Editing Electron transport chain (section)

====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 |pages=687}}</ref>