Kister J, Poyart C, Edelstein S J
Institut National de la Santé et de la Recherche Médicale, Hôpital de Bicêtre, Le Kremlin-Bicetre, France.
Biophys J. 1987 Oct;52(4):527-35. doi: 10.1016/S0006-3495(87)83242-3.
At low concentrations of chloride ions, and in the presence of nonsaturating concentrations of organophosphates, the oxygen equilibrium curves (OEC) for solutions of human adult hemoglobin exhibit a biphasic shape conveniently revealed by graphical analysis of the first derivative of the Hill equation with a characteristic form that we call "the double hump effect." This shape, observed for sub-saturating concentrations of organophosphates, stands in marked contrast to the simple lateral shifts of the OEC represented largely by scaling factors when pH or chloride are varied. In the case of protons or chloride, there is a self-buffering effect due to the presence of a large reservoir of proton or chloride binding sites not necessarily linked to oxygen, whereas such sites do not exist in the case of organophosphates. In addition, in the former case, we are dealing with curves measured at constant activity of the effector, while in the latter, at constant concentration. In the presence of saturating concentrations of inositol hexaphosphate (IHP), at low chloride concentration, the entire OEC is shifted to the right, including both its upper and lower asymptotes, indicating a decrease in the intrinsic oxygen affinities of both the T and R states. Theoretical considerations leading to a successful modeling of OEC obtained under nonsaturating and saturating concentrations of IHP required an expanded two-state allosteric model in which IHP-dependent variations in oxygen association constants for both the T and R conformations are taken into account.
在低浓度氯离子以及非饱和浓度有机磷酸盐存在的情况下，成人血红蛋白溶液的氧平衡曲线（OEC）呈现双相形状，通过对希尔方程一阶导数进行图形分析可方便地揭示这一形状，其具有一种我们称之为“双峰效应”的特征形式。这种在有机磷酸盐亚饱和浓度下观察到的形状，与当pH值或氯离子变化时主要由比例因子表示的OEC简单横向移动形成显著对比。在质子或氯离子的情况下，由于存在大量不一定与氧相连的质子或氯离子结合位点，会产生自缓冲效应，而在有机磷酸盐的情况下不存在此类位点。此外，在前一种情况下，我们处理的是在效应物恒定活性下测量的曲线，而在后一种情况下，是在恒定浓度下测量的曲线。在低氯离子浓度下，当存在饱和浓度的肌醇六磷酸（IHP）时，整个OEC向右移动，包括其上、下渐近线，这表明T态和R态的固有氧亲和力均降低。要成功模拟在非饱和和饱和浓度IHP下获得的OEC，理论上需要一个扩展的二态别构模型，其中要考虑IHP对T态和R态构象的氧结合常数的依赖性变化。
