FIG 20.

Coproporphyrinogen III dehydrogenase (CgdH). (A, left) Crystal structure of E. coli CgdC. (Right) In the active site of the enzyme, the [4Fe-4S] cluster is coordinated by the three cysteine residues of the conserved CX₃CX₂C sequence motif and by one of the two bound SAM molecules. (Adapted from reference 14.) (B) Proposed reaction mechanism by which the radical SAM enzyme CgdH catalyzes two oxidative decarboxylation reactions of the coproporphyrinogen III propionate side chains on rings A and B to the corresponding vinyl groups of protoporphyrinogen without oxygen as the terminal electron acceptor (see the text for a detailed explanation).