SMALL WEAK ACIDS STIMULATE PROTON-TRANSFER EVENTS IN SITE-DIRECTED MUTANTS OF THE 2 IONIZABLE RESIDUES, GLUL212 AND ASPL213, IN THE QB-BINDING SITE OF RHODOBACTER-SPHAEROIDES REACTION CENTER

Mutations of the two ionizable residues, Glu(L212) and Asp(L213), in the secondary quinone (Q(B)) binding site of reaction centers (RCs) from Rhodobacter sphaeroides cause major dysfunctions in the proton transfer processes leading to the formation of quinol. Mutant RCs with Asp(L213)--> Asn are especially severely blocked, and the rate of the proton-limited transfer of the second electron is at least 10(4) times slower than in the wild-type. Small, weak acids, such as azide/hydrazoic acid (N3-/HN3; pK approximately 4.7) accelerated the electron transfer rate in mutant RCs in a pH and concentration-dependent manner, consistent with their functioning as protein-penetrating protonophores, delivering protons to the Q(B) site in a non-specific, diffusive process. Other small weak acids acted similarly with efficacies dependent on their size and pK values. In terms of the concentration of protonated species, the relative effectiveness was: nitrite > cyanate approximately formate > azide > > acetate. The behavior of bacterial RCs containing the Asp(L213)--> Asn mutation resembles that of bicarbonate-depleted photosystem II, and the mutational block is partially alleviated by bicarbonate. The possibility is discussed that bicarbonate acts in PS II as an analogue to the carboxylic acid residues of the bacterial proton conduction pathway.