Figure 4.

Working models for cofactor biogenesis in the (A) wild-type and (B) F₂-Tyr157 hCDO. In the proposed pathway, substrate binding to the ferrous iron center generates an iron-bound superoxide radical, which subsequently oxidizes Cys93 and produces an iron-bound hydroperoxide and a thiol radical. Oxidation of Tyr157 by the thiol radical leads to the crosslink between Cys93 and Tyr157, concomitant with the production of a transient-state radical species in Tyr157. Deprotonation of the hydroxyl group to the iron-bound hydroperoxide results in a ketone species, which drives the C-H bond cleavage. There are two less electrons in the transient-state radical species in F₂-Tyr157 (indicated by brackets) compared with the radical species in Tyr157.