Reaction of Cytochrome-C2 with Photosynthetic Reaction Centers from Rhodopseudomonas-Viridis

by Knaff, David B.; Willie, Anne N.; Long, Joan E.; Kriaucionas, Aidas; Durham, Bill; Millett, Francis Spencer

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-mu-M, 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. The photooxidation of horse cytochrome c by Rps. viridis reaction centers was much slower than that of Rps. viridis cytochrome c2, and obeyed second-order kinetics with a rate constant of 1.7 x 10(5) M-1 s-1. The reaction of cytochrome c2 with reaction centers in membranes prepared from Rps. viridis cells was also studied, with results similar to those reported above using purified reaction centers.

Start Page
1520-4995; 0006-2960