Amino acid residues involved in the coordination and assembly of the manganese cluster of photosystem II. Proton-coupled electron transport of the redox-active tyrosines and its relationship to water oxidation

The combination of site-directed mutagenesis, isotopic labeling, new magnetic resonance techniques and optical spectroscopic methods have provided new insights into cofactor coordination and into the mechanism of electron transport and proton-coupled electron transport in photosystem II. Site-directed mutations in the D1 polypeptide of this photosystem have implicated a number of histidine and carboxylate residues in the coordination and assembly of the manganese cluster, responsible for photosynthetic water oxidation. Many of these are located in the carboxy-terminal region of this polypeptide close to the processing site involved in its maturation. This maturation is a required precondition for cluster assembly. Recent proposals for the mechanism of water oxidation have directly implicated redox-active tyrosine Y-Z in this mechanism and have emphasized the importance of the coupling of proton and electron transfer in the reduction of Y-Z(.) by the Mn cluster. The interaction of both homologous redox-active tyrosines Y-Z and Y-D with their respective homologous proton accepters is discussed in an effort to better understand the significance of such coupling. (C) 2001 Elsevier Science B.V. All rights reserved.