Abstract

Photoinduced water oxidation at the O2-evolving complex (OEC) of photosystem II (PSII) is a complex process involving a tetramanganese-calcium cluster that is surrounded by a hydrogen-bonded network of water molecules, chloride ions, and amino acid residues. Although the structure of the OEC has remained conserved over eons of evolution, significant differences in the chloride-binding characteristics exist between cyanobacteria and higher plants. An analysis of amino acid residues in and around the OEC has identified residue 87 in the D1 subunit as the only significant difference between PSII in cyanobacteria and higher plants. We substituted the D1-Asn87 residue in the cyanobacterium Synechocystis sp. PCC 6803 (wildtype) with alanine, present in higher plants, or with aspartic acid. We studied PSII core complexes purified from D1-N87A and D1-N87D variant strains to probe the function of the D1-Asn87 residue in the water-oxidation mechanism. EPR spectra of the S2 state and flash-induced FTIR spectra of both D1-N87A and D1-N87D PSII core complexes exhibited characteristics similar to those of wildtype Synechocystis PSII core complexes. However, flash-induced O2-evolution studies revealed a decreased cycling efficiency of the D1-N87D variant, whereas the cycling efficiency of the D1-N87A PSII variant was similar to that of wildtype PSII. Steady-state O2-evolution activity assays revealed that substitution of the D1 residue at position 87 with alanine perturbs the chloride-binding site in the proton-exit channel. These findings provide new insight into the role of the D1-Asn87 site in the water-oxidation mechanism and explain the difference in the chloride-binding properties of cyanobacterial and higher-plant PSII.

Highlights

  • Photoinduced water oxidation at the O2-evolving complex (OEC) of photosystem II (PSII) is a complex process involving a tetramanganese-calcium cluster that is surrounded by a hydrogen-bonded network of water molecules, chloride ions, and amino acid residues

  • We studied PSII core complexes purified from D1-N87A and D1-N87D variant strains to probe the function of the D1-Asn87 residue in the water-oxidation mechanism

  • To investigate the significance of the difference in D1 residue 87 between spinach PSII and cyanobacterial PSII, the D1-N87A and D1-N87D mutations were constructed in Synechocystis sp

Read more

Summary

Results

To investigate the significance of the difference in D1 residue 87 between spinach PSII and cyanobacterial PSII, the D1-N87A and D1-N87D mutations were constructed in Synechocystis sp. The mid-frequency FTIR difference spectra induced by four successive flashes given to wildtype and D1-N87D PSII core complexes are compared in Fig. 4 (black and red traces, respectively). The mid-frequency S2-minus-S1 spectrum of D1-N87A PSII core complexes (Fig. S6, upper red trace) showed changes similar to those observed for D1-N87D PSII core complexes. The O–H stretching vibrations of strongly H-bonded OH groups can be observed as very broad positive features between 3200 and 2500 cmϪ1 (13, 26 –29) The oxygen-release kinetics for D1-N87D PSII core complexes was found to be similar to that of His-tagged wildtype PSII core complexes (Fig. S5)

Discussion
14 Ϯ 1 29 Ϯ 3 11 Ϯ 3
Experimental procedures
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call