Abstract
Photosystem II (PSII), which catalyzes photosynthetic water oxidation, is composed of more than 20 subunits, including membrane-intrinsic and -extrinsic proteins. The PSII extrinsic proteins shield the catalytic Mn4CaO5 cluster from the outside bulk solution and enhance binding of inorganic cofactors, such as Ca2+ and Cl-, in the oxygen-evolving center (OEC) of PSII. Among PSII extrinsic proteins, PsbO is commonly found in all oxygenic organisms, while PsbP and PsbQ are specific to higher plants and green algae, and PsbU, PsbV, CyanoQ, and CyanoP exist in cyanobacteria. In addition, red algae and diatoms have unique PSII extrinsic proteins, such as PsbQ′ and Psb31, suggesting functional divergence during evolution. Recent studies with reconstitution experiments combined with Fourier transform infrared spectroscopy have revealed how the individual PSII extrinsic proteins affect the structure and function of the OEC in different organisms. In this review, we summarize our recent results and discuss changes that have occurred in the structural coupling of extrinsic proteins with the OEC during evolutionary history.
Highlights
Photosystem II (PSII) is a key protein complex involved in light-energy conversion reactions in photosynthesis
We focus on the structural coupling of extrinsic proteins with the oxygen-evolving center (OEC) in different photosynthetic organisms, which has been investigated by reconstitution experiments combined with light-induced Fourier transform infrared (FTIR) difference spectroscopy (Tomita et al, 2009; Ido et al, 2012; Uno et al, 2013; Nishimura et al, 2014; Nagao et al, 2015)
The results of FTIR are evaluated in light of the current knowledge on the subunit interactions in PSII and discussed in terms of the changes that occurred during evolution
Summary
Photosystem II (PSII) is a key protein complex involved in light-energy conversion reactions in photosynthesis. PSII converts light energy into the electrochemical potential energy required to split water into H+, electrons, and molecular oxygen (Debus, 1992). Recent X-ray structural analysis of the cyanobacterial PSII complex has revealed the location of most subunits, pigments, and redox cofactors (Ferreira et al, 2004; Guskov et al, 2009; Umena et al, 2011; Suga et al, 2015). The resulting cation radical of P680+ receives electrons via a redox-active tyrosine of D1, YZ, from the Mn4CaO5 cluster. The Mn4CaO5 cluster converts two water molecules into one molecular oxygen and four protons through a light-driven cycle consisting of five intermediates called Si states (i = 0–4; McEvoy and Brudvig, 2006). Molecular oxygen is released during the S3–S4–S0 transition after the transient S4 state (Vinyard et al, 2013)
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