Controlling the pathway of photosynthetic charge separation in bacterial reaction centers

For at least 2 billion years, the structure of the photosynthetic reaction center has maintained an approximate rotational symmetry, in both plants and bacteria, consisting of a heterodimeric core with two ostensibly similar electron-transfer pathways, yet the functional advantage of this symmetry is not clear. This structure/function enigma is nowhere more apparent than in reaction centers isolated from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides. These reaction centers possess two approximately symmetric potential electron transfer pathways (labeled A and B), but stable charge separation is only observed along the A-side in isolated wild type reaction centers. Here we demonstrate that the introduction of two protonatable residues (aspartate and glutamate) in the vicinity of the cofactors involved in initial electron transfer results in pH-dependent switching between A- and B-side charge separation products. At pH 7.2, A-side photochemistry predominates, whereas at pH 9.5, a long-lived B-side charge-separated state is formed almost exclusively. This raises the possibility that a similar control of wild type reaction centers could be mediated either by external factors or by photochemically induced electrostatic changes in vivo.