Substrate inhibition of Lactococcus lactis cytidine 5′-triphosphate synthase by ammonium chloride is enhanced by salt-dependent tetramer dissociation

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 NH3 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 NH4Cl that coincides with the range of NH4Cl concentrations that apparently dissociates tetrameric enzyme into dimers. Even though regular substrate inhibition was observed with NH4Cl 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 NH4Cl 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.