Photosensitized electron transfer reactions of cytochrome c4 from Pseudomonas stutzeri with flavins and methyl viologen

Rate constants for electron transfer reactions of the fully reduced, fully oxidized, and half-reduced forms of the di-heme protein cytochrome c(4) from Pseudomonas stutzeri with several flavin species and methyl viologen were determined by laser flash absorption spectroscopy. The flavin reactions include both triplet and photoreduced semiquinone forms. The rate constant values range from 0.7 x 10(7) to 1.6 x 10(9) M-1 s(-1). The dependence on ionic strength for flavin-mononucleotide semiquinone and methyl viologen follows the behaviour expected from reactions with negatively charged protein sites. Considering these data and those previously reported for electron transfer between cytochrome c(4) and the redox couples [Co(terpy)(2)](2+/3+) and [Co(bipy)(3)](2+/3+) (terpy = 2,2',2 "-terpyridine; bipy = 2,2'-bipyridine), a free energy relation spanning of 0.85 eV was calculated. The total reorganization Gibbs free energy inferred from that relation (E-R = 1-1.2 eV) exceeds the value for electron transfer between several single-heme proteins and flavin species (similar to 0.7-0.9 eV). This could reflect the spin changes associated with electron transfer reactions of cytochrome c(4). The rate constants for cytochrome c(4) at the given driving force are two to five times larger than for single-heme proteins. This implies that the electronic transmission coefficient is about two orders of magnitude larger than for the single-heme electron transfer processes. (C) 1998 Elsevier Science S.A. All rights reserved.