Presteady-State Kinetic Evidence for a Ring-Opening Activity in Fructose-1,6-(bis)phosphate Aldolase

Fructose 1,6-bisphosphate aldolase, a glycolytic enzyme, catalyzes the cleavage of fructose 1,6-bisphosphate, resulting in two three-carbon products. The reaction of the class I enzymes, which utilize a Schiff-base intermediate, requires that the hexose be in the open-chain form. This form comprises only 1?2% of the sugar at equilibrium. The chemical form of the substrate that binds to aldolase and begins the catalytic cycle has not been unequivocally demonstrated. Transient-state kinetics in single-turnover experiments of fructose 1,6-bisphosphate with aldolase in excess reveals the rates of the intermediate steps in the cleavage reaction, including those from initial binding to Schiff-base formation. The rate of hexose Schiff-base formation was faster than the uncatalyzed rate for ring-opening of either the α- or β-furanose at 4 °C. In addition, approach-to-equilibrium experiments reveal that aldolase binds and reacts first with 70% of fructose-1,6-bisphosphate in a fast reaction, consistent with the amount of β-anomer in solution, and with the remaining 30%, presumably the α-anomer, in a slow reaction. These results indicate that aldolase must catalyze the ring-opening step and that there may be a previously unrecognized second active site on the enzyme for catalyzing this reaction.