Ion-dependent activation of AMP nucleosidase from Azotobacter vinelandii

The effect of divalent cations on the purified AMP nucleosidase (AMP phosphoribohydrolase, EC 3.2.2.4) from Azotobacter vinelandii was investigated. All alkaline earth metal-ATP complexes were essential activators of the enzyme, and free alkaline earths also activated the enzyme in an allosteric manner: the apparent Ka for ATP and nH values (Hill interaction coefficient) decreased from 0.45 to 0.05 mM, and from 4 to 2, respectively, with increase in Mg2+ concentration. Transition metal-ATP complex also activated AMP nucleosidase, but a potent activation of the enzyme was followed by a progressive decrease in activity as the concentrations of transition metals increase. The enzyme fully activated in the presence of Mg2+ was inhibited by the higher concentrations of transition metals with the identical I0.5 values when Mg2+ was present. These results suggest the presence of two classes of binding sites for divalent cations. One is the activating site for ATP-metal complex, which is suggested to be commonly occupied by alkaline earths and transition metals. The other sites are those for free metal binding, the sites for free alkaline earths and free transition metals are activating and inhibitory sites, respectively.