Long-Range Electron Transfer from Iron(II)-Cytochrome c to (Zinc-Cytochrome c Peroxidase)(+) within the 1: 1 Complex

Long-range electron transfer within crystallographically defined protein complexes can be studied by substituting zinc protoporphyrin IX for heme in one partner. Electron transfer within such a complex is initiated by flash photoproduction of the zinc protoporphyrin triplet state (³ZnP), which either can decay back to the ground state or can reduce a ferriheme partner, The redox intermediate B returns to the ground state by thermal electron transfer from Fe¹¹P to the cation radical ZnP^.+, Until now, reaction 2 has not been observed directly. This report demonstrates the process in the archetypical 1:1 protein electron transfer complex between yeast cytochrome c peroxidase (CcP) and cytochrome c (Cc) and shows that it exhibits a much more pronounced dependence on the cytochrome c employed than does the photostimulated, forward reaction. It is noteworthy that in this case the thermal reaction (2) is equivalent to the physiological oxidation of Fe¹¹Cc by the H₂O₂-oxidized enzyme.