Tam L T, Riggs A F
J Biol Chem. 1984 Feb 25;259(4):2610-6.
The hemoglobin of the bullfrog, Rana catesbeiana, forms aggregates larger than tetramers in two ways. The first, which results from intermolecular disulfide bonds, can be prevented by treatment with iodoacetamide. The second way results from the association of the deoxygenated forms of the two major components, B and C, to form reversibly an aggregate which is believed to be a trimer, BC2. The sedimentation velocity data show that the stoichiometry of the aggregate cannot be 1:1. The electrophoretic pattern of the deoxygenated B/C mixture suggests that the association is not indefinite. No significant aggregation of the separate deoxygenated tetramers of the components nor of the oxygenated components or mixture occurs. Gel chromatography of the oxygenated forms of components B and C and of mixtures indicates that the B and C tetramers both form dimers upon dilution with a dissociation constant of 2-3 micron. The oxygen-binding data indicate that the B/C aggregate has a much lower oxygen affinity than its constituent tetramers. Dissociation of the low affinity B/C aggregate to higher affinity B and C tetramers with increasing oxygenation gives rise to enhanced cooperativity as measured by the Hill coefficient which is maximal near 75-80% oxygenation and is as high as 4.1 at a heme concentration of 15 mM.
牛蛙（Rana catesbeiana）的血红蛋白以两种方式形成大于四聚体的聚集体。第一种方式是由分子间二硫键导致的，用碘乙酰胺处理可防止这种情况发生。第二种方式是由两种主要成分B和C的脱氧形式缔合，可逆地形成一种聚集体，据信这是一种三聚体BC2。沉降速度数据表明，聚集体的化学计量比不可能是1:1。脱氧的B/C混合物的电泳图谱表明这种缔合并非无限制的。各成分单独的脱氧四聚体、氧化成分或混合物均未发生明显的聚集。对成分B和C的氧化形式以及混合物进行凝胶色谱分析表明，B和C四聚体在稀释时都会形成二聚体，解离常数为2 - 3微米。氧结合数据表明，B/C聚集体的氧亲和力远低于其组成的四聚体。随着氧合作用增强，低亲和力的B/C聚集体解离为高亲和力的B和C四聚体，这导致协同性增强，以希尔系数衡量，在氧合接近75 - 80%时达到最大值，在血红素浓度为15 mM时高达4.1。
