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 reaction center in a Rhodobacter capsulatus mutant in which a His also replaces Leu 212 on the M-polypeptide. The initial stage of charge separation 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 reduction potential of BChl(L), thereby changing the energetics of charge separation. The results demonstrate that an individual amino acid residue can, through its influence on the free energies of the charge-separated states, effectively dictate the balance between the forward electron transfer reactions on the L-side of the RC, the charge-recombination processes, and electron transfer to the M-side chromophores.