Current status of the role of Cl− ion in the oxygen-evolving complex

Abstract

This minireview summarizes the current state of knowledge concerning the role of Cl(-) in the oxygen-evolving complex (OEC) of photosystem II (PSII). The model that proposes that Cl(-) is a Mn ligand is discussed in light of more recent work. Studies of Cl(-) specificity, stoichiometry, kinetics, and retention by extrinsic polypeptides are discussed, as are the results that fail to detect Cl(-) ligation to Mn and results that show a lack of a requirement for Cl(-) in PSII-catalyzed H(2)O oxidation. Mutagenesis experiments in cyanobacteria and higher plants that produce evidence for a correlation between Cl(-) retention and stable interactions among intrinsic and extrinsic polypeptides are summarized, and spectroscopic data on the interaction between PSII and Cl(-) are discussed. Lastly, the question of the site of Cl(-) action in PSII is discussed in connection with the current crystal structures of the enzyme.

Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography

“…However, the 2 Cl Ϫ -binding sites identified here have significant functional consequences. The similar locations of these 2 Cl Ϫ -binding sites and their closeness to the Mn 4 Ca cluster suggest that they both may be important for the normal function of the OEC. This importance is supported by the fact that the 2 anion-binding sites are surrounded by residues that either are coordinated directly with the Mn 4 Ca cluster or are in close proximity to the cluster.…”

“…The catalytic center for water splitting, i.e., the oxygen-evolving complex (OEC), is located in the membrane surface of the luminal side of thylakoid membranes and is composed of 4 Mn atoms and 1 Ca atom in the protein matrix of PSII. Oxygen is produced from 2 molecules of water by sequential, light-induced electron-abstracting events at the Mn 4 Ca cluster that cycle through the Si states with i ϭ 0-4 (2). One or more Cl Ϫ ions are required for the water oxidation cycle to proceed, a requirement not often found in other biological systems (3,4).…”

“…Oxygen is produced from 2 molecules of water by sequential, light-induced electron-abstracting events at the Mn 4 Ca cluster that cycle through the Si states with i ϭ 0-4 (2). One or more Cl Ϫ ions are required for the water oxidation cycle to proceed, a requirement not often found in other biological systems (3,4). Depletion of Cl Ϫ has been shown to inhibit the S 2 3 S 3 and S 3 3 S 0 transitions (5).…”

“…Depletion of Cl Ϫ has been shown to inhibit the S 2 3 S 3 and S 3 3 S 0 transitions (5). The roles of Cl Ϫ proposed for OEC include ligation to Mn or Ca atoms (6)(7)(8), regulation of the redox potential of the Mn 4 Ca cluster (9), maintaining a hydrogen bond network (10), and activation of the substrate water (11). In addition, an association of Cl Ϫ with amino acid residues in the Mn coordination shell has been proposed (12,13).…”

Quantitative Local Photosynthetic Flux Measurements at Isolated Chloroplasts and Thylakoid Membranes Using Scanning Electrochemical Microscopy (SECM)

“…This is consistent with the enhanced photosynthetic activity that has been reported on bulk systems in the presence of the Mg 2+ ions, 56,59,78 and could also be due to the presence of chloride ions or Mn 2+ which play a crucial role in the water splitting complex. 57,79,80 Using the FEM model described earlier, the rate of oxygen evolution over the initial forty seconds of illumination at the chloroplast film, for both HS and HS+ buffers, was calculated. The first 40 seconds was chosen in order to incorporate the initial oxygen concentration spike and its subsequent decrease to a steady-state oxygen evolution current.…”