Different effects of identical symmetry-related mutations near the bacteriochlorophyll dimer in the photosynthetic reaction center of Rhodobacter sphaeroides

Abstract

In the bacterial photosynthetic reaction center (RC), asymmetric protein environment of the bacteriochlorophyll (BChl) dimer largely determines the photophysical and photochemical properties of the primary electron donor. Previously, we noticed significant differences in properties of Rhodobacter sphaeroides RCs with identical mutations in symmetry-related positions - I(M206)H and I(L177)H. The substitution I(L177)H resulted in covalent binding of BChl PA with the L-subunit, as well as in 6-coordination of BChl BB, whereas in RC I(M206)H no such changes of pigment-protein interactions were found. In addition, the yield of RC I(M206)H after its isolation from membranes was significantly lower than the yield of RC I(L177)H. This study shows that replacement of amino acid residues in the M203-M206 positions near BChls PB and BA by symmetry-related residues from the L-subunit near BChls PA and BB leads to further decrease in RC amount in the membranes associated obviously with poor assembly of the complex. Introduction of a new hydrogen bond between BChl PB and its protein environment by means of the F(M197)H mutation stabilized the mutant RC but did not affect its low yield. We suggest that the mutation I(M206)H and substitution of amino acid residues in M203-M205 positions could disturb glycolipid binding on the RC surface near BChl BA that is important for stable assembly of the complex in the membrane.