Proton transfer in reaction centers from photosynthetic bacteria

Proton transfer in the bacterial RC associated with the reduction of the bound QB to the dihydroquinone is an important step in the energetics of photosynthetic bacteria. The binding of two protons by the quinone is associated with the transfer of the second electron to QB at a rate of ca. 10(3) s-1 (pH 7). Mutation of three protonatable residues, GluL212, SerL223, and AspL213, located near QB to nonprotonatable residues (Gln, Ala, and Asn, respectively) resulted in large reductions (by 2 to 3 orders of magnitude) in the rate or proton transfer to QB. These mutations can be grouped into two classes: those that blocked both proton transfer and electron transfer (SerL223, and AspL213) and those that blocked only proton transfer (GluL212). These results were interpreted in terms of a pathway for proton transport in which uptake of the first proton, required for the transfer of the second electron, occurs through a pathway involving AspL213 and SerL223. Uptake of the second proton, which follows electron transfer, occurs through a pathway involving GluL212 and possibly AspL213. Acidic residues near QB affect electron transfer rates via electrostatic interactions. One residue, with a pKa of ca. 10 interacting strongly with the charge on QB (delta pKa greater than 2), was shown to be GluL212. A second residue with a pKa of ca. 6, which interacts more weakly with the charge on QB (delta pK approximately 1), could be either AspL210 or AspL213. Several possible mechanisms for proton transfer are consistent with the observed experimental results and proposed proton pathways. These involve proton transfers from individual amino acid residues or internal water molecules either as single steps or in a concerted fashion. The determination of the dominant mechanism will require evaluation of the energetics of the various steps.