Nucleotide Specificity of Escherichia coli Succinic Thiokinase

Abstract

Abstract The nucleoside triphosphate specificity of Escherichia coli succinic thiokinase (succinate:coenzyme A ligase (ADP), EC 6.2.1.5) was studied. The results showed that ATP, GTP and ITP (in decreasing order of effectiveness) were good substrates, while UTP and CTP were relatively ineffective. A nucleoside triphosphate ⇄ Pi exchange reaction stimulated by succinate and CoA was observed with each nucleoside triphosphate tested. A nucleoside diphosphate kinase activity of the enzyme was observed and found to be different from the ubiquitous nucleoside diphosphate kinase (ATP:nucleoside diphosphate phosphotransferase, EC 2.7.4.6) in its substrate specificity and its marked tendency to be stimulated by succinyl-CoA. Thus, with ATP as the phosphoryl group donor, GDP and IDP were good acceptors, UDP was a relatively poor acceptor, and CDP exhibited almost negligible activity in this respect. The nucleoside diphosphate kinase activity of succinic thiokinase was inhibited at ATP concentrations in excess of 0.5 mm. A comparison of the NDP-kinase to thiokinase activity ratio has revealed that the NDP-kinase activity does not represent a minor catalytic capability of the enzyme.