Reaction of cytochrome c2 with photosynthetic reaction centers from Rhodopseudomonas viridis

Abstract

The reactions of Rhodopseudomonas viridis cytochrome c2 and horse cytochrome c with Rps. viridis photosynthetic reaction centers were studied by using both single- and double-flash excitation. Single-flash excitation of the reaction centers resulted in rapid photooxidation of cytochrome c-556 in the cytochrome subunit of the reaction center. The photooxidized cytochrome c-556 was subsequently reduced by electron transfer from ferrocytochrome c2 present in the solution. The rate constant for this reaction had a hyperbolic dependence on the concentration of cytochrome c2, consistent with the formation of a complex between cytochrome c2 and the reaction center. The dissociation constant of the complex was estimated to be 30 microM, and the rate of electron transfer within the 1:1 complex was 270 s-1. Double-flash experiments revealed that ferricytochrome c2 dissociated from the reaction center with a rate constant of greater than 100 s-1 and allowed another molecule of ferrocytochrome c2 to react. When both cytochrome c-556 and cytochrome c-559 were photooxidized with a double flash, the rate constant for reduction of both components was the same as that observed for cytochrome c-556 alone. The observed rate constant decreased by a factor of 14 as the ionic strength was increased from 5 mM to 1 M, indicating that electrostatic interactions contributed to binding. Molecular modeling studies revealed a possible cytochrome c2 binding site on the cytochrome subunit of the reaction center involving the negatively charged residues Glu-93, Glu-85, Glu-79, and Glu-67 which surround the heme crevice of cytochrome c-554.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonredundant Roles for Cytochrome c 2 and Two High-Potential Iron-Sulfur Proteins in the Photoferrotroph Rhodopseudomonas palustris TIE-1

“…Its reduction potential (ϩ350 mV) is typical of cytochromes c 2 , and our experiments with purified protein and membrane fragments show that the electron transfer rate from cytochrome c 2 to the reaction center is within the range found in other purple bacteria (8,32). A cycA mutant was unable to grow phototrophically, indicating that cytochrome c 2 is the dominant donor to the reaction center during photosynthetic growth and that neither HiPIP can take on this role.…”

A New Cytochrome Subunit Bound to the Photosynthetic Reaction Center in the Purple Bacterium, Rhodovulum sulfidophilum

“…It has been shown that electron transfer reactions from soluble electron donors to the cytochrome subunit are controlled by charge interactions (12,13). The study of site-directed mutagenesis in R. gelatinosus has shown that negatively charged amino acids (Glu) surrounding the heme-1 (positions 82, 113, and 129 in Fig.…”

“…Until now, the following properties of the subunit have been shown: 1) the four hemes are arranged sequentially with high-low-high-low midpoint potentials; 2) the subunit can reduce the photo-oxidized special pair of bacteriochlorophylls faster than the soluble cytochromes; and 3) the heme-1 of the cytochrome is a site involved in the electron flow from soluble electron carriers, indicating that all four hemes of the subunit are likely to be involved in electron transfer toward the photooxidized special pair of bacteriochlorophylls (12,13). The existence of a cytochrome subunit containing only three hemes, including one unusual heme in R. sulfidophilum, suggests that all four hemes and the arrangement of high-low-high-low midpoint potentials are not essential requirements for the functions of the subunit.…”

Shortcut of the photosynthetic electron transfer in a mutant lacking the reaction center‐bound cytochrome subunit by gene disruption in a purple bacterium, Rubrivivax gelatinosus

“…The electron transfer function and characterization of hemes in the cytochrome subunit has been widely studied in various purple bacteria (e.g, R. viridis [2][3][4][5][6][7][8][9][10][11][12], Rhodospirillum molischianum [13], Rubrivivax gelatinosus [14][15][16], Rhodof erax fermentans [17], Chromatium vinosum [18], and Rhodopseudomonas acidophila [19]). These studies have presented that the four hemes in the cytochrome subunit differ in redox midpoint potentials (Em) and peak wavelengths of the a-absorption bands, as shown in a recent review by Nitschke and Dracheva *Corresponding author.…”