Central Cavity of Fructose-1,6-bisphosphatase and the Evolution of AMP/Fructose 2,6-bisphosphate Synergism in Eukaryotic Organisms

The effects of AMP and fructose 2,6-bisphosphate (Fru-2,6-P₂) on porcine fructose-1,6-bisphosphatase (pFBPase) and Escherichia coli FBPase (eFBPase) differ in three respects. AMP/Fru-2,6-P₂ synergism in pFBPase is absent in eFBPase. Fru-2,6-P₂ induces a 13° subunit pair rotation in pFBPase but no rotation in eFBPase. Hydrophilic side chains in eFBPase occupy what otherwise would be a central aqueous cavity observed in pFBPase. Explored here is the linkage of AMP/Fru-2,6-P₂ synergism to the central cavity and the evolution of synergism in FBPases. The single mutation Ser⁴⁵ → His substantially fills the central cavity of pFBPase, and the triple mutation Ser⁴⁵ → His, Thr⁴⁶ → Arg, and Leu¹⁸⁶ → Tyr replaces porcine with E. coli type side chains. Both single and triple mutations significantly reduce synergism while retaining other wild-type kinetic properties. Similar to the effect of Fru-2,6-P₂ on eFBPase, the triple mutant of pFBPase with bound Fru-2,6-P₂ exhibits only a 2° subunit pair rotation as opposed to the 13° rotation exhibited by the Fru-2,6-P₂ complex of wild-type pFBPase. The side chain at position 45 is small in all available eukaryotic FBPases but large and hydrophilic in bacterial FBPases, similar to eFBPase. Sequence information indicates the likelihood of synergism in the FBPase from Leptospira interrogans (lFBPase), and indeed recombinant lFBPase exhibits AMP/Fru-2,6-P₂ synergism. Unexpectedly, however, AMP also enhances Fru-6-P binding to lFBPase. Taken together, these observations suggest the evolution of AMP/Fru-2,6-P₂ synergism in eukaryotic FBPases from an ancestral FBPase having a central aqueous cavity and exhibiting synergistic feedback inhibition by AMP and Fru-6-P.