Suter P, Rosenbusch J P
J Biol Chem. 1977 Nov 25;252(22):8136-41.
The allosteric effectors of aspartate transcarbamoylase from Escherichia coli, CTP and ATP, associate with both the regulatory and the catalytic moieties of the enzyme. Studies with isolated, active subunits yield one binding site per regulatory dimer and one per catalytic trimer. Investigations of effector association with hybrid enzymes, containing either the three regulatory dimers or the two catalytic trimers in inactivated forms, indicate that the data obtained with isolated subunits can be used to analyze the binding patterns of these ligands to the native hexamer. Thus, the nonlinear Scatchard plots, characteristic of the binding of CTP and ATP to the native enzyme, can be interpreted in terms of three effector molecules associating with the regulatory subunits, and two binding to the catalytic moiety of the enzyme. Results with native protein in the presence of saturating concentrations of active site ligands support these assignments. The differences between the binding isotherms of CTP and ATP to the enzyme are due to their different affinities to the two types of subunits. The apparent half-of-the-site saturation of the regulatory moiety of aspartate transcarbamoylase supports the concept that this protein has a tendency to exist in an asymmetric state.
来自大肠杆菌的天冬氨酸转氨甲酰酶的别构效应物CTP和ATP，与该酶的调节部分和催化部分都有关联。对分离出的活性亚基的研究表明，每个调节二聚体有一个结合位点，每个催化三聚体有一个结合位点。对效应物与杂合酶结合的研究，这些杂合酶含有失活形式的三个调节二聚体或两个催化三聚体，结果表明，用分离出的亚基获得的数据可用于分析这些配体与天然六聚体的结合模式。因此，CTP和ATP与天然酶结合时典型的非线性Scatchard图，可以解释为三个效应分子与调节亚基结合，两个与酶的催化部分结合。在活性位点配体饱和浓度存在下对天然蛋白质的研究结果支持了这些归属。CTP和ATP与该酶结合等温线的差异，是由于它们对两种亚基的亲和力不同。天冬氨酸转氨甲酰酶调节部分明显的半位点饱和，支持了这种蛋白质倾向于以不对称状态存在的概念。
