The effect of penta- and hexa-coordination on the reaction between cytochrome c peroxidase and hydrogen peroxide
Britton, Cheryl Ann
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Native cytochrome c peroxidase displays penta- coordination of the heme iron. The spectral characteristics of Soret/380 nm and 620/647 nm yield typical values of 1.56 and .78, respectively. A typical hexa-coordinate form was modeled with a CcP- fluoride complex. In the hexa-coordinated form, there is an intensification of the Soret band relative to the shoulder at 380 nm, and a blue shift in the charge transfer band at 647 nm to 620 nm. The aged samples investigated in this study were varying combinations of the penta- and hexa-coordinate enzymatic forms. The reaction between cytochrome c peroxidase and hydrogen peroxide to produce compound I was dependent upon the coordination state of the heme iron. When monitored over a short time frame, a fast hydrogen peroxide-dependent reaction was observed. With native penta-coordinate CcP, the observed rate of this reaction was linearly dependent upon hydrogen peroxide concentration yielding a bimolecular rate constant of 34 /iff1 s'1. When the aged samples were studied, the observed rate for the fast reaction showed hyperbolic saturation kinetics. The limiting rate at high hydrogen peroxide concentrations was attributed to an internal conformational change from a hydrogen peroxide unreactive to a reactive species. A second reaction was observed when a longer time interval was monitored. This slower reaction was independent of hydrogen peroxide concentration for all samples and gave an observed rate of approximately 1 s'1. The amplitude of this reaction increased as the degree of hexa-coordination increased within the sample.