Reaction of cytochrome c2 with photosynthetic reaction centers from Rhodopseudomonas viridis.
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)
Knaff
D B
DB
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock 79409-1061.
Willie
A
A
Long
J E
JE
Kriauciunas
A
A
Durham
B
B
Millett
F
F
D000818:Animals; D002474:Cell-Free System; D003574:Cytochrome c Group / Q000737:chemistry / Q000378:metabolism*; D045362:Cytochromes c2; D006736:Horses; D066298:In Vitro Techniques; D007700:Kinetics; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D012241:Rhodopseudomonas / Q000378:metabolism*
D003574:Cytochrome c Group; D045322:Photosynthetic Reaction Center Complex Proteins; D045362:Cytochromes c2