volume = 45 | issue = 1 | pages = 1–56 | year = 1985 | pmid = 3881803 | doi = 10.1016/0079-6107(85)90004-5 | doi-access = free }}</ref> This carries only electrons, and these are transferred by the reduction and oxidation of an [[iron]] atom that the protein holds within a [[heme]] group in its structure. Cytochrome c is also found in some bacteria, where it is located within the [[periplasmic space]].<ref>{{cite journal | vauthors = Wood PM | title = Why do c-type cytochromes exist? | journal = FEBS Letters | volume = 164 | issue = 2 | pages = 223–226 | date = December 1983 | pmid = 6317447 | doi = 10.1016/0014-5793(83)80289-0 | s2cid = 7685958 | doi-access = free | bibcode = 1983FEBSL.164..223W }}</ref>

Within the inner mitochondrial membrane, the [[lipid]]-soluble electron carrier [[coenzyme Q10]] (Q) carries both electrons and protons by a [[redox]] cycle.<ref>{{cite journal | vauthors = Crane FL | title = Biochemical functions of coenzyme Q10 | journal = Journal of the American College of Nutrition | volume = 20 | issue = 6 | pages = 591–598 | date = December 2001 | pmid = 11771674 | doi = 10.1080/07315724.2001.10719063 | s2cid = 28013583 }}</ref> This small [[1,4-Benzoquinone|benzoquinone]] molecule is very [[hydrophobe|hydrophobic]], so it diffuses freely within the membrane. When Q accepts two electrons and two protons, it becomes reduced to the ''[[Hydroquinone|ubiquinol]]'' form (QH<sub>2</sub>); when QH<sub>2</sub> releases two electrons and two protons, it becomes oxidized back to the ''ubiquinone'' (Q) form. As a result, if two enzymes are arranged so that Q is reduced on one side of the membrane and QH<sub>2</sub> oxidized on the other, ubiquinone will couple these reactions and shuttle protons across the membrane.<ref>{{cite journal | vauthors = Mitchell P | title = Keilin's respiratory chain concept and its chemiosmotic consequences | journal = Science | volume = 206 | issue = 4423 | pages = 1148–1159 | date = December 1979 | pmid = 388618 | doi = 10.1126/science.388618 | bibcode = 1979Sci...206.1148M }}</ref> Some bacterial electron transport chains use different quinones, such as [[vitamin K|menaquinone]], in addition to ubiquinone.<ref name="Søballe">{{cite journal | vauthors = Søballe B, Poole RK | title = Microbial ubiquinones: multiple roles in respiration, gene regulation and oxidative stress management | journal = Microbiology | volume = 145 | issue = 8 | pages = 1817–1830 | date = August 1999 | pmid = 10463148 | doi = 10.1099/13500872-145-8-1817 | url = http://mic.sgmjournals.org/cgi/reprint/145/8/1817.pdf | url-status = live | doi-access = free | archive-url = https://web.archive.org/web/20080529070520/http://mic.sgmjournals.org/cgi/reprint/145/8/1817.pdf | archive-date = 2008-05-29 }}</ref>

Within proteins, electrons are transferred between [[Flavin group|flavin]] cofactors,<ref name=Schultz/><ref name=Johnson>{{cite journal | vauthors = Johnson DC, Dean DR, Smith AD, Johnson MK | title = Structure, function, and formation of biological iron-sulfur clusters | journal = Annual Review of Biochemistry | volume = 74 | pages = 247–281 | year = 2005 | pmid = 15952888 | doi = 10.1146/annurev.biochem.74.082803.133518 }}</ref> [[iron–sulfur cluster]]s and cytochromes. There are several types of iron–sulfur cluster. The simplest kind found in the electron transfer chain consists of two iron atoms joined by two atoms of inorganic [[sulfur]]; these are called [2Fe–2S] clusters. The second kind, called [4Fe–4S], contains a cube of four iron atoms and four sulfur atoms. Each iron atom in these clusters is coordinated by an additional [[amino acid]], usually by the sulfur atom of [[cysteine]]. Metal ion cofactors undergo redox reactions without binding or releasing protons, so in the electron transport chain they serve solely to transport electrons through proteins. Electrons move quite long distances through proteins by hopping along chains of these cofactors.<ref>{{cite journal | vauthors = Page CC, Moser CC, Chen X, Dutton PL | title = Natural engineering principles of electron tunnelling in biological oxidation-reduction | journal = Nature | volume = 402 | issue = 6757 | pages = 47–52 | date = November 1999 | pmid = 10573417 | doi = 10.1038/46972 | s2cid = 4431405 | bibcode = 1999Natur.402...47P }}</ref> This occurs by [[quantum tunnelling]], which is rapid over distances of less than 1.4{{e|−9}} m.<ref>{{cite journal | vauthors = Leys D, Scrutton NS | title = Electrical circuitry in biology: emerging principles from protein structure | journal = Current Opinion in Structural Biology | volume = 14 | issue = 6 | pages = 642–647 | date = December 2004 | pmid = 15582386 | doi = 10.1016/j.sbi.2004.10.002 }}</ref>