Function of Extrinsic Proteins in Stabilization of the Photosynthetic Oxygen-Evolving Complex

Oxygenic phototrophs convert photon energy into chemical energy through a series of lightinduced electron transfer reactions initiated with charge separation of chlorophyll (Chl) special pairs located in the central part of photosystem I and II (PSI and PSII) (Fig. 1). The reducing power is transferred from PSII to PSI through cytochrome b6f, and finally utilized for reduction of NADP+ to assimilate CO2. The oxidized equivalents accumulated on the PSII donor side are neutralized by substrate water molecules to release protons for driving ATP synthase and O2 molecules as a by-product. This water oxidation takes place in the oxygen-evolving complex (OEC) of PSII [McEvoy & Brudvig, 2006, Renger & Renger, 2008]. The OEC assembly is largely similar between cyanobacteria and higher plants, except for a critical difference in the composition of extrinsic proteins [Roose et al., 2007]. In cyanobacteria, PsbO, PsbV, and PsbU residing on the lumenal side of PSII play significant roles in the regulation and stabilization of the water oxidation machinery. Higher plants possess major nuclear gene-encoded extrinsic proteins named PsbO, PsbP, and PsbQ. PsbO is a common extrinsic protein highly conserved among the oxygenic phototrophs. PsbP and PsbQ are indigenous to plant PSII and have been proposed as the functional equivalents of PsbV and PsbU in bacterial PSII, having replaced them during the course of evolution from ancestral cyanobacteria to higher plants. These proteins play significant roles in the regulation and stabilization of the photosynthetic water oxidation [Roose et al., 2007, Seidler, 1996, Williamson, 2008] although the details of their function(s) are still a matter of debate.