The iron‐quinone electron‐acceptor complex of photosystem II

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Abstract
The iron quinone‐complex of the reaction centers of photosystem II and the purple non‐sulphur photosynthetic bacteria contains two quinones, QAand QBconnected in series with respect to electron transfer, and separated by a non‐heme iron coordinated by amino acid residues. It is the site of inhibition of many of the common photosynthetic herbicides, which act by displacing QBfrom the center. The complex is responsible for reducing QBto QBH2in two successive one‐electron photo acts. OBH2dissociates from the center, to be replaced by a new QBmolecule and reduces the following membrane‐bound electron‐transfer complex (cytochromeb6/for b/c1). The energetic, kinetics and mechanism of complex function are reviewed here. Recent crystallographic, spectroscopic and molecular biological evidence has produced a considerable quantity of structural information on this complex. These data have given a less formal and more molecular view of how the complex functions. They have also revealed fundamental differences between the photo system II and bacterial complexes, particularly with respect to the coordination of the iron and its chemistry. The comparative anatomy of the complexes is reviewed and its implications for function discussed.

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