Ikeda-Saito M, Yonetani T, Gibson Q H
J Mol Biol. 1983 Aug 15;168(3):673-86. doi: 10.1016/s0022-2836(83)80308-8.
Oxygen equilibrium curves of the purified hemoglobin component I from the Atlantic bluefin tuna (Thunnus thynnus) have been determined between pH 6.5 and 8.75 at 25 degrees C, and for five temperatures between 10 and 30 degrees C at pH 7.0 and 7.5. From the equilibrium data oxygen equilibrium constants for four oxygenation steps, Ki (i = 1 to 4) were estimated. The number of the Bohr protons released on the ith oxygenation (delta Hi+), and the enthalpy and entropy changes at each oxygenation step (delta Hi and delta Si) were calculated. The Hill plot for oxygenation below neutral pH is biphasic; the top asymptote lies to the right of the bottom one and the linking limb between them exhibits a slope less than unity, exhibiting apparent negative co-operativity. The values of K1 and K2 exhibit little pH dependence, while those of K3 and K4 increase by two orders of magnitudes as the pH is changed from 6.5 to 8.75. In consequence, oxygen equilibrium above neutral pH exhibits a normal positive co-operativity. The oxygen equilibrium at lower temperature is biphasic as is that below neutral pH. The shape of the Hill plot is temperature-dependent. The affinity at low saturation decreases, and that at high saturation increases upon raising the temperature from 10 to 30 degrees C, resulting in crossing of the middle portion of the equilibrium curves at different temperatures. The delta H1 and delta H2 values are negative as are those of most other hemoglobins, but the delta H3 and delta H4 values are positive. Consideration of these results in a framework of the allosteric model extended to take account of differences between subunits has indicated that the deoxy quaternary structure is stabilized at low pH or low temperature, and that subunit heterogeneity gives rise to the biphasic oxygen equilibrium curve. An analysis of delta Hi+ suggests that the large number of the Bohr groups is responsible for the biased allosteric equilibrium towards the deoxy quaternary structure. The positive delta H3 and delta H4 values are also considered to arise from the large endothermic contribution of the Bohr protons released at the third and fourth steps of oxygenation.
已测定了大西洋蓝鳍金枪鱼（Thunnus thynnus）纯化血红蛋白成分I在25℃下pH值6.5至8.75之间以及在pH值7.0和7.5时10至30℃之间五个温度下的氧平衡曲线。根据平衡数据估算了四个氧合步骤的氧平衡常数Ki（i = 1至4）。计算了第i次氧合时释放的玻尔质子数量（δHi+）以及每个氧合步骤的焓变和熵变（δHi和δSi）。中性pH以下的氧合希尔图是双相的；顶部渐近线位于底部渐近线的右侧，它们之间的连接部分斜率小于1，表现出明显的负协同性。K1和K2的值对pH依赖性较小，而K3和K4的值在pH从6.5变为8.75时增加两个数量级。因此，中性pH以上的氧平衡表现出正常的正协同性。较低温度下的氧平衡与中性pH以下的情况一样是双相的。希尔图的形状取决于温度。从10℃升高到30℃时，低饱和度时的亲和力降低，高饱和度时的亲和力增加，导致平衡曲线中间部分在不同温度下交叉。δH1和δH2值与大多数其他血红蛋白一样为负，但δH3和δH4值为正。在扩展的变构模型框架内考虑这些结果以考虑亚基之间的差异表明，脱氧四级结构在低pH或低温下稳定，亚基异质性导致双相氧平衡曲线。对δHi+的分析表明，大量的玻尔基团导致变构平衡偏向脱氧四级结构。正的δH3和δH4值也被认为是由于在氧合第三步和第四步释放的玻尔质子的大量吸热贡献所致。
