[Interaction of metal ions with the carboxyl group of Asp-5 in the active site of uridine phosphorylase from Escherichia coli K-12].

The effects of La3+ ions on enzymatic activity and difference absorption spectra of native and fluorescein isothiocyanate (FITC) modified uridine phosphorylase from E. coli K-12 have been studied. Excess La3+, unlike Ag+, only slightly decreases the enzyme activity but provokes similar changes in the absorption spectra of both native and modified proteins. The Kd value for La3+ ions (0.2 mM) coincides with that obtained earlier for Ag+. La3+ ions (0.2 mM) have no effect on the rate of the enzyme inactivation by diethylpyrocarbonate or tetranitromethane but increases the rate of its inactivation by Woodward's reagent K (WRK). Binding of La3+ (Kd = 0.2 mM) markedly decreases the thermal stability of the enzyme which increases with a further rise in the La3+ concentration. The values of Kd (0.2 mM) as well as the difference spectra and specific interactions with WRK indicate that one of the ligands interacting with metal ions is the carboxyl group of the Asp-5 residue. According to X-ray analysis data, this residue is involved in the formation of the active center of uridine phosphorylase.