FIGURE 7.

Proposed intermediates of IscS formed in E. coli cells with deficiency of accessible iron for iron-sulfur cluster biogenesis. In E. coli cells with deletion of IscA/SufA or depletion of intracellular iron, IscS accumulates persulfide in Cys-328, which may transfer a hydride from the thiol group of l-cysteine to produce H₂S and a thiocarbonyl group in l-cysteine. The thiocarbonyl group may form a highly conjugated quinonoid intermediate in red IscS that has an absorption peak at 528 nm. Oxidation of red IscS with H₂O₂ may modify the thiocarbonyl group and generate an unstable intermediate with an absorption peak at 510 nm.