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Electron transfer in cytochrome c depends upon the structure of the intervening medium
T B Karpishin1, M W Grinstaff, S Komar-Panicucci
1Beckman Institute, California Institute of Technology, Pasadena 91125.
Background:
Long-distance electron-transfer (ET) reactions through proteins are involved in a great many biochemical processes; however, the way in which the protein structure influences the rates of these reactions is not well understood. We have therefore measured the rates of intramolecular ET from the ferroheme to a bis(2,2'-bipyridine)imidazoleruthenium(III) acceptor at histidine 39 or 54 in derivatives of yeast iso-1-cytochrome c, and studied the effect of an asparagine to isoleucine mutation at position 52, a residue situated between the heme and the electron acceptor.
Results:
The Fe2+-->Ru3+ rate constants demonstrate that residue 52 affects ET from the heme to His54 (Ile52 > Asn52), but not to His39 (Ile52 = Asn52). The enhanced Fe(2+)-Ru3+(His54) electronic coupling for the N52I/K54H protein is in good agreement with sigma-tunneling calculations, which predict the length of the ET pathways between the heme and His54.
Conclusion:
The structure of the intervening medium between the heme and electron acceptors at the protein surface influences the donor-acceptor couplings in cytochrome c.