Through site-directed mutagenesism the roles of the amino acids H392, H395, D197, Q419 in heme conversion were studied. H392 mutants which disable the His-Tyr bond contain only heme b as their prosthetic group, and lose specific activity, thermostability and sensitivity to the inhibitors NaCN, NaN3, NH2OH, CH3ONH2, C2 H5ONH2 compared to wild type HPII. Populations of D197S/H395Q and 0419A mutant HPII contain both heme b and heme d as their prosthetic group, and are less thermostable compared to wild type HPII but retain wild type HPII characteristics regarding enzyme kinetics and inhibitor sensitivity. H395A, H395Q, D 97A, D197S and Q419H mutants showed no significant differences when compared to wild type HPII. It therefore appears that, while His392, His395, Asp 197 and Gln419 are all involved in heme conversion, only the presence of His392 is absolutely critical for the reaction to proceed. These results support the mechanistic relationship between the proposed autocatalytic conversion of heme b to heme d mechanism proposed by Bravo 'et al'. (1997a). (Abstract shortened by UMI.)