Kinetic hysteresis for fructose bisphosphatase: A change in substrate configuration specificity

Kinetic hysteresis for rabbit liver fructose bisphosphatase in the presence of Mg²⁺ (pH 7.6) is exhibited by the varied rates at which product formation is reduced on the addition of different inhibitors under cycling conditions. Two different states of the enzyme are detected: the initial resting state which binds α-, β- and keto analogs of fructose 1,6-bisphosphate; and the active cycling state which binds, and is inhibited by, only the α-analog. Both enzyme states, however, bind the allosteric modifier, AMP, and a product analog, (α+β)methyl-D-fructofuranoside 6-phosphate to the same extent so that the resulting inhibition is state independent. A relatively slow first-order transition (0.13 min^-1) characterizes the reversion of the active enzyme to its resting state. The implications of this phenomenon for regulating fructose bisphosphatase activity in vivo are discussed.