INHIBITOR-ENHANCED ELECTRON-TRANSFER - COPPER CYTOCHROME-C AS A REDOX-INERT PROBE OF TERNARY COMPLEXES

Copper-substituted cytochrome c (CuCc) has been used as a structurally faithful, redox-inert inhibitor to probe the mechanism of electron tr ansfer (ET) between Cc molecules and cytochrome c peroxidase (CcP). Th is inhibitor enhances photoinduced ET quenching of the triplet excited state of a zinc-substituted protein (ZnCcP or ZnCc) by its iron(lll) partner (Fe(3+)Cc or Fe(3+)CcP). These results show that CcP and Cc fo rm a ternary complex in which one Cc molecule binds tightly at a surfa ce domain of CcP having low ET reactivity, whereas the second Cc molec ule binds weakly to the 1:1 complex at a second domain I with markedly greater (similar to 10(3)) reactivity. These results also rule out th e possibility that Cc bound at the second domain cooperatively enhance s ET to Cc at the first domain. The multiphasic kinetics observed for the photoproduced ET intermediate do not reflect electron self-exchang e between two Cc molecules within the ternary complex.