A distal histidine mutant (H52Q) of yeast cytochrome c peroxidase catalyzes the oxidation of H2O2 instead of its reduction

A H52Q variant of yeast cytochrome c peroxidase (CcP), in which the distal histidine is replaced by glutamine, catalyzes oxidation of H2O2 instead of reduction. This redirection of catalytic action is detected by protein film voltammetry. In the presence of H2O2, wild-type CcP, adsorbed on a graphite electrode, shows a strong catalytic reduction wave commencing at about 0.8V (pH 5.4); by contrast, H52Q does not exhibit this activity but instead shows a catalytic oxidation current at potentials in the region of 0.9 V. The oxidation current is partly suppressed in the presence of tetranitromethane (a superoxide scavenger) and is not observed for other mutants studied, including H52A. The only significant structural change in the H52Q variant is that the Q-52 side chain occupies the space vacated by the H-52 imidazole; specifically, the N-epsilon atom that is believed to transfer a proton and induce O-O cleavage is replaced, to within 0.75 Angstrom, by the carbamide-O. Thus, while the weakly basic an-dde functionality is unable to serve in the reorganization of bound H2O2, it is able to facilitate its oxidation, most obviously by serving as a H-bond acceptor to assist formation of a labile superoxide intermediate.