Figure 5.

(A) Recently proposed mechanism for the cleavage of tyrosine by HydG: Tyrosine binds to the auxiliary C-terminal [Fe₄S₄] cluster. Ado• generated by N-terminal [Fe₄S₄] cluster reacts with tyrosine, producing a tyrosyl radical. Cleavage of the tyrosyl radical results in 4-oxidobenzyl radical and dehydroglycine bound to the auxiliary cluster that is subsequently cleave to produce CO and CN^– ligands to the H cluster of HydA. (B) Proposed mechanism for the conversion of GTP to 3′, 8-cH₂GTP by MoaA; MoaC is now established to catalyze the further steps necessary to produce the pyranopterin ring system.