Effect of Single-Site Charge-Reversal Mutations on the Catalytic Properties of Yeast Cytochrome c Peroxidase: Mutations Near the High-Affinity Cytochrome c Binding Site

摘要

Fifteen single-site charge-reversal mutations of yeast cytochrome c peroxidase (CcP) have been constructed in order to determine the effect of localized charge on the catalytic properties of the enzyme. The mutations are located on the front face of CcP, near the cytochrome c binding site identified in the crystallographic structure of the yeast cytochrome c/CcP complex (Pelletier and Kraut (1992) Science 258, 1748–1755). The mutants are characterized by absorption spectroscopy and hydrogen peroxide reactivity at both pH 6.0 and 7.5 and by steady-state kinetic studies using recombinant yeast iso-1 ferrocytochrome c(C102T) as substrate at pH 7.5. Some of the charge-reversal mutations cause detectable changes in the absorption spectrum, especially at pH 7.5, reflecting changes in the equilibrium between penta-and hexa-coordinate heme species in the enzyme. An increase in the amount of hexa-coordinate heme in the mutant enzymes correlates with an increase in the fraction of enzyme that does not react with hydrogen peroxide. Steady-state velocity measurements indicate that five of the fifteen mutations cause large increases in the Michaelis constant (R31E, D34K, D37K, E118K, and E290K). These data support the hypothesis that the cytochrome c/CcP complex observed in the crystal is the dominant catalytically active complex in solution.