Human hypoxanthine guanine phosphoribosyltransferase. The role of magnesium ion in a phosphoribosylpyrophosphate-utilizing enzyme.
The role of Mg ions in the hypoxanthine guanine phosphoribosyltransferase-catalyzed reaction have been studied using accurate values of proton and Mg stability constants of phosphoribosylpyrophosphate (P-Rib-PP) determined from pH titration data. The results obtained favor the conclusion that the dimagnesium salt of P-Rib-PP is the true substrate of the enzyme. The other species of P-Rib-PP do not appreciably affect the initial reaction rate. The inhibition of the hypoxanthine guanine phosphoribosyltransferase-catalyzed reaction observed at high MgCl2 concentration can be attributed to a competitive inhibition of Mg2+ with respect to the dimagnesium salt of P-Rib-PP, suggesting that these ionic species bind to the same enzyme form. At a fixed [P-Rib-PPtot], the concentration of its dimagnesium complex is a sigmoidal function of MgCl2 concentration, suggesting that caution must be employed in the interpretation of sigmoidal saturation curves for P-Rib-PP-utilizing enzymes when low and not constant concentrations of the divalent cation are used.
D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D007041:Hypoxanthine Phosphoribosyltransferase / Q000097:blood*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D010754:Phosphoribosyl Pyrophosphate
D010754:Phosphoribosyl Pyrophosphate; D007041:Hypoxanthine Phosphoribosyltransferase; D008274:Magnesium