Abstract
Photosynthetic oxygen evolution is catalyzed by a Mn4Ca cluster which is putatively coordinated by amino acid residues, mostly from the D1 protein of photosystem II. To understand the detailed ligation structure of the Mn4Ca cluster, we examined the interaction between Mn ions and putative amino acid ligands by means of flash-induced FTIR difference spectroscopy in combination with specific isotope labeling and site-directed mutagenesis using Synechocystis sp. PCC 6803. Histidine-specific 15N-labeling studies showed the S-state dependent structural coupling between the Mn4Ca cluster and histidine ligand(s). Alanine-specific 13C-labeling studies revealed the unidentate ligation of the D1-Ala344 carboxylate to a redox-active Mn ion. D1-Glu189Gln mutation studies indicated the bidentate ligation of the Glu189 carboxylate to the cluster, while the reported D1-Asp170His mutation studies showed no effect on the FTIR difference spectra. Based on these findings together with recent X-ray crystallographic models, the ligation structure of the Mn4Ca cluster is discussed.
