The C1-C2 interface residue lysine 50 of pig kidney fructose-1, 6-bisphosphatase has a crucial role in the cooperative signal transmission of the AMP inhibition

Fructose 1,6-bisphosphatase: getting the message across

Fructose 1,6-bisphosphatase (FBPase) is a key enzyme in gluconeogenesis. It is a potential drug target in the treatment of type II diabetes. The protein is also associated with a rare inherited metabolic disease and some cancer cells lack FBPase activity which promotes glycolysis facilitating the Warburg effect. Thus, there is interest in both inhibiting the enzyme (for diabetes treatment) and restoring its activity (in relevant cancers). The mammalian enzyme is tetrameric, competitively inhibited by Fructose 2,6-bisphosphate and negatively allosterically regulated by AMP. This allosteric regulation requires information transmission between the AMP binding site and the active site of the enzyme. A recent paper by Topaz et al. (Bioscience Reports (2019) 39, pii:BSR20180960) has added additional detail to our understanding of this information transmission process. Two residues in the AMP binding site (Lys¹¹² and Tyr¹¹³) were shown to be involved in initiating the message between the two sites. This tyrosine residue has recently be shown to be important with protein’s interaction with the antidiabetic drug metformin. A variant designed to increase metal ion affinity (M248D) resulted in a five-fold increase in enzymatic activity. Interestingly alterations of two residues at the subunit interfaces (Tyr¹⁶⁴ and Met¹⁷⁷) resulted in increased responsiveness to AMP. Overall, these findings may have implications in the design of novel FBPase inhibitors or activators.

Arsenite-induced changes in hepatic protein abundance in cynomolgus monkeys (Macaca fascicularis)

Arsenic is an environmental pollutant, and its liver toxicity has long been recognized. The effect of arsenic on liver protein expression was analyzed using a proteomic approach in monkeys. Monkeys were orally administered sodium arsenite (SA) for 28 days. As shown by 2D-PAGE in combination with MS, the expression levels of 16 proteins were quantitatively changed in SA-treated monkey livers compared to control-treated monkey livers. Specifically, the levels of two proteins, mortalin and tubulin beta chain, were increased, and 14 were decreased, including plastin-3, cystathionine-beta-synthase, selenium-binding protein 1, annexin A6, alpha-enolase, phosphoenolpyruvate carboxykinase-M, erlin-2, and arginase-1. In view of their functional roles, differential expression of these proteins may contribute to arsenic-induced liver toxicity, including cell death and carcinogenesis. Among the 16 identified proteins, four were selected for validation by Western blot and immunohistochemistry. Additional Western blot analyses indicated arsenic-induced dysregulation of oxidative stress related, genotoxicity-related, and glucose metabolism related proteins in livers from SA-treated animals. Many changes in the abundance of toxicity-related proteins were also demonstrated in SA-treated human hepatoma cells. These data on the arsenic-induced regulation of proteins with critical roles may help elucidate the specific mechanisms underlying arsenic-induced liver toxicity.

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.

Nuclear accumulation of fructose 1,6-bisphosphatase is impaired in diabetic rat liver

Using a streptozotocin-induced type 1 diabetic rat model, we analyzed and separated the effects of hyperglycemia and hyperinsulinemia over the in vivo expression and subcellular localization of hepatic fructose 1,6-bisphosphatase (FBPase) in the multicellular context of the liver. Our data showed that FBPase subcellular localization was modulated by the nutritional state in normal but not in diabetic rats. By contrast, the liver zonation was not affected in any condition. In healthy starved rats, FBPase was localized in the cytoplasm of hepatocytes, whereas in healthy re-fed rats it was concentrated in the nucleus and the cell periphery. Interestingly, despite the hyperglycemia, FBPase was unable to accumulate in the nucleus in hepatocytes from streptozotocin-induced diabetic rats, suggesting that insulin is a critical in vivo modulator. This idea was confirmed by exogenous insulin supplementation to diabetic rats, where insulin was able to induce the rapid accumulation of FBPase within the hepatocyte nucleus. Besides, hepatic FBPase was found phosphorylated only in the cytoplasm, suggesting that the phosphorylation state is involved in the nuclear translocation. In conclusion, insulin and not hyperglycemia plays a crucial role in the nuclear accumulation of FBPase in vivo and may be an important regulatory mechanism that could account for the increased endogenous glucose production of liver of diabetic rodents.

Unraveling multistate unfolding of pig kidney fructose-1,6-bisphosphatase using single tryptophan mutants

Pig kidney fructose-1,6-bisphosphatase is a homotetrameric enzyme which does not contain tryptophan. In a previous report the guanidine hydrochloride-induced unfolding of the enzyme has been described as a multistate process [Reyes, A. M., Ludwig, H. C., Yañez, A. J., Rodriguez, P. H and Slebe, J. C. (2003) Biochemistry 42, 6956-6964]. To monitor spectroscopically the unfolding transitions, four mutants were constructed containing a single tryptophan residue either near the C1-C2 or the C1-C4 intersubunit interface of the tetramer. The mutants were shown to retain essentially all of the structural and kinetic properties of the enzyme isolated from pig kidney. The enzymatic activity, intrinsic fluorescence, size-exclusion chromatographic profiles and 1-anilinonaphthalene-8-sulfonate binding by the mutants were studied under unfolding equilibrium conditions. The unfolding profiles were multisteps, and formation of hydrophobic structures was detected. The enzymatic activity of wild-type and mutant FBPases as a function of guanidine hydrochloride concentration showed an initial enhancement (maximum approximately 30%) followed by a biphasic decay. The activity and fluorescence results indicate that these transitions involve conformational changes in the fructose-1,6-bisphosphate and AMP domains. The representation of intrinsic fluorescence data as a 'phase diagram' reveals the existence of five intermediates, including two catalytically active intermediates that have not been previously described, and provides the first spectroscopic evidence for the formation of dimers. The intrinsic fluorescence unfolding profiles indicate that the dimers are formed by selective disruption of the C1-C2 interface.

Different involvement for aldolase isoenzymes in kidney glucose metabolism: aldolase B but not aldolase A colocalizes and forms a complex with FBPase

The expression of aldolase A and B isoenzyme transcripts was confirmed by RT-PCR in rat kidney and their cell distribution was compared with characteristic enzymes of the gluconeogenic and glycolytic metabolic pathway: fructose-1,6-bisphosphatase (FBPase), phosphoenol pyruvate carboxykinase (PEPCK), and pyruvate kinase (PK). We detected aldolase A isoenzyme in the thin limb and collecting ducts of the medulla and in the distal tubules and glomerula of the cortex. The same pattern of distribution was found for PK, but not for aldolase B, PEPCK, and FBPase. In addition, co-localization studies confirmed that aldolase B, FBPase, and PEPCK are expressed in the same proximal cells. This segregated cell distribution of aldolase A and B with key glycolytic and gluconeogenic enzymes, respectively, suggests that these aldolase isoenzymes participate in different metabolic pathways. In order to test if FBPase interacts with aldolase B, FBPase was immobilized on agarose and subjected to binding experiments. The results show that only aldolase B is specifically bound to FBPase and that this interaction was specifically disrupted by 60 microM Fru-1,6-P2. These data indicate the presence of a modulated enzyme-enzyme interaction between FBPase and isoenzyme B. They affirm that in kidney, aldolase B specifically participates, along the gluconeogenic pathway and aldolase A in glycolysis.

R-state AMP complex reveals initial steps of the quaternary transition of fructose-1,6-bisphosphatase

AMP transforms fructose-1,6-bisphosphatase from its active R-state to its inactive T-state; however, the mechanism of that transformation is poorly understood. The mutation of Ala(54) to leucine destabilizes the T-state of fructose-1,6-bisphosphatase. The mutant enzyme retains wild-type levels of activity, but the concentration of AMP that causes 50% inhibition increases 50-fold. In the absence of AMP, the Leu(54) enzyme adopts an R-state conformation nearly identical to that of the wild-type enzyme. The mutant enzyme, however, grows in two crystal forms in the presence of saturating AMP. In one form, the AMP-bound tetramer is in a T-like conformation, whereas in the other form, the AMP-bound tetramer is in a R-like conformation. The latter reveals conformational changes in two helices due to the binding of AMP. Helix H1 moves toward the center of the tetramer and displaces Ile(10) from a hydrophobic pocket. The displacement of Ile(10) exposes a hydrophobic surface critical to interactions that stabilize the T-state. Helix H2 moves away from the center of the tetramer, breaking hydrogen bonds with a buried loop (residues 187-195) in an adjacent subunit. The same hydrogen bonds reform but only after the quaternary transition to the T-state. Proposed here is a model that accounts for the quaternary transition and cooperativity in the inhibition of catalysis by AMP.

Broad expression of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase provide evidence for gluconeogenesis in human tissues other than liver and kidney

The importance of renal and hepatic gluconeogenesis in glucose homeostasis is well established, but the cellular localization of the key gluconeogenic enzymes liver fructose-1,6-bisphosphatase (FBPase) and cytosolic phosphoenolpyruvate carboxykinase (PEPCK) in these organs and the potential contribution of other tissues in this process has not been investigated in detail. Therefore, we analyzed the human tissue localization and cellular distribution of FBPase and PEPCK immunohistochemically. The localization analysis demonstrated that FBPase was expressed in many tissues that had not been previously reported to contain FBPase activity (e.g., prostate, ovary, suprarenal cortex, stomach, and heart). In some multicellular tissues, this enzyme was detected in specialized areas such as epithelial cells of the small intestine and prostate or lung pneumocytes II. Interestingly, FBPase was also present in pancreas and cortex cells of the adrenal gland, organs that are involved in the control of carbohydrate and lipid metabolism. Although similar results were obtained for PEPCK localization, different expression of this enzyme was observed in pancreas, adrenal gland, and pneumocytes type I. These results show that co-expression of FBPase and PEPCK occurs not only in kidney and liver, but also in a variety of organs such as the small intestine, stomach, adrenal gland, testis, and prostate which might also contribute to gluconeogenesis. Our results are consistent with published data on the expression of glucose-6-phosphatase in the human small intestine, providing evidence that this organ may play an important role in the human glucose homeostasis.

Anilinoquinazoline inhibitors of fructose 1,6-bisphosphatase bind at a novel allosteric site: synthesis, in vitro characterization, and X-ray crystallography

The synthesis and in vitro structure-activity relationships (SAR) of a novel series of anilinoquinazolines as allosteric inhibitors of fructose-1,6-bisphosphatase (F16Bpase) are reported. The compounds have a different SAR as inhibitors of F16Bpase than anilinoquinazolines previously reported. Selective inhibition of F16Bpase can be attained through the addition of appropriate polar functional groups at the quinazoline 2-position, thus separating the F16Bpase inhibitory activity from the epidermal growth factor receptor tyrosine kinase inhibitory activity previously observed with similar structures. The compounds have been found to bind at a symmetry-repeated novel allosteric site at the subunit interface of the enzyme. Inhibition is brought about by binding to a loop comprised of residues 52-72, preventing the necessary participation of these residues in the assembly of the catalytic site. Mutagenesis studies have identified the key amino acid residues in the loop that are required for inhibitor recognition and binding.

A novel candidate for the true fructose-1,6-bisphosphatase in archaea

Fructose-1,6-bisphosphatase (FBPase) is one of the key enzymes of the gluconeogenic pathway. Although enzyme activity had been detected in Archaea, the corresponding gene had not been identified until a presumable inositol monophosphatase gene from Methanococcus jannaschii was found to encode a protein with both inositol monophosphatase and FBPase activities. Here we display that a gene from the hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1, which does not correspond to the inositol monophosphatase gene from M. jannaschii, displays high FBPase activity. The FBPase from strain KOD1 was partially purified, its N-terminal amino acid sequence was determined, and the gene (Tk-fbp) was cloned. Tk-fbp encoded a protein of 375 amino acid residues with a molecular mass of 41,658 Da. The recombinant Tk-Fbp was purified and characterized. Tk-Fbp catalyzed the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate following Michaelis-Menten kinetics with a K(m) value of 100 microm toward fructose 1,6-bisphosphate, and a k(cat) value of 17 s(-1) subunit(-1) at 95 degrees C. Unlike the inositol monophosphatase from M. jannaschii, Tk-Fbp displayed strict substrate specificity for fructose 1,6-bisphosphate. Activity was enhanced by Mg(2+) and dithioerythritol, and was slightly inhibited by fructose 2,6-bisphosphate. AMP did not inhibit the enzyme activity. We examined whether expression of Tk-fbp was regulated at the transcription level. High levels of Tk-fbp transcripts were detected in cells grown on pyruvate or amino acids, whereas no transcription was detected when starch was present in the medium. Orthologue genes corresponding to Tk-fbp with high similarity are present in all the complete genome sequences of thermophilic Archaea, including M. jannaschii, Pyrococcus furiosus, Sulfolobus solfataricus, and Archaeoglobus fulgidus, but are yet to be assigned any function. Taking into account the high FBPase activity of the protein, the strict substrate specificity, and its sugar-repressed gene expression, we propose that Tk-Fbp may represent the bona fide FBPase in Archaea.

Hybrid tetramers of porcine liver fructose-1,6-bisphosphatase reveal multiple pathways of allosteric inhibition

Fructose-1,6-bisphosphatase is a square planar tetramer of identical subunits, which exhibits cooperative allosteric inhibition of catalysis by AMP. Protocols for in vitro subunit exchange provide three of five possible hybrid tetramers of fructose-1,6-bisphosphatase in high purity. The two hybrid types with different subunits in the top and bottom halves of the tetramer co-purify. Hybrid tetramers, formed from subunits unable to bind AMP and subunits with wild-type properties, differ from the wild-type enzyme only in regard to their properties of AMP inhibition. Hybrid tetramers exhibit cooperative, potent, and complete (100%) AMP inhibition if at least one functional AMP binding site exists in the top and bottom halves of the tetramer. Furthermore, titrations of hybrid tetramers with AMP, monitored by a tryptophan reporter group, reveal cooperativity and fluorescence changes consistent with an R- to T-state transition, provided that again at least one functional AMP site exists in the top and bottom halves of the tetramer. In contrast, hybrid tetramers, which have functional AMP binding sites in only one half (top/bottom), exhibit an R- to T-state transition and complete AMP inhibition, but without cooperativity. Evidently, two pathways of allosteric inhibition of fructose-1,6-bisphosphatase are possible, only one of which is cooperative.

The N-terminal segment of recombinant porcine fructose-1,6-bisphosphatase participates in the allosteric regulation of catalysis

Residues 1--10 of porcine fructose-1,6-bisphosphatase (FBPase) are poorly ordered or are in different conformations, sensitive to the state of ligation of the enzyme. Deletion of the first 10 residues of FBPase reduces k(cat) by 30-fold and Mg(2+) affinity by 20-fold and eliminates cooperativity in Mg(2+) activation. Although a fluorescent analogue of AMP binds with high affinity to the truncated enzyme, AMP itself potently inhibits only 50% of the enzyme activity. Additional inhibition occurs only when the concentration of AMP exceeds 10 mm. Deletion of the first seven residues reduces k(cat) and Mg(2+) affinity significantly but has no effect on AMP inhibition. The mutation of Asp(9) to alanine reproduces the weakened affinity for Mg(2+) observed in the deletion mutants, and the mutation of Ile(10) to aspartate reproduces the AMP inhibition of the 10-residue deletion mutant. Changes in the relative stability of the known conformational states for loop 52--72, in response to changes in the quaternary structure of FBPase, can account for the phenomena above. Some aspects of the proposed model may be relevant to all forms of FBPase, including the thioredoxin-regulated FBPase from the chloroplast.

Mutations in the hinge of a dynamic loop broadly influence functional properties of fructose-1,6-bisphosphatase

Loop 52-72 of porcine fructose-1,6-bisphosphatase may play a central role in the mechanism of catalysis and allosteric inhibition by AMP. The loop pivots between different conformational states about a hinge located at residues 50 and 51. The insertion of proline separately at positions 50 and 51 reduces k(cat) by up to 3-fold, with no effect on the K(m) for fructose 1,6-bisphosphate. The K(a) for Mg(2+) in the Lys(50) --> Pro mutant increases approximately 15-fold, whereas that for the Ala(51) --> Pro mutant is unchanged. Although these mutants retain wild-type binding affinity for AMP and the fluorescent AMP analog 2'(3')-O-(trinitrophenyl)adenosine 5'-monophosphate, the K(i) for AMP increases 8000- and 280-fold in the position 50 and 51 mutants, respectively. In fact, the mutation Lys(50) --> Pro changes the mechanism of AMP inhibition with respect to Mg(2+) from competitive to noncompetitive and abolishes K(+) activation. The K(i) for fructose 2,6-bisphosphate increases approximately 20- and 30-fold in the Lys(50) --> Pro and Ala(51) --> Pro mutants, respectively. Fluorescence from a tryptophan introduced by the mutation of Tyr(57) suggests an altered conformational state for Loop 52-72 due to the proline at position 50. Evidently, the Pro(50) mutant binds AMP with high affinity at the allosteric site, but the mechanism of allosteric regulation of catalysis has been disabled.