Pap S, Sjöberg B, Mortensen K
Department of Medical Biochemistry, University of Göteborg, Sweden.
Eur J Biochem. 1990 Jul 20;191(1):41-5. doi: 10.1111/j.1432-1033.1990.tb19091.x.
The dissociation of the tetrameric alpha 2-macroglobulin molecule into two half-molecular fragments, which occurs at pH less than 4.5, has been investigated using the small-angle neutron scattering method, and test of trypsin binding activity. Best fit with the relative forward scattering of neutrons is obtained for a model where the dissociation of the protein is driven by the uptake of H+ on altogether four acid-base groups, one per monomeric subunit of alpha 2-macroglobulin. These groups are not (or only slightly) accessible in the native tetramer, but become exposed to the solvent after dissociation of the protein. The H(+)-binding constant obtained for these groups, after dissociation of the protein, log K1 in the range 4.2-4.5, suggests that they are most probably carboxylate groups. From the about 10% increase in the radius of gyration, which occurs when lowering the pH from 4.5 to 2.0, we can conclude that the dissociation is associated with a change in structure of the protein. Tests of trypsin binding show that there is also an irreversible loss in trypsin binding activity, which is directly related to the fraction of dissociated protein. Thus, at pH less than 4.5, there is a transition of alpha 2-macroglobulin which results simultaneously in dissociation, disorganisation of the conformation of the subunits and loss in activity.
利用小角中子散射法和胰蛋白酶结合活性测试，研究了四聚体α2-巨球蛋白分子在pH小于4.5时解离为两个半分子片段的过程。对于一个模型，中子的相对前向散射得到了最佳拟合，在该模型中，蛋白质的解离是由总共四个酸碱基团摄取H+驱动的，α2-巨球蛋白的每个单体亚基有一个基团。这些基团在天然四聚体中不可接近（或仅略有可及性），但在蛋白质解离后会暴露于溶剂中。蛋白质解离后，这些基团的H(+)-结合常数log K1在4.2 - 4.5范围内，表明它们很可能是羧基。当pH从4.5降至2.0时，回转半径增加约10%，由此我们可以得出结论，解离与蛋白质结构的变化有关。胰蛋白酶结合测试表明，胰蛋白酶结合活性也存在不可逆损失，这与解离的蛋白质部分直接相关。因此，在pH小于4.5时，α2-巨球蛋白会发生转变，同时导致解离、亚基构象紊乱和活性丧失。
