McPartland R P, Weinfeld H
J Biol Chem. 1979 Nov 25;254(22):11394-8.
In all previous kinetics studies of calf liver CTP synthetase, simple Michaelis-Menten hyperbolic plots were obtained. In this study it was shown that calf liver CTP synthetase could generate sigmoidal kinetic plots as a function of the substrate UTP when in the presence of the product of the reaction, CTP. The Hill number was estimated to be 2.8. The enzyme did not generate sigmoidal plots as a function of the other substrates (L-glutamine and ATP) either in the presence or absence of CTP. Thus, CTP apparently induced changes in the liver enzyme which altered the binding of UTP to the enzyme by acting at a site distinct from the UTP binding site (allosteric site). This concept was further strengthened by the fact that 3-deazaUTP, a known competitive inhibitor of the liver enzyme, did not induce sigmoidal kinetic plots. It was also shown that CTP had no effect upon the dimerization of the enzyme, thus ruling out monomer to dimer transitions as a potential mechanism for the observed sigmoidal kinetics.
在之前所有关于小牛肝脏CTP合成酶的动力学研究中，均得到了简单的米氏双曲线图。在本研究中发现，当存在反应产物CTP时，小牛肝脏CTP合成酶作为底物UTP的函数可生成S形动力学图。希尔系数估计为2.8。无论有无CTP，该酶作为其他底物（L-谷氨酰胺和ATP）的函数均未生成S形图。因此，CTP显然诱导了肝脏酶的变化，通过作用于与UTP结合位点不同的位点（别构位点）改变了UTP与酶的结合。已知的肝脏酶竞争性抑制剂3-脱氮UTP不会诱导S形动力学图，这一事实进一步强化了这一概念。研究还表明，CTP对该酶的二聚化没有影响，因此排除了单体到二聚体转变作为观察到的S形动力学的潜在机制。
