Cytidine Triphosphate Synthetase of Escherichia coli B

Abstract

Cytidine triphosphate synthetase (EC 6.3.4.2, UTP: ammonia ligase (ADP)) of Escherichia coli B has been purified about 300-fold. Kinetics of the reactions with both glutamine (plus GTP) and ammonia as nitrogen donors have been studied. Dependence of the rate on the concentration of each substrate approximately follows the Michaelis equation when all other substrates are at near saturation. But when other substrates are at low concentrations, the kinetics are strongly sinusoidal. Hill plots produce straight lines under all conditions, the slopes of which (n) vary from 1 to 4 depending on concentrations of other substrates. The product CTP has been found to inhibit competitively with UTP and to activate the enzyme, probably by replacing GTP. A simple model consistent with these results assumes that the enzyme is a dimer in which all sites must be filled to permit catalytic activity. The activity of the enzyme appears to be regulated both by its substrates and by its product.