The peroxide complex of yeast cytochrome c peroxidase contains two distinct radical species, neither of which resides at methionine 172 or tryptophan 51.

The nature of the free radical species observed in the peroxide complex of yeast cytochrome c peroxidase is described for protein variants containing amino acid substitutions at Met-172 and Trp-51. As was the case with Met-172 mutations (Goodin, D.B., Mauk, A.G., and Smith, M. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 1295-1299), Trp-51 can be substituted to give active enzyme. Phe-51-containing enzyme has a higher turnover rate than the original enzyme and exhibits an altered pH dependence. The properties of the isotropic and axial components (Hoffman, B.M., Roberts, J.E., Kang, C.H., and Margoliash, E. (1981) J. Biol. Chem. 256, 6556-6564; Hori, H., and Yonetani, T. (1985) J. Biol. Chem. 260, 349-355) of the EPR signal of the wild-type enzyme-peroxide complex, studied as a function of H2O2 stoichiometry, support proposals (Goodin et al. (1985) and Hori and Yonetani (1985), see above) that two distinct radical species are formed, and spin quantification shows that the isotropic radical is always formed in substoichiometric amounts. The peroxide complexes for proteins containing amino acid substitutions at either Met-172 or Trp-51 exhibit somewhat larger than normal levels of the isotropic radical signal. In addition, these mutants are unlike wild-type enzyme in that the axial EPR signal associated with the peroxide complex is seen only at 10 K and not at 90 K. Thus, neither amino acid can be considered to be the molecular species responsible for either radical signal, but both mutations appear to affect the physical properties of the axial signal representing the major radical species.