Characterization of Bacterial Reaction Centers Having Mutations of Aromatic Residues in the Binding Site of the Bacteriopheophytin Intermediary Electron Carrier

We report the initial characterization of a series of reaction centers (RCs) from the photosynthetic bacterium Rhodobacter capsulatus having single or double mutations of phenylalanines 97 and 121 on the L polypeptide. Substitution of these aromatic amino acids, which may interact with the photoactive bacteriopheophytin associated with the L polypeptide (BPhO, was carried out to examine their possible roles in electron transfer, charge stabilization, and/or BPh_L binding. In some mutant RCs, the wild-type pigment content is obtained while in certain others a bacteriochlorophyll (BChl) replaces BPh_L. The mutant RCs with wild-type pigment content are found to have overall photochemistry effectively identical to that of wild-type RCs. This indicates that aromatic residues at L97 and L121 are not critical factors in the charge separation process, although an approximate 2-fold increase in the rate of electron transfer from BPh_L- to Q_A is observed in two mutants where residue L121 is leucine. In two double mutants where L121 is histidine and L97 is either valine or cysteine, BPhL is replaced with a BChl (denoted β). This pigment content is surprising since in the native RC structure amino acid LI21 is not in optimum geometry for coordination to the Mg in the center of the pigment macrocycle. Charge separation takes place in the βcontaining mutants with an ~70% yield of P+Q_A ^- at 285 K compared to ~100% for wild-type. The photochemistry of these new beta-type RCs is very similar to that reported previously for the beta RC from Rhodobacter sphaeroides wherein the same pigment change was induced by a mutation in the M polypeptide.