Tam L T, Manning D, Cox D J, Riggs A F
Department of Zoology, University of Texas, Austin 78712.
J Biol Chem. 1993 Dec 25;268(36):26972-7.
Hemolysates from the adult bullfrog, Rana catesbeiana, show an unusually high degree of cooperativity of oxygen binding with Hill coefficients greater than 4. The principal components of the tetrameric hemoglobin, B and C, do not show this high cooperativity when isolated, but it reappears when the components are mixed. Sedimentation velocity measurements show that the unusual behavior results from the mixed association of components B and C to form complexes larger than tetramers. Computer simulation of the sedimentation behavior of mixtures of deoxygenated B and C components shows that the gradient profiles can be satisfactorily described in terms of an equilibrium between the B and C tetramers and a BC2 trimer. The simplest model consistent with the results is the mixed association: B + C<-->BC and BC + C<-->BC2, with the second binding constant being higher than the first, indicating significant cooperativity. The extent of association is highest at low pH and low temperature. The dissociation of the B.C complex with low oxygen affinity to higher affinity B and C molecules during oxygenation results in greatly increased cooperativity of oxygen binding with higher Hill coefficients than possessed by either component alone in equilibria measured between 5 and 25 degrees C and between pH 6 and 8.
成年牛蛙（牛蛙）的溶血产物显示出异常高的氧结合协同性，希尔系数大于4。四聚体血红蛋白的主要成分B和C在分离时不显示这种高协同性，但当这些成分混合时又会重新出现。沉降速度测量表明，这种异常行为是由于成分B和C的混合缔合形成了大于四聚体的复合物。对脱氧B和C成分混合物的沉降行为进行计算机模拟表明，梯度分布可以用B和C四聚体与BC2三聚体之间的平衡来令人满意地描述。与结果一致的最简单模型是混合缔合：B + C<-->BC和BC + C<-->BC2，第二个结合常数高于第一个，表明存在显著的协同性。缔合程度在低pH和低温下最高。在5至25摄氏度和pH 6至8之间测量的平衡中，在氧合过程中具有低氧亲和力的B.C复合物解离为具有高亲和力的B和C分子，导致氧结合的协同性大大增加，希尔系数高于单独任何一种成分。
