Oxygenic Photosynthesis Photosystem II

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas- 

PS II reduces plastoquinone (PQ) at the acceptor side, the required electrons are withdrawn from water leading to release of molecular oxygen and protons on the inside of the membrane. Conceptually, the complex can be divided into 2 parts, the photochemical one with the light-driven electron-transport chain, and the catalytic one, which is responsible for water oxidation. 

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