Hopping in the electron-transfer photocycle of the 1:1 complex of Zn-cytochrome c peroxidase with cytochrome c

The physiological electron-transfer (ET) partners, cytochrome c peroxidase (CcP) and cytochrome c (Cc)1, can be modified to exhibit photoinitiated ET through substitution of Zn (or Mg) for Fe in either partner. Laser excitation of the Zn-porphyrin (ZnP) to its triplet excited state (3ZnP) initiates direct heme-heme ET to the ferriheme center of its partner across the protein-protein interface. This photoinitiated ET produces the charge-separated intermediate, I = ZnP+CcP, Fe2+Cc], with a metalloporphyrin pi-cation radical (ZnP+) in the donor protein and a ferroheme acceptor protein. I, in general, is thought to return to the ground state by a thermal ET process that involves direct heme-heme back-ET to complete a simple photocycle. We here contrast intracomplex ET between yeast iso-1 Cc and ZnCcP(WT) (wild-type) with that for two ZnCcP(X) variants: X = W191F, with redox-active W191 replaced by Phe; WYM4, a W191F mutant with further replacement of four other potentially redox-active sites (W51F, Y187F, Y229F, and Y236F). The results show that W191 acts as an ET mediator, which "short-circuits" the direct heme-heme back-ET through a two-step, hopping process in which the ZnP+ cation radical formed by photoinitiated ET rapidly oxidizes W191, and the resultant W191+, in turn, rapidly oxidizes Fe2+Cc.