Participation of the C-terminal region of the D1-polypeptide in the first steps in the assembly of the Mn4Ca cluster of photosystem II

Abstract

Amino acid residue D1-Asp170 of the D1-polypeptide of photosystem II was previously shown to be implicated in the binding and oxidation of the first manganese to be assembled into the Mn4Ca cluster of the oxygen-evolving complex (OEC). According to recent x-ray crystallographic structures of photosystem II, D1-Glu333 is proposed to participate with D1-Asp170 in the coordination of Mn4 of the OEC. Other residues in the C-terminal region of the D1-polypeptide are proposed to coordinate nearby manganese of the cluster. Site-directed replacements in Synechocystis sp. PCC 6803 at D1-His332, D1-Glu333, D1-Asp342, D1-Ala344, and D1-Ser345 were examined with regard to their ability to influence the binding and oxidation of the first manganese in manganese-depleted photosystem II core complexes. Direct and indirect measurements reveal in all mutants, but most marked in D1-Glu 333 replaced by His, an impaired ability of Mn2+ to reduce YZ·, indicating a reduced ability (elevated Km) compared with WT to bind and oxidize the first manganese of the OEC. The effect on the Km of these mutations is, however, considerably weaker than some of those constructed at D1-Asp170 (replacement by Asn, Ala, and Ser). These observations imply that the C-terminal residues ultimately involved in manganese coordination contribute to the high affinity binding at D1-Asp 170 likely through electrostatic interactions. That these residues are far from D1-Asp170 in the primary structure of the D1-polypeptide, imply that the C terminus of the D1-polypeptide is already close to its mature conformation at the first stages of assembly of the Mn 4Ca cluster. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.