Proton release upon oxidation of tyrosine in reaction centers from Rhodobacter sphaeroides

Markedly different light-induced protonational changes were measured in two reaction center mutants of Rhodobacter sphaeroides. A quadruple mutant containing alterations, at residues L131, M160, M197, and M210, that elevate the midpoint potential of the bacteriochlorophyll dimer was compared to the Y_M mutant, which contains these alterations plus a tyrosine at M164 serving as a secondary electron donor [Kálmán et al., Nature 402 (1999) 696]. In the quadruple mutant, a proton uptake of 0.1-0.3 H⁺/reaction center between pH 6 and 10 resulted from formation of the oxidized bacteriochlorophyll donor and reduced primary quinone. In the Y_M mutant, a maximal proton release of -0.5 H⁺/reaction center at pH 8 was attributed to formation of the tyrosyl radical and modeled using electrostatic and direct proton-releasing mechanisms.