On the mechanism of alkaline and neutral fructose 1,6-diphosphatase: Inhibition by substrate analogs at neutral pH

Inhibitor characteristics at pH 7.6 are reported for rabbit liver fructose-1,6-diphosphatase (EC 3.1.3.11) with the substrate analogs, α- and β-methyl-d-fructo-furanoside-1,6-diphosphate and 2,5-anhydro-d-mannitol-1,6-diphosphate. The α-analog yields competitive inhibition at all concentrations examined, whereas the β-analogs result in competitive-noncompetitive inhibition with the latter mode becoming increasingly important at higher concentrations. The data obtained from the kinetics of inhibition are quantitatively in accord with increasing substrate inhibition of FDPase at neutral pH and strongly implicate as responsible a site specific for a β-configuration. Incubation of enzyme with Mn²⁺ and α-methyl-d-fructofuranoside-1,6-diphosphate at pH 7.6 apparently leads to increased levels of a more active form of the enzyme while similar incubation with the β-analog at pH 7.6 results in only a small change. Incubation at pH 9.4 is without effect. Enzyme with high activity in the neutral pH range and enzyme with an alkaline pH optimum respond similarly to the substrate analogs suggesting there are no gross differences in substrate specificity for the two forms.