Drastic changes in the ligand structure of the oxygen-evolving Mn cluster upon Ca depletion as revealed by FTIR difference spectroscopy |
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Authors: | Yuta Taguchi |
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Institution: | Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan |
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Abstract: | A Fourier transform infrared (FTIR) difference spectrum of the oxygen-evolving Mn cluster upon the S1-to-S2 transition was obtained with Ca2+-depleted photosystem II (PSII) membranes to investigate the structural relevance of Ca2+ to the Mn cluster. Previously, Noguchi et al. Biochim. Biophys. Acta 1228 (1995) 189] observed drastic changes in the carboxylate stretching region of the S2/S1 FTIR spectrum upon Ca2+ depletion, whereas Kimura and co-workers Biochemistry 40 (2001) 14061; ibid. 41 (2002) 5844] later claimed that these changes were not ascribed to Ca2+ depletion itself but caused by the interaction of EDTA to the Mn cluster and/or binding of K+ at the Ca2+ site. In the present study, the preparation of the Ca2+-depleted PSII sample and its FTIR measurement were performed in the absence of EDTA and K+. The obtained S2/S1 spectrum exhibited the loss of carboxylate bands at 1587/1562 and 1364/1403 cm− 1 and diminished amide I intensities, which were identical to the previous observations in the presence of EDTA and K+. This result indicates that the drastic FTIR changes are a pure effect of Ca2+ depletion, and provides solid evidence for the general view that Ca2+ is strongly coupled with the Mn cluster. |
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Keywords: | DCMU 3-(3 4-dichlorophenyl)-1 1-dimethylurea FTIR Fourier transform infrared Mes 2-(N-morpholino)ethanesulfonic acid OEC oxygen evolving center PpBQ phenyl-p-benzoquinone PSII photosystem II QA primary quinone electron acceptor |
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