Structures of mammalian and bacterial fructose-1,6-bisphosphatase reveal the basis for synergism in AMP/fructose 2,6-bisphosphate inhibition

Fructose-1,6-bisphosphatase (FBPase) operates at a control point in mammalian gluconeogenesis, being inhibited synergistically by fructose 2,6-bisphosphate (Fru-2,6- P-2) and AMP. AMP and Fru-2,6-P-2 bind to allosteric and active sites, respectively, but the mechanism responsible for AMP/Fru-2,6-P-2 synergy is unclear. Demonstrated here for the first time is a global conformational change in porcine FBPase induced by Fru-2,6-P-2 in the absence of AMP. The Fru-2,6-P-2 complex exhibits a subunit pair rotation of 13 degrees from the R-state ( compared with the 15 rotation of the T-state AMP complex) with active site loops in the disengaged conformation. A three-state thermodynamic model in which Fru2,6-P-2 drives a conformational change to a T-like intermediate state can account for AMP/Fru-2,6-P-2 synergism in mammalian FBPases. AMP and Fru-2,6-P-2 are not synergistic inhibitors of the Type I FBPase from Escherichia coli, and consistent with that model, the complex of E. coli FBPase with Fru-2,6-P-2 remains in the R-state with dynamic loops in the engaged conformation. Evidently in porcine FBPase, the actions of AMP at the allosteric site and Fru-2,6- P2 at the active site displace engaged dynamic loops by distinct mechanisms, resulting in similar quaternary end-states. Conceivably, Type I FBPases from alleukaryotes may undergo similar global conformational changes in response to Fru-2,6-P-2 ligation.