Lue P F, Aitken D M, Kaplan J G
Biochimie. 1976;58(1-2):19-25. doi: 10.1016/s0300-9084(76)80352-5.
Kinetic studies of the carbamyl phosphate synthetase activity (CPSase) of bakers' yeast revealed an absolute requirement for K+ ions ; KM values for two of the substrates, glutamine and bicarbonate, were found to be 5 X 10(-4) M and 3 X 10(-3) M respectively. CPSase activity of the purified enzyme aggregate (M.W. 800,000) was extremely sensitive to UTP with a Ki of 2.4 X 10(-4) M. The purine nucleotide intermediate, XMP, was a strong activator of CPSase, acting at a site different from the regulatory site at which UTP binds ; XMP activation diminished at high concentrations of the substrate Mg-ATP. Studies of the reaction mechanism of CPSase revealed that it involved the sequential addition of the substrates bicarbonate and Mg-ATP, liberation of ADP, addition of glutamine, binding of ATP and then release of ADP and the product carbamyl phosphate. Studies of the reaction mechanism of the aspartate transcarbamylase (ATCase) of the aggregate yielded data which were not compatible with any of the usual models ; whichever reaction mechanism is ultivately found to fit the data, it will probably prove applicable both to the ATCase of the aggregate and to the disaggregated ATCase subunit (MW 138,000).
对面包酵母的氨甲酰磷酸合成酶（CPSase）活性进行的动力学研究表明，该酶绝对需要K+离子；发现两种底物谷氨酰胺和碳酸氢盐的KM值分别为5×10（-4）M和3×10（-3）M。纯化的酶聚集体（分子量800,000）的CPSase活性对UTP极为敏感，其抑制常数（Ki）为2.4×10（-4）M。嘌呤核苷酸中间体XMP是CPSase的强效激活剂，作用于与UTP结合的调节位点不同的位点；在高浓度底物Mg-ATP存在下，XMP的激活作用减弱。对CPSase反应机制的研究表明，其反应过程包括依次添加底物碳酸氢盐和Mg-ATP、释放ADP、添加谷氨酰胺、结合ATP，然后释放ADP和产物氨甲酰磷酸。对该聚集体的天冬氨酸转氨甲酰酶（ATCase）反应机制的研究得出的数据与任何常见模型均不相符；无论最终发现哪种反应机制符合这些数据，它可能都适用于该聚集体的ATCase和已解离的ATCase亚基（分子量138,000）。
