Photosystem II: commonality and diversity with emphasis on the extrinsic subunits.

Photosystem II (PSII) is a multi-subunit complex embedded in the thylakoid membranes of all oxygenic photosynthetic organisms, ranging from cyanobacteria to algae and plants. PSII converts solar energy to chemical energy and produces oxygen by oxidizing water, thereby sustaining life on Earth. The basic structures of the PSII core and the fundamental mechanisms of light-driven water oxidation are well-conserved among the diverse oxyphototrophs. Meanwhile, the compositions of the extrinsic subunits, which have critical roles in supporting water oxidation, have largely changed during evolution. The light-harvesting antenna systems of PSII are even more diverse. In this review, we comprehensively summarize the commonality of PSII, while highlighting the diversity of PSII among various oxyphototrophs. This includes summaries on the overall PSII core structure, PSII assembly and repair, charge separation and electron transfer in PSII, water oxidation by PSII, peripheral light-harvesting antennas of PSII, and PSII-antenna supercomplex structures, as well as a summary on the extrinsic subunits. Special emphasis is given to the extrinsic subunits, updating our understanding of their roles, and discussing the structural and functional complementation of the different sets of extrinsic subunits in cyanobacterial, red-lineage, and green plant PSII.