Multiple Molecular Forms of Uridine Diphosphate Glucose Pyrophosphorylase from Salmonella typhimurium

UDP-glucose pyrophosphorylase (EC 2.7.7.9) of a wild type Salmonella typhimurium strain was eluted as four distinct bands (Enzymes I, II, IIIa, and IIIb) from a column of DEAE-cellulose.In contrast, the enzyme from mutants with extensive deletions in the rjb region of the chromosome was eluted as a single band (Enzyme IV).Enzyme I had a lower K, and higher V,m,, for dTDP-glucose and dTTP than for UDP-glucose and UTP, and was coded for by a gene, rjbA, which is different from the gene (or genes) coding for UDP-glucose pyrophosphorylase (or pyrophosphorylases).Enzyme I thus corresponds to the dTDP-glucose pyrophosphorylase (EC 2.7.7.24), which happens to have a broad substrate specificity.Examination of substrate specificity of enzymes in the four remaining peaks indicated, however, that these peaks do not represent cross-reactions by other nucleotide-sugar pyrophosphorylases, and that they are, in fact, UDP-glucose pyrophosphorylase (or phosphorylases).Enzyme IV showed a significantly different K, for UDPglucose from Enzymes II and IIIa, and Enzyme IIIb showed a kinetic behavior that widely deviated from the Michaelis-Menten equation.UDP-glucose acted as an apparently competitive inhibitor of UTP for Enzymes II, IIIa, and IV, but inhibition of Enzyme IIIb was more or less noncompetitive.The various enzymes were also significantly different in their pH optima, stability against heat inactivation, etc.These results establish that there are real differences in catalytic properties among the multiple forms of UDP-glucose pyrophosphorylase found in S. typhimurium.