Electron transfer reactions of high-potential iron-sulfur proteins and c-type cytochromes

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Abstract
Studies of electron transfer by biological oxidation-reduction proteins frequently focus on the interaction of a particular protein with nonphysiological oxidants and/or reductants. This approach is limited by the size and chemistry of the nonphysiological reactants. To further the understanding of biological electron transfer, the interaction of 2 examples of high-potential iron-sulfur proteins (HIPIP''s) with mitochondrial cytochrome c (horse heart) and bacterial cytochrome c2 from Rhodosporillum rubrum, Rhodopseudomonas palustris, Rhodopseudomonas capsulata and Rhodopseudomonas sphaeroides was studied. On the basis of the kinetics of electron transfer between the various HIPIP''s and cytochromes, the interactions are apparently more complex than those observed with nonphysiological reactants. Specific sites on both the HIPIP''s and the cytochromes mediate electron transfer with the effect of ionic strength different from that expected on the basis of the interaction of the various proteins with the iron hexacyanides, the interaction of HIPIP with some of the cytochromes investigated is heterogeneous, resulting from at least 2 possible orientations (cytochrome dependent) for interaction leading to electron transfer, and no long-lived complexes between the HIPIP''s and cytochromes are formed due to rapid equilibrium between the 2 proteins. The measured 2nd-order rate constant may be the product of the association constant (for any HIPIP and a particular cytochrome) and a 1st-order rate constant reflecting the rate-limiting step leading to products.