Reactivity ferryl compounds

The peroxidases produce a reactive intermediate, Compound I, whose reactions are controlled by the protein environment. In conventional peroxidases with a histidine iron ligand, access to the Compound I ferryl species is restricted by the protein, favoring the transfer of single electrons from the substrate to an exposed heme edge. If the protein has a suitably placed oxidizable residue such as a tyrosine or... 

Two current alternative views are available as to how remotely boimd NADPH may work. One sees its action as involving two successive one-electron oxidations (52, 53). The effectiveness of NADPH in preventing compound II formation is then due to the high reactivity of the NADP intermediate as reductant of the compound II generated in the first one-electron step. The other model (47) prefers to see NADPH as a hydride donor responsible for the almost simultaneous reduction of the ferryl iron and the protein radical species. 

In view of the formation of a highly reactive Compound I ferryl species, and the fact that the porphyrin radical cation of this intermediate is reduced in enzymes such as CcP by a protein residue, it is not surprising that permanent covalent modifications are autocatalytically introduced into some protein frameworks. Two examples of autocatalytic protein modification, those of LiP and the catalase-peroxidases, are summarized here to illustrate the maturation of peroxidase protein structures that can have important functional consequences. 

Many other species also contain catalases. In bacteria these can contain either haem or dihydroporphyrin (chlorin) prosthetic groups [52], However, the presence of a weak tyrosine-ligation to the iron appears to be present in all cases. This, combined with the lack (present in peroxidases) of an H-bond between an arginine residue and the ferryl oxygen, may explain why catalase compound I is uniquely reactive to H2O2. 

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