Characteristic changes of the S2/S1 difference FTIR spectrum induced by Ca2+ depletion and metal cation substitution in the photosynthetic oxygen-evolving complex.
Characteristic changes of the S2/S1 difference FTIR spectrum induced by Ca2+ depletion and metal cation substitution in the photosynthetic oxygen-evolving complex.
复制标题
光合放氧复合物中 Ca2 消耗和金属阳离子取代引起的 S2/S1 差值 FTIR 光谱的特征变化。
作者：
Y. Kimura;K. Hasegawa;T. Ono
使用傅里叶变换红外 (FTIR) 光谱检查了 Ca2+ 耗尽和其他金属阳离子取代对释氧复合物 (OEC) 蛋白质基质结构的影响及其在 S1 到 S2 转变时的相应变化。 Ca2+耗尽和进一步补充Li+、Na+、Mg2+、Ca2+或Sr2+并没有显着影响双差S2/S1谱中的典型振动特征，包括羧酸配体和酰胺I的对称[1365(+)/1404(-) cm(-1)]和不对称[1587(+)/1566(-) cm(-1)]拉伸模式和骨架多肽的II模式。另一方面，补充 K+、Rb+、Cs+ 或 Ba2+ 显着改变了 S2/S1 光谱，其中羧酸盐模式消失，酰胺 I 和 II 模式被改变。结果表明，离子半径大于 Ca2+ 的金属阳离子与 Ca2+ 位点的结合会引起 Mn 簇附近蛋白质基质的扰动，从而中断 Mn 簇氧化时伴随 S1 到 S2 转变的特征性结构和/或构象变化。添加 Cd2+ 也改变了光谱，Cd2+ 的离子半径与 Ca2+ 相当。单脉冲诱导的 S2/S1 差异谱表明，在补充 K+ 的膜中未诱导分配给 Y(Z) 酪氨酸和 Mn 簇的组氨酸配体的振动模式的带，尽管组氨酸带可能在 Ca2+ 耗尽的膜中保留。在 Ca2+ 耗尽和阳离子取代的膜中，Y(Z) 带在双差 S2/S1 光谱中相当小，但在 Sr2+- 或 Ca2+ 补充的膜中明显存在。此外，补充阳离子诱导了几个新条带，这些条带在补充 Ca2+ 后消失。这些结果表明，水氧化化学中 OEC 内氢键网络的正确组织需要 Ca2+ 离子，并表明 Ca2+ 的作用在结构上并不纯粹由离子的物理性质（例如化合价和离子半径）决定。基于这些和其他发现，我们提出 Ca2+ 对于水氧化反应步骤中涉及的氢键网络的形成是必需的。
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 Y(Z) 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 Y(Z) 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.