INTERACTION BETWEEN CYTOCHROME-C(2) AND REACTION CENTERS FROM PURPLE BACTERIAL

The kinetics of electron transfer of cytochrome c(2) from Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodospirillum centenum to re action centers from Rb. sphaeroides and Rb. capsulatus have been measu red. Observed in the kinetics of decay of the oxidized donor are a rap id first-order rate and one or more slower rates that are due to diffu sion-limited complex formation. For reaction centers from Rb. sphaeroi des, the fast component had time constants of 1.0 and 0.5 mu s for cyt ochrome c(2) from Rb. sphaeroides and Rb. capsulatus, respectively, bu t only a slow component was observed for cytochrome c(2) from Rs. cent enum. For reaction centers from Rb. capsulatus, the kinetics from all three cytochromes had a fast component with time constants of 1.0, 0.7 , and 1.9 mu s for cytochrome ct from Rb. sphaeroides, Rb. capsulatus, and Rs. centenum, respectively, although the dissociation constant fo r cytochrome c(2) from Rs. centenum was approximately 20 times larger than that of the other cytochromes. The observation of the fast compon ent for cytochrome c(2) from Rs. centenum in reaction centers from Rb. capsulatus but not Rb. sphaeroides demonstrates that the binding inte ractions for the two reaction centers differ, and the involvement of a mino acid residues in the binding is discussed. The kinetics of electr on transfer from cytochrome c(2) to reaction centers of Rb. sphaeroide s from wild type and three mutant strains that have altered carboxyl-t erminal regions of the M subunit of the reaction center have also been measured. For cytochrome c(2) from Rb. sphaeroides, the kinetics are very similar between the mutants and wild type. In contrast, for cytoc hrome c(2) from Rb. capsulatus, the dissociation constants vary from 2 .4 to 18 mu M in the mutants compared to 6.3 mu M for wild type. A gre ater involvement for the M carboxyl region in the binding of the cytoc hrome c(2) from Rb. capsulatus is proposed.