Fructose 1,6-bisphosphate: isomeric composition, kinetics, and substrate specificity for the aldolases

13C NMR shows fructose 6-phosphate and fructose 1,6-bisphosphate to contain respectively 4.1 and 2.0% keto isomer at room temperature. The lower value for fructose 1,6-bisphosphate can be attributed to the electron-withdrawing effect of the C-1 phosphate. Measurements of the ring-opening rates of the alpha and beta anomers of fructose, 1,6-bisphosphate by an NMR line-broadening technique show them to be about 8 and 35 S-1, respectively, at pH 7.2, and 25degreesC. The value for the predominant beta anomer is threefold greater than the turnover rate of muscle aldolase so that, if the kinetic properties of the keto form were favorable, the reaction could proceed entirely through the keto form in solution. The kinetic properties of a fructose 1,6-bisphosphate(keto) analogue, 5-deoxyfructose, 1,6-bisphosphate, in the muscle aldolase reaction are more favorable (Vmax = 2.6, Km = 0.11 X 10(-6) M) than those of fructose 1,6-bisphosphate total (Vmax = 1, Km = 2.3 X 10(-6)M), giving a value of Vmax/Km that is 56 times greater for the 5-deoxy analogue. At the 2.0% concentration of the keto form this is sufficient to account for the steady-state rate and requires that the beta form, present at 40 times greater concentration, contributes little to the cleavage rate. With yeast aldolase the cleavage rate can be explained by the rapid spontaneous ring opening and reaction of the keto form with the enzyme. In view of the high rate of ring opening and the excellent properties of the keto form, previous rapid kinetic studies favoring action of cyclic forms may require reevaluation.