Editing Respiratory complex I (section)

==Active/inactive transition==

The catalytic properties of eukaryotic complex I are not simple. Two catalytically and structurally distinct forms exist in any given preparation of the enzyme: one is the fully competent, so-called “active” A-form and the other is the catalytically silent, dormant, “inactive”, D-form. After exposure of idle enzyme to elevated, but physiological temperatures (>30&nbsp;°C) in the absence of substrate, the enzyme converts to the D-form. This form is catalytically incompetent but can be activated by the slow reaction (k~4 min<sup>−1</sup>) of NADH oxidation with subsequent ubiquinone reduction. After one or several turnovers the enzyme becomes active and can catalyse physiological NADH:ubiquinone reaction at a much higher rate (k~10<sup>4</sup> min<sup>−1</sup>). In the presence of divalent cations (Mg<sup>2+</sup>, Ca<sup>2+</sup>), or at alkaline pH the activation takes much longer.

The high [[activation energy]] (270 kJ/mol) of the deactivation process indicates the occurrence of major conformational changes in the organisation of the complex I. However, until now, the only conformational difference observed between these two forms is the number of [[cysteine]] residues exposed at the surface of the enzyme. Treatment of the D-form of complex I with the sulfhydryl reagents [[N-Ethylmaleimide]] or [[DTNB]] irreversibly blocks critical cysteine residues, abolishing the ability of the enzyme to respond to activation, thus inactivating it irreversibly. The A-form of complex I is insensitive to sulfhydryl reagents.<ref>{{cite journal | vauthors = Babot M, Birch A, Labarbuta P, Galkin A | title = Characterisation of the active/de-active transition of mitochondrial complex I | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1837 | issue = 7 | pages = 1083–92 | date = July 2014 | pmid = 24569053 | pmc = 4331042 | doi = 10.1016/j.bbabio.2014.02.018 }}</ref><ref>{{cite journal | vauthors = Dröse S, Stepanova A, Galkin A | title = Ischemic A/D transition of mitochondrial complex I and its role in ROS generation | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1857 | issue = 7 | pages = 946–57 | date = July 2016 | pmid = 26777588 | pmc = 4893024 | doi = 10.1016/j.bbabio.2015.12.013 }}</ref>

It was found that these conformational changes may have a very important physiological significance. The inactive, but not the active form of complex I was susceptible to inhibition by nitrosothiols and [[peroxynitrite]].<ref name="pmid17956863">{{cite journal | vauthors = Galkin A, Moncada S | title = S-nitrosation of mitochondrial complex I depends on its structural conformation | journal = The Journal of Biological Chemistry | volume = 282 | issue = 52 | pages = 37448–53 | date = December 2007 | pmid = 17956863 | doi = 10.1074/jbc.M707543200 | doi-access = free }}</ref> It is likely that transition from the active to the inactive form of complex I takes place during pathological conditions when the turnover of the enzyme is limited at physiological temperatures, such as during [[Hypoxia (medical)|hypoxia]], ischemia <ref>{{cite journal | vauthors = Kim M, Stepanova A, Niatsetskaya Z, Sosunov S, Arndt S, Murphy MP, Galkin A, Ten VS | display-authors = 6 | title = Attenuation of oxidative damage by targeting mitochondrial complex I in neonatal hypoxic-ischemic brain injury | journal = Free Radical Biology & Medicine | volume = 124 | pages = 517–524 | date = August 2018 | pmid = 30037775 | pmc = 6389362 | doi = 10.1016/j.freeradbiomed.2018.06.040 }}</ref><ref>{{cite journal | vauthors = Stepanova A, Konrad C, Guerrero-Castillo S, Manfredi G, Vannucci S, Arnold S, Galkin A | title = Deactivation of mitochondrial complex I after hypoxia-ischemia in the immature brain | journal = Journal of Cerebral Blood Flow and Metabolism | volume = 39 | issue = 9 | pages = 1790–1802 | date = September 2019 | pmid = 29629602 | pmc = 6727140 | doi = 10.1177/0271678X18770331 }}</ref> or when the tissue [[nitric oxide]]:oxygen ratio increases (i.e. metabolic hypoxia).<ref name="pmid11994742">{{cite journal | vauthors = Moncada S, Erusalimsky JD | title = Does nitric oxide modulate mitochondrial energy generation and apoptosis? | journal = Nature Reviews. Molecular Cell Biology | volume = 3 | issue = 3 | pages = 214–20 | date = March 2002 | pmid = 11994742 | doi = 10.1038/nrm762 | s2cid = 29513174 }}</ref>