Willemoës Martin, Larsen Sine
Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Universitetetsparken 5, Denmark.
Arch Biochem Biophys. 2003 May 1;413(1):17-22. doi: 10.1016/s0003-9861(03)00085-7.
Cytidine 5(')-triphosphate (CTP) synthase (EC 6.4.3.2) catalyzes the transfer of an amino group to the 4 position of uridine 5(')-triphosphate (UTP) to yield CTP. The reaction proceeds by activation of the base moiety of UTP by adenosine 5(')-triphosphate (ATP)-dependent phosphorylation. The activated intermediate reacts with NH(3) in the solution or is obtained by hydrolysis of glutamine. The Lactococcus lactis CTP synthase shows significant differences from the enzymes from Escherichia coli, yeast, and mammals. One is the apparent stability of the L. lactis CTP synthase tetramer in the absence of the nucleotides ATP and UTP. This condition causes the E. coli, yeast, and mammal enzymes to dissociate into dimers. However, the L. lactis CTP synthase shows substrate inhibition by NH(4)Cl that coincides with the range of NH(4)Cl concentrations that apparently dissociates tetrameric enzyme into dimers. Even though regular substrate inhibition was observed with NH(4)Cl when the ionic strength was held constant, a significant part of the inhibition could be shown to be due to the increase in ionic strength with increasing substrate concentration. Since the substrate inhibition by NH(4)Cl was relieved by increasing the equimolar ATP and UTP concentrations, it appeared that the substrate nucleotides stabilized the tetramer in a manner similar to that found in the absence of salt for other CTP synthases. In contrast to the suggested hydrophobic nature of the tetramer interactions in E. coli CTP synthase, the dissociation of the L. lactis CTP synthase tetramer in response to an increase in ionic strength suggests that the tetramer is stabilized by ionic interactions.
胞苷5'-三磷酸（CTP）合酶（EC 6.4.3.2）催化将一个氨基转移至尿苷5'-三磷酸（UTP）的4位，生成CTP。该反应通过三磷酸腺苷（ATP）依赖性磷酸化激活UTP的碱基部分来进行。活化的中间体与溶液中的NH₃反应，或通过谷氨酰胺水解获得。乳酸乳球菌CTP合酶与来自大肠杆菌、酵母和哺乳动物的酶存在显著差异。其一，在不存在核苷酸ATP和UTP的情况下，乳酸乳球菌CTP合酶四聚体具有明显的稳定性。这种情况会导致大肠杆菌、酵母和哺乳动物的酶解离成二聚体。然而，乳酸乳球菌CTP合酶表现出受氯化铵的底物抑制，这与氯化铵浓度范围一致，该浓度范围显然会使四聚体酶解离成二聚体。尽管当离子强度保持恒定时，观察到氯化铵对底物有常规抑制作用，但可以证明很大一部分抑制作用是由于底物浓度增加导致离子强度增加所致。由于通过增加等摩尔的ATP和UTP浓度可缓解氯化铵的底物抑制作用，因此似乎底物核苷酸以类似于其他CTP合酶在无盐情况下的方式稳定了四聚体。与大肠杆菌CTP合酶中四聚体相互作用的推测疏水性不同，乳酸乳球菌CTP合酶四聚体因离子强度增加而解离表明，四聚体是通过离子相互作用稳定的。
