HO Catalytic Mechanism

Once the oxy complex is formed, a second electron transfer to the HO heme effectively reduces the oxy complex to the peroxide level. From this point many heme enzymes catalyze the heterolytic fission of the peroxide 0-0 bond, leaving behind the well known oxyferryl center, (Fe-0) +, characteristic of peroxidase compound 1 and similar to the active hydroxylating intermediate thought to operate in P450s. However, in HO the active oxidizing intermediate is peroxide. Peracids that form the (Fe-0) + intermediate do not support the HO reaction, whereas H2O2 addition to Fe + HO does support substrate hydroxylation 187, 188). EPR and ENDOR spectroscopy have been used to analyze the cryo-genically reduced oxy-HO complex 189). In these studies reduction of 

This step in the reaction cycle requires one O2 molecule and two electrons to give Fe + biliverdin with a third electron needed to reduce Fe + to Fe + followed by release of the iron from biliverdin 176, 188, 198,199). A hydrolytic mechanism for the incorporation of oxygen in the conversion of verdoheme to biliverdin is ruled out because O labeling experiments show incorporation of into biliverdin 200). Details on the mechanism remain sketchy, and one possible pathway is illustrated in Fig. 21, taken after Ref 176). 

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