Kinetics and equilibria of pyrimidine nucleoside monophosphate kinase from human erythrocytes

The common type of pyrimidine nucleoside monophosphate kinase (ATP:CMP phosphotransferase, EC 2.7.4.14), purified 50 000-fold from human erythrotes, reacted with a wide variety of nucleotides, but only ATP, dATP, UMP and CMP were good substrates. The optimum Mg2+ concentration, 2-3 mM, was generally independent of substrate concentration, of the nature of the substrate, and of the direction of the reaction. Kinetic studies indicated that a ternary complex was formed, that the substrates were bound at two unlike sites, and that the order of addition of substrates was random. Equilibrium constants were ATP + UMP 0.98, ATP + CMP 1.59, dATP + UMP 1.13, and ATP + AMP 1.20.