Fig 10.

Proposed reaction mechanisms for holoCcmE (His130) (A) and cytochrome c (B) linkage to heme vinyl groups. Noted are the oxidation states of iron (Fe³⁺ or Fe²⁺); red half arrows are one-electron transfers, and full red arrows are two-electron transfers. Transfer of the proton from the imidazolium to the alpha carbon in (A) is probably solvent- or protein-mediated (i.e., the proton may be abstracted at an early step, with a solvent- or protein-mediated protonation of the alpha carbon occurring at a later step). Reduction of heme (from Fe³⁺ to Fe²⁺) could favor ejection of the CcmE His130 imidazole adduct (reverse blue arrow) and is required for holocytochrome c formation. For simplicity, only a single vinyl of heme is shown. Adapted from (Kranz, Richard-Fogal et al. 2009).