Dugal B S, Louis B M
Enzyme. 1975;20(2):98-110. doi: 10.1159/000458925.
Kinetic methods have been used to determine whether Mg2+ and MgATP2- play an important role in regulating pigeon liver pyruvate carboxylase [pyruvate: CO2 ligase (ADP), EC 6.4.1.1]. Mg2+ not only forms a complex with ATP4- (MgATP2-) but is also required for the enzyme activation (and probably for the binding of MgATP2- to this enzyme). Contrary to the results of other investigators, the MgATP2- complex was not found to activate pigeon liver pyruvate carboxylase. We could not demonstrate homotropic cooperativity with MgATP2-. Excess Mg2+ induced allosteric stimulation of the enzymatic activity at different concentrations of MgATP2-. With different Mg2+ concentrations, changes also occurred in the apparent Km, Vmax and Rs values. Without excess of Mg2+ (heterotropic effector) only about 2% of the total enzymic activity available could be demonstrated in the presence of MgATP2-. It is concluded that Mg2+ exhibits a homotropic cooperative effect and is required for the activation of this enzyme. Mg2+ may bind either to a specific effector site, at the active site, or at the binding site for MgATP2- which is capable of functioning as an effector site and in this way facilitates the carboxylation of pyruvate.
动力学方法已被用于确定Mg2+和MgATP2-在调节鸽肝丙酮酸羧化酶[丙酮酸：二氧化碳连接酶（ADP），EC 6.4.1.1]中是否起重要作用。Mg2+不仅能与ATP4-形成复合物（MgATP2-），而且酶的激活也需要Mg2+（可能还需要MgATP2-与该酶结合）。与其他研究者的结果相反，未发现MgATP2-复合物能激活鸽肝丙酮酸羧化酶。我们未能证明MgATP2-存在同促协同效应。过量的Mg2+在不同浓度MgATP2-的情况下会引起酶活性的变构刺激。在不同的Mg2+浓度下表观Km、Vmax和Rs值也会发生变化。在没有过量Mg2+（异促效应物）的情况下，在MgATP2-存在时只能检测到约2%的总酶活性。得出的结论是，Mg2+表现出同促协同效应，且该酶的激活需要Mg2+。Mg2+可能与活性位点上的特定效应物位点结合，也可能与MgATP2-的结合位点结合，而该结合位点能够作为效应物位点发挥作用，从而促进丙酮酸的羧化反应。
