Kister J, Poyart C, Edelstein S J
J Biol Chem. 1987 Sep 5;262(25):12085-91.
Oxygen binding curves (OEC) for red cell suspensions have a biphasic shape and reduced n50 values when the concentration of 2,3-diphosphoglycerate (DPG) is lowered by aging or experimental procedures. The mechanism for the abnormal shape of the OEC has been related to variations in the activity of free DPG. DPG binds to tetrameric Hb at a single site, and in red cells its normal concentration is equivalent to that of tetrameric Hb. This equivalence renders the oxygen affinity of Hb and the shape of the OEC very sensitive to small changes in the activity of DPG. The OEC for stripped Hb solutions in the presence of nonsaturating concentrations of DPG also exhibit a biphasic shape but with much larger changes in the n values than observed for red cells. Upon addition of chloride, a known competitor of DPG binding to Hb, the shape of the OEC becomes similar to that of red cell suspensions with the same DPG/Hb ratio. Studies on Hb solutions in the presence of varying concentrations of DPG, but without chloride, have revealed that the cofactor shifts the entire OEC to the right, including both its upper and lower asymptotes. This finding indicates that DPG lowers the intrinsic oxygen affinity for both the T and R states. Theoretical considerations leading to a successful modeling of OEC obtained under varying conditions of DPG and chloride require an expanded two-state allosteric model in which allowance is made for DPG-dependent variations in the dissociation constants of oxygen for both the T and R conformations.
当通过老化或实验程序降低2,3 - 二磷酸甘油酸（DPG）的浓度时，红细胞悬液的氧结合曲线（OEC）呈双相形状且n50值降低。OEC异常形状的机制与游离DPG活性的变化有关。DPG在单个位点与四聚体血红蛋白结合，在红细胞中其正常浓度与四聚体血红蛋白的浓度相当。这种等效性使得血红蛋白的氧亲和力和OEC的形状对DPG活性的微小变化非常敏感。在存在非饱和浓度DPG的情况下，脱铁血红蛋白溶液的OEC也呈现双相形状，但n值的变化比红细胞中观察到的要大得多。加入氯离子（一种已知的DPG与血红蛋白结合的竞争者）后，OEC的形状变得与具有相同DPG/血红蛋白比率的红细胞悬液相似。对存在不同浓度DPG但无氯离子的血红蛋白溶液的研究表明，辅因子将整个OEC向右移动，包括其上、下渐近线。这一发现表明DPG降低了T态和R态的固有氧亲和力。要成功模拟在不同DPG和氯离子条件下获得的OEC，理论上需要一个扩展的二态变构模型，其中要考虑到DPG对T态和R态氧解离常数的依赖性变化。
