Haire R N, Hedlund B E
Proc Natl Acad Sci U S A. 1977 Oct;74(10):4135-8. doi: 10.1073/pnas.74.10.4135.
Oxygen isotherms of human hemoglobin measured in distilled water and in solutions of sodium chloride in the concentration range from 0.02 to 3.0 M indicate that the oxygen affinity decreases up to about 1 M salt and then begins to increase. The isotherms obtained in the range from 0.02 to 0.6 M sodium chloride, at 37 degrees and pH 7.4, have been analyzed in terms of changes in Gibbs free energy of heme ligation, resulting from the differential interaction between the chloride ion and the two forms of hemoglobin. The maximal theoretical change in Gibbs free energy that chloride ion can exert on the oxygen binding of hemoglobin amounts to 4.9 +/- 0.2 kcal/mol (21 +/- 0.8 kJ/mol) of hemoglobin tetramer. A plot of the logarithm of oxygen concentration at half saturation versus the logarithm of the chloride concentration has a slope of 0.40, suggesting 1.6 apparent chloride sites per hemoglobin tetramer. Because the interaction between chloride and hemoglobin is dependent on pH, the apparent thermodynamic linkage between chloride and oxygen binding will also include the salt dependence of the Bohr effect at pH 7.4. The fractional change in Gibbs free energy, measured as a function of the chloride concentration, can be approximated by the binding isotherm between a protein and a ligand, using an association constant of 11 M(-1). Thus, if the number of oxygen-linked chloride sites is more than one per hemoglobin tetramer, these sites must be considered independent.
在蒸馏水以及浓度范围为0.02至3.0 M的氯化钠溶液中测得的人血红蛋白的氧等温线表明，氧亲和力在盐浓度达到约1 M之前会降低，然后开始升高。在37摄氏度和pH值为7.4的条件下，在0.02至0.6 M氯化钠范围内获得的等温线，已根据氯离子与两种形式的血红蛋白之间的差异相互作用所导致的血红素连接吉布斯自由能的变化进行了分析。氯离子能够对血红蛋白的氧结合施加的最大理论吉布斯自由能变化量为每摩尔血红蛋白四聚体4.9±0.2千卡/摩尔（21±0.8千焦/摩尔）。半饱和时氧浓度的对数与氯离子浓度的对数的关系图斜率为0.40，这表明每个血红蛋白四聚体有1.6个表观氯离子结合位点。由于氯离子与血红蛋白之间的相互作用取决于pH值，因此在pH值为7.4时，氯离子与氧结合之间明显的热力学联系也将包括波尔效应的盐依赖性。以氯离子浓度为函数测量的吉布斯自由能分数变化，可以使用11 M⁻¹的缔合常数，通过蛋白质与配体之间的结合等温线来近似。因此，如果每个血红蛋白四聚体的氧连接氯离子位点数量超过一个，那么这些位点必须被视为相互独立的。
