Han K H, La Mar G N
J Mol Biol. 1986 Jun 5;189(3):541-52. doi: 10.1016/0022-2836(86)90323-2.
Proton nuclear magnetic resonance spectroscopy has been used to investigate the rates and mechanism of exchange with deuterium of the proximal histidyl imidazole labile ring proton in deoxy and oxy-hemoglobin A. The resolved signals for the two subunits indicate dynamic heterogeneity, with the exchange rate always faster in the alpha than the beta subunits, suggesting a lower dynamic stability for the alpha subunit. The activation energy for the exchange in both subunits (approximately 25 kcal; 1 cal = 4.184 J) indicates that exchange proceeds via an intermediate far from denaturation or global unfolding. The pH profiles for both hemoglobin states reflect the EX2 mechanism for both subunits. While the base catalysis expected for an iron-bound imidazole is observed in all cases, there are important differences in both rates and mechanisms between the subunits. In deoxy-hemoglobin, both base-catalyzed and water-assisted exchange contribute to the alpha subunit, but only the former to the beta subunit. For oxy-hemoglobin, the base-catalysis is retained for both subunits, but the slope is considerably less for the alpha relative to the beta subunit. Thus the two subunits in the two states of hemoglobin differ both in mechanisms and in the inherent dynamic stability reflected in any one mechanism. The relationships of the proximal histidyl ring NH exchange rates to previously characterized subsets of allosterically responsive protons in hemoglobin A is briefly discussed.
质子核磁共振波谱已被用于研究脱氧血红蛋白A和氧合血红蛋白A中近端组氨酸咪唑不稳定环质子与氘的交换速率及机制。两个亚基的分辨信号表明存在动态异质性，α亚基的交换速率总是比β亚基快，这表明α亚基的动态稳定性较低。两个亚基中交换的活化能（约25千卡；1卡 = 4.184焦耳）表明交换是通过一个远离变性或整体解折叠的中间体进行的。两种血红蛋白状态的pH曲线反映了两个亚基的EX2机制。虽然在所有情况下都观察到了与铁结合的咪唑预期的碱催化作用，但两个亚基在速率和机制上都存在重要差异。在脱氧血红蛋白中，碱催化交换和水辅助交换都对α亚基有贡献，但对β亚基只有前者有贡献。对于氧合血红蛋白，两个亚基都保留了碱催化作用，但α亚基相对于β亚基的斜率要小得多。因此，血红蛋白两种状态下的两个亚基在机制上以及在任何一种机制中所反映的固有动态稳定性上都存在差异。本文简要讨论了近端组氨酸环NH交换速率与先前表征的血红蛋白A变构响应性子集之间的关系。
