Kinetic differences between two interconvertible forms of fructose-1,6-bisphosphatase from Saccharomyces cerevisiae

Abstract

Fructose-1,6-bisphosphatase (FbPase) can be partially inactivated in vivo by addition of glucose to a yeast suspension. Some kinetic properties of the active and inactivated enzymes have been studied in freshly prepared extracts. The ratio of activities found when the enzyme is assayed with 2 mm Mn2+ and with 2 mm Mg2+ is around 0.6 for active FbPase and increases three times for inactivated FbPase. Both forms of FbPase are inhibited by AMP noncompetitively with fructose-l,6-bisphosphate (F1,6P2), however, active FbPase is less inhibited by AMP when Mn2+ is present, while the opposite behavior is shown by the inactivated enzyme. Fructose-2,6-bisphosphate (F2,6P2) is an inhibitor of both forms of FbPase the inhibition reached being dependent on the substrate concentration. The active form of FbPase is the most sensitive to F2,6P2 inhibition (Ki in the range of 5 nm). When assayed with Mg2+ both forms of the enzyme were less inhibited by F2,6P2 if AMP was present. In the presence of Mn2+ AMP reinforced slightly the inhibition by F2,6P2. Inactivated FbPase has been tested at the concentrations of AMP, F2,6P2, and F1,6P2 that are present in yeast treated with glucose. In these conditions the inhibition due to F2,6P2 is only about 30%.