Prx Sp HO S
Figure 2. Catalytic and overoxidation cycles of Prxs. The peroxidatic Cys of the Prx is depicted as a thiol (SpH) or sulfenic acid (SpOH), or in a disulfide with the resolving Cys (SRH). In 2-Cys Prxs both Cys residues are involved in the mechanism (black and grey), whereas catalysis in 1-Cys Prxs involves only the black species. The box to the right illustrates the inactivation shunt which can occur in the presence of excess substrate. In select Prxs, this overoxidation can be reversed by specialized sulfiredoxin (Srx) or sestrin proteins (grey, dotted line). The disulfide reductase system (lower left) that returns oxidized Prxs to their activated state varies with the organism and isoform, often being a thioredoxin (Trx) or glutaredoxin homologue and/or flavoenzyme (e.g. AhpF or thiore-doxin reductase), and generally utilizing at least one, and usually more, CxxC-containing proteins or domains
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