Origin of cooperativity in the activation of fructose-1,6-bisphosphatase by Mg2+

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 Ser(45) → His substantially fills the central cavity of pFBPase, and the triple mutation Ser(45) → His, Thr(46) → Arg, and Leu(186) → 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.

Crystal structures of human muscle fructose-1,6-bisphosphatase: novel quaternary states, enhanced AMP affinity, and allosteric signal transmission pathway

Fructose-1,6-bisphosphatase, a key enzyme in gluconeogenesis, is subject to metabolic regulation. The human muscle isozyme is significantly more sensitive towards the allosteric inhibitor, AMP, than the liver isoform. Here we report crystal structures and kinetic studies for wild-type human muscle Fru-1,6-Pase, the AMP-bound (1.6 Å), and product-bound complexes of the Q32R mutant, which was firstly introduced by an error in the cloning. Our high-resolution structure reveals for the first time that the higher sensitivity of the muscle isozyme towards AMP originates from an additional water-mediated, H-bonded network established between AMP and the binding pocket. Also present in our structures are a metaphosphate molecule, alternate conformations of Glu97 coordinating Mg²⁺, and possible metal migration during catalysis. Although the individual subunit is similar to previously reported Fru-1,6-Pase structures, the tetrameric assembly of all these structures deviates from the canonical R- or T-states, representing novel tetrameric assemblies. Intriguingly, the concentration of AMP required for 50% inhibition of the Q32R mutant is increased 19-fold, and the cooperativity of both AMP and Mg²⁺ is abolished or decreased. These structures demonstrate the Q32R mutation affects the conformations of both N-terminal residues and the dynamic loop 52–72. Also importantly, structural comparison indicates that this mutation in helix α2 is detrimental to the R-to-T conversion as evidenced by the absence of quaternary structural changes upon AMP binding, providing direct evidence for the critical role of helix α2 in the allosteric signal transduction.

Mechanism of displacement of a catalytically essential loop from the active site of mammalian fructose-1,6-bisphosphatase

AMP triggers a 15° subunit-pair rotation in fructose-1,6-bisphosphatase (FBPase) from its active R state to its inactive T state. During this transition, a catalytically essential loop (residues 50-72) leaves its active (engaged) conformation. Here, the structures of Ile(10) → Asp FBPase and molecular dynamic simulations reveal factors responsible for loop displacement. The AMP/Mg(2+) and AMP/Zn(2+) complexes of Asp(10) FBPase are in intermediate quaternary conformations (completing 12° of the subunit-pair rotation), but the complex with Zn(2+) provides the first instance of an engaged loop in a near-T quaternary state. The 12° subunit-pair rotation generates close contacts involving the hinges (residues 50-57) and hairpin turns (residues 58-72) of the engaged loops. Additional subunit-pair rotation toward the T state would make such contacts unfavorable, presumably causing displacement of the loop. Targeted molecular dynamics simulations reveal no steric barriers to subunit-pair rotations of up to 14° followed by the displacement of the loop from the active site. Principal component analysis reveals high-amplitude motions that exacerbate steric clashes of engaged loops in the near-T state. The results of the simulations and crystal structures are in agreement: subunit-pair rotations just short of the canonical T state coupled with high-amplitude modes sterically displace the dynamic loop from the active site.

cpFBPaseII, a novel redox-independent chloroplastic isoform of fructose-1,6-bisphosphatase

A full-length FBPase cDNA has been isolated from Fragaria x ananassa (strawberry) corresponding to a novel putative chloroplastic FBPase but lacking the regulatory redox domain, a characteristic of the plastidial isoenzyme (cpFBPaseI). Another outstanding feature of this novel isoform, called cpFBPaseII, is the absence of the canonical active site. Enzymatic assays with cpFBPaseII evidenced clear Mg(2+)-dependent FBPase activity and a K(m) for fructose-1,6-bisphosphate (FBP) of 1.3 mM. Immunolocalization experiments and chloroplast isolation confirmed that the new isoenzyme is located in the stroma. Nevertheless, unlike cpFBPaseI, which is redox activated, cpFBPaseII did not increase its activity in the presence of either DTT or thioredoxin f (TRX f) and is resistant to H(2)O(2) inactivation. Additionally, the novel isoform was able to complement the growth deficiency of the yeast FBP1 deletion fed with a non-fermentable carbon source. Furthermore, orthologues are restricted to land plants, suggesting that cpFBPaseII is a novel and an intriguing chloroplastic FBPase that emerged late in the evolution of photosynthetic organisms, possibly because of a pressing need of land plants.

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 degrees rotation of the T-state AMP complex) with active site loops in the disengaged conformation. A three-state thermodynamic model in which Fru-2,6-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-P(2) at the active site displace engaged dynamic loops by distinct mechanisms, resulting in similar quaternary end-states. Conceivably, Type I FBPases from all eukaryotes may undergo similar global conformational changes in response to Fru-2,6-P(2) ligation.