Drastic changes in the ligand structure of the oxygen-evolving Mn cluster upon Ca2+ depletion as revealed by FTIR difference spectroscopy

A Fourier transform infrared (FTIR) difference spectrum of the oxygen-evolving Mn cluster upon the S-1-to-S-2 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 S-2/S-1 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 S-2/S-1 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. (c) 2006 Elsevier B.V. All rights reserved.