Effects of the Location of Distal Histidine in the Reaction of

To clarify how the location of distal histidine affects the activation process of H2O2 by heme proteins, we have characterized reactions with H2O2 for the L29H/ H64L and F43H/H64L mutants of sperm whale myoglo-bin (Mb), designed to locate the histidine farther from the heme iron. Whereas the L29H/H64L double substitu-tion retarded the reaction with H2O2, an 11-fold rate increase versus wild-type Mb was observed for the F43H/ H64L mutant. The Vmax values for 1-electron oxidations by the myoglobins correlate well with the varied reac-tivities with H2O2. The functions of the distal histidine as a general acid-base catalyst were examined based on the reactions with cumene hydroperoxide and cyanide, and only the histidine in F43H/H64L Mb was suggested to facilitate heterolysis of the peroxide bond. The x-ra