Yeast cytochrome c with phenylalanine or tyrosine at position 87 transfers electrons to (zinc cytochrome c peroxidase)+ at a rate ten thousand times that of the serine-87 or glycine-87 variants.

Of the many factors known to influence the rate of electron transfer between two metalloproteins, it is particularly difficult to assess the role of the polypeptide matrix intervening between the donor and acceptor sites. To determine whether the phylogenetically conserved Phe-87 of yeast iso-1-cytochrome c helps to mediate electron transfer between cytochrome c and cytochrome c peroxidase, we have constructed mutants of cytochrome c that are altered at this position and now have studied the kinetics of long-range electron transfer within their complexes with zinc-substituted cytochrome c peroxidase. We find that the rate of electron transfer from reduced cytochrome c to the zinc cytochrome c peroxidase pi-cation radical is four orders of magnitude greater when phenylalanine or tyrosine is present at position 87 than when serine or glycine is present.