Properties of mutant reaction centers of Rhodobacter sphaeroides with substitutions of histidine L153, the axial Mg2+ ligand of bacteriochlorophyll B(A).

Mutant reaction centers (RC) from Rhodobacter sphaeroides have been studied in which histidine L153, the axial ligand of the central Mg atom of bacteriochlorophyll B(A) molecule, was substituted by cysteine, methionine, tyrosine, or leucine. None of the mutations resulted in conversion of the bacteriochlorophyll B(A) to a bacteriopheophytin molecule. Isolated H(L153)C and H(L153)M RCs demonstrated spectral properties similar to those of the wild-type RC, indicating the ability of cysteine and methionine to serve as stable axial ligands of the Mg atom of bacteriochlorophyll B(A). Because of instability of mutant H(L153)L and H(L153)Y RCs, their properties were studied without isolation of these complexes from the photosynthetic membranes. The most prominent effect of the mutations was observed with substitution of histidine by tyrosine. According to the spectral data and the results of pigment analysis, the B(A) molecule is missing in the H(L153)Y RC. Nevertheless, being associated with the photosynthetic membrane, this RC can accomplish photochemical charge separation with quantum yield of approximately 7% of that characteristic of the wild-type RC. Possible pathways of the primary electron transport in the H(L153)Y RC in absence of photochemically active chromophore are discussed.