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The provision of inorganic sulfide ions for construction of 2Fe–2S and 4Fe–4S clusters from the thiol group of cysteine (Cys) presumably represents an ancient intersection between organosulfur and inorganic sulfur metabolism as microbes evolved. The cysteine desulfurylases convert cysteine to alanine in a pyridoxal-phosphate-dependent β-replacement of the C3–SH by H. The sulfur released is not the usual eight-electron electron-rich sulfide ion S2− but instead a six-electron sulfane atom, captured by Cys-thiolate side chains as a Cys persulfide –S-SH. The thiane sulfurs are mobilized on chaperone proteins as persulfides and then deposited on the proteins in mitochondria, cytoplasm and nucleus that, along with iron cations, assemble into 2Fe–2S clusters, some of which are matured into 4Fe–4S clusters. The Fe clusters are obligate one-electron redox agents. Eight of them function in tandem to connect the flavin-containing mitochondrial NADH dehydrogenase to the electron chain components.

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