Tyrosine oxidation in modified reaction centers from Rhodobacter sphaeroides

Abstract

A highly oxidizing bacteriochlorophyll dimer was generated by multiple amino acid substitution in reaction centers from Rhodobacter sphaeroides. Tyrosine residues were then introduced into these modified reaction centers at positions that are analogous to those of YZ and YD in photosystem II. Optical and EPR spectra show changes that are consistent with the oxidation of the tyrosine residues by the bacteriochlorophyll dimer. The formation of the tyrosyl radical was found to be pH-dependent, indicating an important role of the surrounding protein environment. Subsequent amino acid substitutions at nearby positions resulted in altered pK values for the pH dependence of the optical and EPR signals characteristic of the oxidized tyrosine. The pH dependence is explained by models of the mechanism of tyrosine oxidation in proteins, in which electron transfer is coupled to the release of the phenolic proton to a proton acceptor, resulting in a neutral tyrosyl radical. These studies provide insight into how electron transfer pathways evolved in photosynthetic complexes.