Interaction and Inhibitory Effect of Ammonium Cation in the Oxygen Evolving Center of Photosytem II

详细信息    查看全文

• 作者：Masaya Tsuno ; Hiroyuki Suzuki ; Toru Kondo ; Hiroyuki Mino ; Takumi Noguchi
• 刊名：Biochemistry
• 出版年：2011
• 出版时间：April 5, 2011
• 年：2011
• 卷：50
• 期：13
• 页码：2506-2514
• 全文大小：1023K
• 年卷期：v.50,no.13(April 5, 2011)
• ISSN：1520-4995

文摘

Photosynthetic O₂ evolution takes place at the Mn cluster in photosystem II (PSII) by oxidation of water. It has been proposed that ammonia, one of water analogues, functions as an inhibitor of O₂ evolution at alkaline pH. However, the detailed mechanism of inhibition has not been understood yet. In this study, we investigated the mechanism of ammonia inhibition by examining the NH₄Cl-induced inhibition of O₂ evolution in a wide pH range (pH 5.0鈭?.0) and by detecting the interaction site using Fourier transform infrared (FTIR) spectroscopy. In addition to intact PSII membranes from spinach, PSII membranes depleted of the PsbP and PsbQ extrinsic proteins were used as samples to avoid the effect of the release of these proteins by salt treatments. In both types of samples, oxygen evolution activity decreased by approximately 40% by addition of 100 mM NH₄Cl in the range of pH 5.0鈭?.0. The presence of inhibition at acidic pH without significant pH dependence strongly suggests that NH₄⁺ cation functions as a major inhibitor in the acidic pH region, where neutral NH₃ scarcely exists in the buffer. The NH₄Cl treatment at pH 6.5 and 5.5 induced prominent changes in the COO^{鈭?/sup> stretching regions in FTIR difference spectra upon the S₁ 鈫?S₂ transition measured at 283 K. The NH₄Cl concentration dependence of the amplitude of the spectral changes showed a good correlation with that of the inhibition of O₂ evolution. From this observation, it is proposed that NH₄+ cation interacts with carboxylate groups coupled to the Mn cluster as direct ligands or proton transfer mediators, causing inhibition of the O₂ evolving reaction.}