Noguchi T, Inoue Y, Tang X S
Photosynthesis Research Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama, Japan.
Biochemistry. 1999 Aug 3;38(31):10187-95. doi: 10.1021/bi990631+.
Fourier transform infrared (FTIR) signals of a histidine side chain were identified in flash-induced S(2)/S(1) difference spectra of the oxygen-evolving complex (OEC) of photosystem II (PS II) using PS II membranes from globally (15)N-labeled spinach and PS II core complexes from Synechocystis cells in which both the imidazole nitrogens of histidine were selectively labeled with (15)N. A negative band at 1113-1114 cm(-1) was downshifted by 7 cm(-1) upon both global (15)N-labeling and selective [(15)N]His labeling, and assigned to the C-N stretching mode of the imidazole ring. This band was unaffected by H-D exchange in the PS II preparations. In addition, several peaks observed at 2500-2850 cm(-1) all downshifted upon global and selective (15)N-labeling. These were ascribed to Fermi resonance peaks on a hydrogen-bonding N-H stretching band of the histidine side chain. FTIR measurements of model compounds of the histidine side chain showed that the C-N stretching band around 1100 cm(-)(1) can be a useful IR marker of the protonation form of the imidazole ring. The band appeared with frequencies in the following order: Npi-protonated (>1100 cm(-1)) > imidazolate > imidazolium > Ntau-protonated (<1095 cm(-1)). The frequency shift upon N-deuteration was occurred in the following order: imidazolium (15-20 cm(-1)) > Ntau-protonated (5-10 cm(-1)) > Npi-protonated approximately imidazolate ( approximately 0 cm(-1)). On the basis of these findings together with the Fermi resonance peaks at >2500 cm(-1) as a marker of N-H hydrogen-bonding, we concluded that the histidine residue in the S(2)/S(1) spectrum is protonated at the Npi site and that this Npi-H is hydrogen bonded. This histidine side chain probably ligated the redox-active Mn ion at the Ntau site, and thus, oxidation of the Mn cluster upon S(2) formation perturbed the histidine vibrations, causing this histidine to appear in the S(2)/S(1) difference spectrum.
利用来自全球（15）N标记菠菜的光系统II（PS II）膜以及来自集胞藻细胞的PS II核心复合物（其中组氨酸的两个咪唑氮均被（15）N选择性标记），在光系统II（PS II）放氧复合物（OEC）的闪光诱导S(2)/S(1)差光谱中鉴定出组氨酸侧链的傅里叶变换红外（FTIR）信号。在1113 - 1114 cm(-1)处的一个负带在全局（15）N标记和选择性[(15)N]His标记后均下移了7 cm(-1)，并被指定为咪唑环的C - N伸缩模式。该带在PS II制剂中不受H - D交换的影响。此外，在2500 - 2850 cm(-1)处观察到的几个峰在全局和选择性（15）N标记后均下移。这些被归因于组氨酸侧链氢键N - H伸缩带上的费米共振峰。组氨酸侧链模型化合物的FTIR测量表明，1100 cm(-)(1)附近的C - N伸缩带可以作为咪唑环质子化形式的有用红外标记。该带出现在频率顺序如下：Nπ - 质子化（>1100 cm(-1)）>咪唑盐>咪唑鎓>Nτ - 质子化（<1095 cm(-1)）。N - 氘代后的频率位移顺序如下：咪唑鎓（15 - 20 cm(-1)）>Nτ - 质子化（5 - 10 cm(-1)）>Nπ - 质子化≈咪唑盐（≈0 cm(-1)）。基于这些发现以及>2500 cm(-1)处的费米共振峰作为N - H氢键的标记，我们得出结论，S(2)/S(1)光谱中的组氨酸残基在Nπ位点质子化且该Nπ - H形成氢键。该组氨酸侧链可能在Nτ位点与氧化还原活性锰离子配位，因此，在S(2)形成时锰簇的氧化扰乱了组氨酸振动，导致该组氨酸出现在S(2)/S(1)差光谱中。
