Changes in the Functional and Structural Properties of the Mn Cluster Induced by Replacing the Side Group of the C-Terminus of the D1 Protein of Photosystem II

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Abstract
A free alpha-COO(-) in the C-terminal alanine-344 (Ala344) in the D1 protein of photosystem II is thought to be responsible for ligating the Mn cluster. The effects of the side group of the C-terminus of the D1 protein on the functional and structural properties of the oxygen-evolving complex (OEC) were comprehensively studied by replacing Ala344 with glycine (Gly), valine (Val), aspartate (Asp), or asparagine (Asn). All the mutants grew photoautotrophically under low-light conditions with lower O(2) evolution activity depending on the mutants when compared with the activity of the control wild type. The Gly-, Asp-, and Asn-substituted mutants did not grow under high-light conditions, while the Val-substituted mutant grew even under the high-light conditions. S(2)-state thermoluminescence bands appeared at slightly elevated temperatures when compared with those of the wild type in the Asp- and Gly-substituted mutants, but at almost normal temperatures in the Val- and Asn-substituted mutants. The oxygen-evolving core particles isolated from the mutants showed little change in protein composition. The Gly-, Asp-, and Asn-substituted core particles exhibited low-temperature electron spin resonance (ESR) spectra with reduced S(2) multiline and enhanced g = 4.1 ESR signals, while the Val-substituted particles showed a spectrum similar to that of the control particles. Mid-frequency Fourier transform infrared difference spectra showed distinctive changes in several bands arising from the putative carboxylate ligands for the Mn cluster in all substituted particles, but the bands for the putative C-terminal alpha-carboxylate did not seem to change in the substituted spectra. The changes induced by the Asp and Asn substitution resembled each other except for the amide I region, and showed some similarity to those induced by the Gly substitution in the symmetric carboxylate stretching region. The results were interpreted to mean that similar types of changes of the carboxylate ligands are induced by these substitutions. The band from a putative histidine ligand for the Mn cluster was similarly affected in the Gly-, Asp-, and Asn-substituted spectra, but not in the Val-substituted spectrum. Notably, marked changes in the amide I, amide II, and carboxylate bands were observed in the Val-substituted spectrum, which was different from the Gly-, Asp-, and Asn-substituted spectra. The results indicated that the structural perturbations induced by the Val substitution include large changes of the protein backbone and are considerably different from those induced by the other substitutions. Possible amino acid ligands participating in the changes deduced by Ala344 replacement in the D1 C-terminal and the effects of the changes of the side group on these ligands were considered on the basis of the available X-ray model of the OEC.

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