Acquirement of water-splitting ability and alteration of the charge-separation mechanism in photosynthetic reaction centers

Significance In photosynthetic reaction centers from purple bacteria (PbRC) and the water-splitting enzyme, photosystem II (PSII), light-induced electron transfer occurs only in one of the two branches irrespective of the apparent symmetry in the structures. In PSII, the protein components that are involved in the Mn 4 CaO 5 cluster or the proceeding proton-transfer pathway facilitate electron transfer along the active branch. In PbRC, most of these components are not conserved and polar residues on the active side facilitate electron transfer. The energy profile suggests that the initial electron donor differs between PbRC and PSII. It seems likely that the acquirement of oxygen-evolving ability alters the protein environment near the accessory chlorophyll and makes it the initial electron donor in PSII.