Interaction between Cytochrome c₂ and Reaction Centers from Purple Bacteria

The kinetics of electron transfer of cytochrome c₂ from Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodospirillum centenum to reaction centers from Rb. sphaeroides and Rb. capsulatus have been measured. Observed in the kinetics of decay of the oxidized donor are a rapid first-order rate and one or more slower rates that are due to diffusion-limited complex formation. For reaction centers from Rb. sphaeroides, the fast component had time constants of 1.0 and 0.5 µs for cytochrome c₂ from Rb. sphaeroides and Rb. capsulatus, respectively, but only a slow component was observed for cytochrome c₂ from Rs. centenum. 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 µs for cytochrome c₂ from Rb. sphaeroides, Rb. capsulatus, and Rs. centenum, respectively, although the dissociation constant for cytochrome c₂ from Rs. centenum was approximately 20 times larger than that of the other cytochromes. The observation of the fast component for cytochrome c₂ from Rs. centenum in reaction centers from Rb. capsulatus but not Rb. sphaeroides demonstrates that the binding interactions for the two reaction centers differ, and the involvement of amino acid residues in the binding is discussed. The kinetics of electron transfer from cytochrome c₂ to reaction centers of Rb. sphaeroides from wild type and three mutant strains that have altered carboxyl-terminal regions of the M subunit of the reaction center have also been measured. For cytochrome c₂ from Rb. sphaeroides, the kinetics are very similar between the mutants and wild type. In contrast, for cytochrome c₂ from Rb. capsulatus, the dissociation constants vary from 2.4 to 18 µM in the mutants compared to 6.3 µM for wild type. A greater involvement for the M carboxyl region in the binding of the cytochrome c₂ from Rb. capsulatus is proposed.