D1, D2 subunits of photosystem II (PSII); M, L subunits of bacterial photosynthetic reaction center
This protein superfamily contains the D1, D2 subunits of the photosystem II (PS II) and the M, L subunits of the bacterial photosynthetic reaction center (RC). These four proteins are highly homologous and share a common fold. PS II is a multi-subunit protein found in the photosynthetic membranes of plants, algae, and cyanobacteria. It utilizes light-induced electron transfer and water-splitting reactions to produce protons, electrons, and molecular oxygen. The protons generated are instrumental in ATP formation. Bacterial photosynthetic reaction center (RC) complex is found in photosynthetic bacteria, such as purple bacteria and other proteobacteria species. It couples light-induced electron transfer to proton pumping across the membrane by reactions of a quinone molecule (QB) that binds two electrons and two protons at the active site. Protons are translocated from the bacterial cytoplasm to the periplasmic space, generating an electrochemical gradient of protons (the protonmotive force) that can be used to power reactions such as the synthesis of ATP.
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