Characteristic Changes of the S2/S1 Difference FTIR Spectrum Induced by Ca2+ Depletion and Metal Cation Substitution in the Photosynthetic Oxygen-Evolving Complex

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Abstract
Effects of Ca2+ depletion and substitution with other metal cations on the structure of the protein matrices of the oxygen-evolving complex (OEC) and their corresponding changes upon the S1 to S2 transition were examined using Fourier transform infrared (FTIR) spectroscopy. Ca2+ depletion and further supplementation with Li+, Na+, Mg2+, Ca2+, or Sr2+ did not significantly affect the typical vibrational features in the double difference S2/S1 spectrum, including the symmetric [1365(+)/1404(−) cm-1] and the asymmetric [1587(+)/1566(−) cm-1] stretching modes of the carboxylate ligand and the amide I and II modes of the backbone polypeptides. On the other hand, supplementation with K+, Rb+, Cs+, or Ba2+ significantly modified the S2/S1 spectrum, in which the carboxylate modes disappeared and the amide I and II modes were modified. Results indicate that the binding of metal cations that have ionic radii larger than that of Ca2+ to the Ca2+ site induces perturbations in the protein matrices in the vicinity of the Mn cluster to interrupt the characteristic structural and/or conformational changes upon the oxidation of the Mn cluster accompanied with the S1 to S2 transition. The spectrum was also altered by the supplementation of Cd2+, which has an ionic radius comparable to that of Ca2+. A single-pulse-induced S2/S1 difference spectrum revealed that bands that have been assigned to the vibrational modes for the YZ tyrosine and the histidine ligand for the Mn cluster were not induced in the K+-supplemented membranes, although the histidine band is likely to be preserved in the Ca2+-depleted membranes. The YZ band was considerably small in the double difference S2/S1 spectrum in the Ca2+-depleted and the cation-substituted membranes but distinctively present in the Sr2+- or Ca2+-replenished membranes. Furthermore, cation supplementation induced several new bands that disappeared following the Ca2+ replenishment. These results suggest that the proper organization of the hydrogen bond network within OEC for the water oxidation chemistry requires the Ca2+ ion and indicate that the role of Ca2+ is not purely structurally defined by the physical properties of the ion, such as valence and ionic radius. On the basis of these and other findings, we propose that Ca2+ is necessary for the formation of the hydrogen bond network that is involved in the reaction step of water oxidation.