Specific Alteration of the Oxidation Potential of the Electron Donor in Reaction Centers from Rhodobacter Sphaeroides

The effects of multiple changes in hydrogen bond interactions between the electron donor, a bacteriochlorophyll dimer, and histidine residues in the reaction center from Rhodobacter sphaeroides have been investigated. Site-directed mutations were designed to add or remove hydrogen bonds between the 2-acetyl groups of the dimer and histidine residues at the symmetry-related sites His-L168 and Phe-M197, and between the 9-keto groups and Leu-L131 and Leu-M160. The addition of a hydrogen bond was correlated with an increase in the dimer midpoint potential. Measurements on double and triple mutants showed that changes in the midpoint potential due to alterations at the individual sites were additive. Midpoint potentials ranging from 410 to 765 mV, compared with 505 mV for wild type, were achieved by various combinations of mutations. The optical absorption spectra of the reaction centers showed relatively minor changes in the position of the donor absorption band, indicating that the addition of hydrogen bonds to histidines primarily destabilized the oxidized state of the donor and had little effect on the excited state relative to the ground state. Despite the change in energy of the charge-separated states by up to 260 meV, the mutant reaction centers were still capable of electron transfer to the primary quinone. The increase in midpoint potential was correlated with an increase in the rate of charge recombination from the primary quinone, and a fit of these data using the Marcus equation indicated that the reorganization energy for this reaction is approximately 400 meV higher than the change in free energy in wild type. The mutants were still capable of photosynthetic growth, although at reduced rates relative to the wild type. These results suggest a role for protein-cofactor interactions--in particular, histidine-donor interactions--in establishing the redox potentials needed for electron transfer in biological systems.