PATHWAY OF PROTON-TRANSFER IN BACTERIAL REACTION CENTERS - FURTHER INVESTIGATIONS ON THE ROLE OF SER-L223 STUDIED BY SITE-DIRECTED MUTAGENESIS

The role of Ser-L223 in proton transfer to reduced Q(B) in the reaction center (RC) from Rhodobacter sphaeroides was studied by site-directed replacement of Ser with residues having different proton donor properties, e.g., the aliphatic residues Ala and Gly, the hydroxyl residue Thr, the amide residue Asn, the sulhydryl residue Cys, the imidazole residue His, and the carboxylic acid residue Asp. Compared to native reaction centers, RCs with Ala or Asn at L223 had greatly reduced (similar to 300-fold) proton-coupled electron transfer rates, k(AB)((2)), associated with the second electron reduction of Q(B) (Q(A)(-)Q(B)(-)+H+ --> Q(A)Q(B)H(-)). In contrast, RCs containing Thr, Asp, or Gly at L223 retained fast proton-coupled electron transfer rates. RCs with His or Cys at L223 did not bind the secondary quinone Q(B). These results show that k(AB)((2)) is larger when a good proton transfer group, e.g., a hydroxyl residue (Ser, Thr) or a carboxylic acid (Asp), occupies the L223 site, supporting the proposal that Ser-L223 is a component of a proton transfer chain [Paddock, M. L., McPherson, P. H., Feher, G., & Okamura, M. Y. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6803-6807]. The surprising result that k(AB)((2)) is not significantly reduced in RCs with Gly at L223 suggests that a water molecule functionally replaces the missing Ser hydroxyl group in the mutant RCs. The importance of Ser-L223 in internal proton transfer reactions within the RC is discussed.