Structure of the Reaction Center from Rhodopseudomonas sphaeroides R-26: Protein-Cofactor (Quinones and Fe^(2+)) Interactions

The three-dimensional structure of the reaction center (RC) from Rhodobacter sphaeroides has been determined by x-ray diffraction to a resolution of 2.8 Å with an R value of 24%. The interactions of the protein with the primary quinone, Q_A, secondary quinone, Q_B, and the nonheme iron are described and compared to those of RCs from Rhodopseudomonas viridis. Structural differences between the Q_A and Q_B environments that contribute to the function of the quinones (the electron transfer from Q_A- to Q_B and the charge recombination of Q_A-, Q_B- with the primary donor) are delineated. The protein residues that may be involved in the protonation of Q_B are identified. A pathway for the doubly reduced Q_B to dissociate from the RC is proposed. The interactions between QB and the residues that have been changed in herbicide-resistant mutants are described. The environment of the nonheme iron is compared to the environments of metal ions in other proteins