Manganese oxidation by modified reaction centers from Rhodobacter sphaeroides

The transfer of an electron from exogenous manganese (II) ions to the bacteriochlorophyll dimer, P, of bacterial reaction centers was characterized for a series of mutants that have P/P⁺ midpoint potentials ranging from 585 to 765 mV compared to 505 mV for wild type. Light-induced changes in optical and EPR spectra of the mutants were measured to monitor the disappearance of the oxidized dimer upon electron donation by manganese in the presence of bicarbonate. The extent of electron transfer was strongly dependent upon the P/P⁺ midpoint potential. The midpoint potential of the Mn²⁺/Mn³⁺ couple was calculated to decrease linearly from 751 to 623 mV as the pH was raised from 8 to 10, indicating the involvement of a proton. The electron donation had a second order rate constant of approximately 9 × 10⁴ M^-1 s^-1, determined from the linear increase in rate for Mn²⁺ concentrations up to 200 μM. Weak dissociation constants of 100-200 μM were found. Quantitative EPR analysis of the six-line free Mn²⁺ signal revealed that up to seven manganese ions were associated with the reaction centers at a 1 mM concentration of manganese. The association and the electron transfer between manganese and the reaction centers could be inhibited by Ca ²⁺ and Na⁺ ions. The ability of reaction centers with high potentials to oxidize manganese suggests that manganese oxidation could have preceded water oxidation in the evolutionary development of photosystem II.