This chapter discusses the high-resolution crystal structures of bacterial photosynthetic reaction centers. Photosynthetic Reaction Centers (RCs) are membrane-spanning complexes of polypeptide chains and cofactors that catalyze the first steps in the conversion of light energy to chemical energy during photosynthesis. Absorption of a photon in the RC, or energy transfer from light-harvesting complexes to the RC, causes rapid, efficient electron transfer from a primary donor along a chain of acceptors leading across the photosynthetic membrane. In all types of RCs, the primary donor has been shown, or is assumed to be, a closely associated pair of chlorophyll molecules. The chemical nature of the acceptor molecule at the end of the electron transfer chain within RCs depends on the type of RC: in photosynthetic purple bacteria; and in photosystem II of green plants and cyanobacteria, it is a quinone that is converted to a quinol by two successive electron transfer events and protonation.
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