Structure of the divalent cation.nucleotide complex at the active site of phosphoribosylpyrophosphate synthetase

When Mg2+ was used as the activating cation, the phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) of Salmonella typhimurium showed absolute specificity for the A(S) enantiomer of adenosine 5'-O-(1-thiotriphosphate), which gave a Km of 72 +/- 10 microM and a Vmax of 111 +/- 5 mumol/min/mg. The corresponding values for ATP were 46 +/- 3 microM and approximately 107 mumol/min/mg. Under the same conditions the B(R) isomer was a linearly competitive inhibitor (Ki = 54 +/- 11 microM) with respect to ATP. When Cd2+ replaced Mg2+, the two isomers reacted at comparable rates (Vmax (A)/Vmax (B) approximately equal to 0.8). This change in specificity suggests that the alpha-phosphate of ATP is liganded to a divalent cation during catalysis. Adenosine 5'-O-thiomonophosphate was 34-fold more effective as a product inhibitor when Cd2+ replaced Mg2+, while the effectiveness of AMP was not altered. This result suggests a divalent cation bridge between the enzyme and the alpha-phosphate of nucleotides. The results of these and previously published experiments enable us to propose a structure and stereochemical configuration for the divalent cation.ATP complex at the active site of phosphoribosylpyrophosphate synthetase.