Binding of fructose-6-P and Pi to rabbit liver fructose bisphosphatase has been analyzed in terms of four negatively cooperative binding sites per enzyme tetramer. The association of fructose-6-P occurs in the absence of divalent metal ion, although the extent of binding is increased in the order Mg2+ < Zn2+ < Mn2+. The binding of Pi shows an absolute requirement for divalent metal ion with Mn2+ being more effective than Mg2+. The interaction of the enzyme with the substrate analog, (α + β) methyl-d-fructofuranoside-1,6-P2 in the presence of Mn2+ closely resembles that found for fructose-1,6-P2 in the absence of Mn2+, although the measured constants are on average an order of magnitude smaller. Combination experiments with the three ligands show that the binding follows an identical ordered sequence, i.e., the tighter sites are initially occupied regardless of the ligand's identity. The binding of Pi or fructose-6-P is not altered by the presence of the other. Comparison of binding constant with Ki values obtained from steady-state assays permits identification of the catalytic sites expressed in the latter. The association of Mn2+ at the catalytic site can be induced by fructose-6-P or the substrate analog suggesting that a 1-phosphoryl group enhances but is not necessary for Mn2+ binding at this site. The binding of AMP is decreased in the presence of substrate analog relative to fructose-1,6-P2, suggesting that the 2-hydroxyl serves as a "molecular signal." From the single and combined binding experiments, a calculation of the equilibrium constant for the overall hydrolysis reaction on the enzyme surface in the presence of Mn2+ has been carried out and an estimate made for the Mg2+ case.
All Science Journal Classification (ASJC) codes
- Molecular Biology