CONTROL OF ELECTRON-TRANSFER BETWEEN THE L-SIDE AND M-SIDE OF PHOTOSYNTHETIC REACTION CENTERS

An aspartic acid residue has been introduced near ring V of the L-side accessory bacteriochlorophyll (BChl(L)) of the photosynthetic reactio n center in a Rhodobacter capsulatus mutant in which a His also replac es Leu 212 on the M-polypeptide. The initial stage of charge separatio n in the G(M201)D/L(M212)H double mutant yields similar to 70 percent electron transfer to the L-side cofactors, similar to 15 percent rapid deactivation to the ground state, and similar to 15 percent electron transfer to the so-called inactive M-side bacteriopheophytin (BPh(M)). It is suggested here that the Asp introduced at M201 modulates the re duction potential of BChl(L), thereby changing the energetics of charg e separation. The results demonstrate that an individual amino acid re sidue can, through its influence on the free energies of the charge-se parated states, effectively dictate the balance between the forward el ectron transfer reactions on the L-side of the RC, the charge-recombin ation processes, and electron transfer to the M-side chromophores.