Electron-transfer reactions of photoreduced flavin analogues with c-type cytochromes: quantitation of steric and electrostatic factors.

We have found correlations between rate constants and the difference in redox potential of the reactants for electron-transfer reactions between oxidized cytochromes and either photoproduced riboflavin or flavin mononucleotide (FMN) semiquinones (the latter rate constants extrapolated to infinite ionic strength). The riboflavin-cytochrome rate constants are about 70% of those for reduction by lumiflavin, probably because of steric interference by the ribityl side chain. Reduction of cytochromes by FMN semiquinone was ionic strength dependent in all cases, due to electrostatic interactions. Extrapolation of rate constants to infinite ionic strength shows that the phosphate exerts a significant steric effect as well (rate constants average about 27% of those for lumiflavin, although part of this decrease is due to a difference in the semiquinone pK value). Differences in the magnitude of the FMN steric effect correlate well with surface topology differences for those cytochromes whose three-dimensional structures are known. Mitochondrial cytochromes c and the cytochromes c2 all showed attractive (plus-minus) interaction with FMN in spite of the fact that some of these proteins have large net negative charges. Four small c-type cytochromes (including Pseudomonas cytochrome c-551) show a weak repulsive interaction with FMN semiquinone. We conclude that flavosemiquinones interact at a site on the cytochromes that is near the exposed heme edge. There is a large positive electrostatic field at this site in mitochondrial cytochrome c and the cytochromes c2, but this region is primarily hydrophobic in Pseudomonas cytochrome c-551 and in the other small bacterial cytochromes.(ABSTRACT TRUNCATED AT 250 WORDS)