Evidence for a phosphoenzyme intermediate in the reaction pathway of rat hepatic fructose-2,6-bisphosphatase

Enzymatic preparation of high-specific-activity beta-D-[6,6'-3H]fructose-2,6-bisphosphate: Application to a sensitive assay for fructose-2,6-bisphosphatase

beta-D-Fructose-2,6-bisphosphate (Fru-2,6-P(2)) is an important regulator of eukaryotic glucose homeostasis, functioning as a potent activator of 6-phosphofructo-1-kinase and inhibitor of fructose-1,6-bisphosphatase. Pharmaceutical manipulation of intracellular Fru-2,6-P(2) levels, therefore, is of interest for the treatment of certain diseases, including diabetes and cancer. [2-(32)P]Fru-2,6-P(2) has been the reagent of choice for studying the metabolism of this effector molecule; however, its short half-life necessitates frequent preparation. Here we describe a convenient, economical, one-pot enzymatic preparation of high-specific-activity tritium-labeled Fru-2,6-P(2). The preparation involves conversion of readily available, carrier-free d-[6,6'-(3)H]glucose to [6,6'-(3)H]Fru-2,6-P(2) using hexokinase, glucose-6-phosphate isomerase, and 6-phosphofructo-2-kinase. The key reagent in this preparation, bifunctional 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from human liver, was produced recombinantly in Escherichia coli and purified in a single step using an appendant C-terminal hexa-His affinity tag. Following purification by anion exchange chromatography using triethylammonium bicarbonate as eluant, radiochemically pure [6,6'-(3)H]Fru-2,6-P(2) having a specific activity of 50 Ci/mmol was obtained in yields averaging 35%. [6,6'-(3)H]Fru-2,6-P(2) serves as a stable, high-specific-activity substrate in a facile assay capable of detecting fructose-2,6-bisphosphatase in the range of 10(-14) to 10(-15) mol, and it should prove to be useful in many studies of the metabolism of this important biofactor.

PFK-2/FBPase-2: maker and breaker of the essential biofactor fructose-2,6-bisphosphate

Fructose-2,6-bisphosphate is responsible for mediating glucagon-stimulated gluconeogenesis in the liver. This discovery has led to the realization that this compound plays a significant role in directing carbohydrate fluxes in all eukaryotes. Biophysical studies of the enzyme that both synthesizes and degrades this biofactor have yielded insight into its molecular enzymology. Moreover, the metabolic role of fructose-2,6-bisphosphate has great potential in the treatment of diabetes.

The difference in the carboxy-terminal sequence is responsible for the difference in the activity of chicken and rat liver fructose-2,6-bisphosphatase

The fructose-2,6-bisphosphatase domain of the bifunctional chicken liver enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase shares approximately 95% amino acid sequence homology with that of the rat enzyme. However, these two enzymes are significantly different in their phosphatase activities. In this report, we show that the COOH-terminal 25 amino acids of the two enzymes are responsible for the different enzymatic activities. Although these 25 amino acids are not required for the phosphatase activity, their removal diminishes the differences in the activities between the two enzymes. In addition, two chimeric molecules (one consisting of the catalytic core of the chicken bisphosphatase domain and the rat COOH-terminal 25 amino acids, and the other consisting of most of the intact chicken enzyme and the rat COOH-terminal 25 amino acids) showed the same kinetic properties as the rat enzyme. Furthermore, substitution of the residues Pro456Pro457Ala458 of the chicken enzyme with GluAlaGlu, the corresponding sequence in the rat liver enzyme, yields a chicken enzyme that behaves like the rat enzyme. These results demonstrate that the different bisphosphatase activities of the chicken and rat liver bifunctional enzymes can be attributed to the differences in their COOH-terminal amino acid sequences, particularly the three residues.

Reaction mechanism of fructose-2,6-bisphosphatase. A mutation of nucleophilic catalyst, histidine 256, induces an alteration in the reaction pathway

A bifunctional enzyme, fructose-6-phosphate,2-kinase/fructose 2, 6-bisphosphatase (Fru-6-P,2-kinase/Fru-2,6-Pase), catalyzes synthesis and degradation of fructose 2,6-bisphosphate (Fru-2,6-P2). Previously, the rat liver Fru-2,6-Pase reaction (Fru-2,6-P2 --> Fru-6-P + Pi) has been shown to proceed via a phosphoenzyme intermediate with His258 phosphorylated, and mutation of the histidine to alanine resulted in complete loss of activity (Tauler, A., Lin, K., and Pilkis, S. J. (1990) J. Biol. Chem. 265, 15617-15622). In the present study, it is shown that mutation of the corresponding histidine (His256) of the rat testis enzyme decreases activity by less than a factor of 10 with a kcat of 17% compared with the wild type enzyme. Mutation of His390 (in close proximity to His256) to Ala results in a kcat of 12.5% compared with the wild type enzyme. Attempts to detect a phosphohistidine intermediate with the H256A mutant enzyme were unsuccessful, but the phosphoenzyme is detected in the wild type, H390A, R255A, R305S, and E325A mutant enzymes. Data demonstrate that the mutation of His256 induces a change in the phosphatase hydrolytic reaction mechanism. Elimination of the nucleophilic catalyst, H256A, results in a change in mechanism. In the H256A mutant enzyme, His390 likely acts as a general base to activate water for direct hydrolysis of the 2-phosphate of Fru-2,6-P2. Mutation of Arg255 and Arg305 suggests that the arginines probably have a role in neutralizing excess charge on the 2-phosphate and polarizing the phosphoryl for subsequent transfer to either His256 or water. The role of Glu325 is less certain, but it may serve as a general acid, protonating the leaving 2-hydroxyl of Fru-2,6-P2.

Crystal structure of the H256A mutant of rat testis fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase. Fructose 6-phosphate in the active site leads to mechanisms for both mutant and wild type bisphosphatase activities

Fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase (Fru-6-P, 2-kinase/Fru-2,6-Pase) is a bifunctional enzyme, catalyzing the interconversion of beta-D-fructose- 6-phosphate (Fru-6-P) and fructose-2,6-bisphosphate (Fru-2,6-P2) at distinct active sites. A mutant rat testis isozyme with an alanine replacement for the catalytic histidine (H256A) in the Fru-2,6-Pase domain retains 17% of the wild type activity (Mizuguchi, H., Cook, P. F., Tai, C-H., Hasemann, C. A., and Uyeda, K. (1998) J. Biol. Chem. 274, 2166-2175). We have solved the crystal structure of H256A to a resolution of 2. 4 A by molecular replacement. Clear electron density for Fru-6-P is found at the Fru-2,6-Pase active site, revealing the important interactions in substrate/product binding. A superposition of the H256A structure with the RT2K-Wo structure reveals no significant reorganization of the active site resulting from the binding of Fru-6-P or the H256A mutation. Using this superposition, we have built a view of the Fru-2,6-P2-bound enzyme and identify the residues responsible for catalysis. This analysis yields distinct catalytic mechanisms for the wild type and mutant proteins. The wild type mechanism would lead to an inefficient transfer of a proton to the leaving group Fru-6-P, which is consistent with a view of this event being rate-limiting, explaining the extremely slow turnover (0. 032 s-1) of the Fru-2,6-Pase in all Fru-6-P,2-kinase/Fru-2,6-Pase isozymes.

Fructose-2,6-bisphosphate and control of carbohydrate metabolism in eukaryotes

Fructose-2,6-bisphosphate is an important intracellular biofactor in the control of carbohydrate metabolic fluxes in eukaryotes. It is generated from ATP and fructose-6-phosphate by 6-phosphofructo-2-kinase and degraded to fructose-6-phosphate and phosphate ion by fructose-2,6-bisphosphatase. In most organisms these enzymatic activities are contained in a single polypeptide. The reciprocal modulation of the kinase and bisphosphatase activities by post-translational modifications places the level of the biofactor under the control of extra-cellular signals. In general, these signals are generated in response to changing nutritional states, therefore, fructose-2,6-bisphosphate plays a role in the adaptation of organisms, and the tissues within them, to changes in environmental and metabolic states. Although the specific mechanism of fructose-2,6-bisphosphate action varies between species and between tissues, most involve the allosteric activation of 6-phosphofructo-1-kinase and inhibition of fructose-1,6-bisphosphatase. These highly conserved enzymes regulate the fructose-6-phosphate/fructose-1,6-bisphosphate cycle, and thereby, determine the carbon flux. It is by reciprocal modulation of these activities that fructose-2,6-bisphosphate plays a fundamental role in eukaryotic carbohydrate metabolism.

Labeling of recombinant protein for NMR spectroscopy: global and specific labeling of the rat liver fructose 2,6-bisphosphatase domain

Methods for the efficient use of the 13C-labeled nutrients, glucose and histidine, in the production of recombinant protein were developed to provide the large amount of sample required for NMR studies. The nutrient requirements were reduced by determining the minimum amount of these metabolites needed during both the growth and the induction phases of the BL21(DE3) and newly constructed BL21(DE3) histidine auxotrophic Escherichia coli cultures. These methods were developed using the separate bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/ fructose-2,6-bisphosphatase, which is expressed to high levels in the pET3a/BL21 (DE3) bacterial system. Use of the optimized expression methods reduced the requirements for the labeled nutrients, glucose and histidine, by 90 and 93.8%, respectively. The savings realized by use of the minimized media and modified induction protocols were obtained without significant reduction of the yield of purified protein. Comprehensive study of the bisphosphatase domain by NMR spectroscopy requires large amounts of protein because of its low solubility and the short lifetime (2-3 days) of the NMR samples. The significant reduction in the costs of labeled protein samples realized by the optimized expression methods can meet these sample requirements in a cost-effective way, and thereby, allow NMR studies of the bisphosphatase domain to proceed.

Crystal structure of the rat liver fructose-2,6-bisphosphatase based on selenomethionine multiwavelength anomalous dispersion phases

The crystal structure of the recombinant fructose-2,6-bisphosphatase domain, which covers the residues between 251 and 440 of the rat liver bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, was determined by multiwavelength anomalous dispersion phasing and refined at 2.5 A resolution. The selenomethionine-substituted protein was induced in the methionine auxotroph, Escherichia coli DL41DE3, purified, and crystallized in a manner similar to that of the native protein. Phase information was calculated using the multiwavelength anomalous dispersion data collected at the X-ray wavelengths near the absorption edge of the K-shell alpha electrons of selenium. The fructose-2,6-bisphosphatase domain has a core alpha/beta structure which consists of six stacked beta-strands, four parallel and two antiparallel. The core beta-sheet is surrounded by nine alpha-helices. The catalytic site, as defined by a bound phosphate ion, is positioned near the C-terminal end of the beta-sheet and close to the N-terminal end of an alpha-helix. The active site pocket is funnel-shaped. The narrow opening of the funnel is wide enough for a water molecule to pass. The key catalytic residues, including His7, His141, and Glu76, are near each other at the active site and probably function as general acids and/or bases during a catalytic cycle. The inorganic phosphate molecule is bound to an anion trap formed by Arg6, His7, Arg56, and His141. The core structure of the Fru-2,6-P2ase is similar to that of the yeast phosphoglycerate mutase and the rat prostatic acid phosphatase. However, the structure of one of the loops near the active site is completely different from the other family members, perhaps reflecting functional differences and the nanomolar range affinity of Fru-2,6-P2ase for its substrate. The imidazole rings of the two key catalytic residues, His7 and His141, are not parallel as in the yeast phosphoglycerate mutase. The crystal structure is used to interpret the existing chemical data already available for the bisphosphatase domain. In addition, the crystal structure is compared with two other proteins that belong to the histidine phosphatase family.

Covalent control of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: insights into autoregulation of a bifunctional enzyme

The hepatic bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF-2-K/Fru-2,6-P2ase), E.C. 2.7-1-105/E.C. 3-1-3-46, is one member of a family of unique bifunctional proteins that catalyze the synthesis and degradation of the regulatory metabolite fructose-2,6-bisphosphate (Fru-2,6-P2). Fru-2,6-P2 is a potent activator of the glycolytic enzyme 6-phosphofructo-1-kinase and an inhibitor of the gluconeogenic enzyme fructose-1,6-bisphosphatase, and provides a switching mechanism between these two opposing pathways of hepatic carbohydrate metabolism. The activities of the hepatic 6PF-2-K/Fru-2,6-P2ase isoform are reciprocally regulated by a cyclic AMP-dependent protein kinase (cAPK)-catalyzed phosphorylation at a single NH2-terminal residue, Ser-32. Phosphorylation at Ser-32 inhibits the kinase and activates the bisphosphatase, in part through an electrostatic mechanism. Substitution of Asp for Ser-32 mimics the effects of cAPK-catalyzed phosphorylation. In the dephosphorylated homodimer, the NH2- and COOH-terminal tail regions also have an interaction with their respective active sites on the same subunit to produce an autoregulatory inhibition of the bisphosphatase and activation of the kinase. In support of this hypothesis, deletion of either the NH2- or COOH-terminal tail region, or both regions, leads to a disruption of these interactions with a maximal activation of the bisphosphatase. Inhibition of the kinase is observed with the NH2-truncated forms, in which there is also a diminution of cAPK phosphorylation to decrease the Km for Fru-6-P. Phosphorylation of the bifunctional enzyme by cAPK disrupts these autoregulatory interactions, resulting in inhibition of the kinase and activation of the bisphosphatase. Therefore, effects of cyclic AMP-dependent phosphorylation are mediated by a combination of electrostatic and autoregulatory control mechanisms.

Regulation of the fructose 6-phosphate/fructose 2,6-bisphosphate cycle by enzyme phosphorylation and sn-glycerol 3-phosphate

The regulation of the Fru-6-P/Fru-2,6-P2 cycle by the cooperation of allosteric and covalent mechanisms was investigated in a reconstituted enzyme system under in vitro conditions. Phosphorylation of the bifunctional enzyme exerts a much stronger effect than sn-glycerol 3-phosphate in lowering the quasi-stationary concentration of fructose 2,6-bisphosphate and in increasing the critical concentration of the fructose phosphates, respectively. However, sn-glycerol 3-phosphate is able to strongly amplify the decrease of the quasi-stationary concentration of fructose 2,6-bisphosphate due to phosphorylation. The experiments can be described by a mathematical model involving rate equations for the dephosphorylated and the phosphorylated PFD-2 and FBPase-2. The results are compared with data from the literature obtained under in vivo conditions.

Vanadate inhibits liver fructose-2,6-bisphosphatase

Vanadate was found to be a reversible non-competitive inhibitor of chicken liver fructose-2,6-bisphosphatase. The inhibition was best observed in the presence of glycerol 2- or 3-phosphate and half-maximal effect was obtained with about 0.15 mM vanadate. Vanadate decreased the extent of phosphorylation of the enzyme (E-P) by fructose 2,6-[2-32P]bisphosphate. This did not result from an increased rate of E-P breakdown, as is the case with phosphoglycerate mutase, an enzyme which shares structural and functional similarity to fructose-2,6-bisphosphate. The data were consistent with the formation of a dead-end transition state analogue of phosphate in the active site. Inhibition of fructose-2,6-bisphosphatase by vanadate offers a likely explanation for the increase in fructose 2,6-bisphosphate concentration brought about by vanadate in isolated rat hepatocytes.

Purification and properties of spinach leaf phosphofructokinase 2/fructose 2,6-bisphosphatase

Phosphofructokinase 2 was purified from spinach leaves by fractionation with poly(ethylene glycol) and by chromatography on blue Sepharose, anion exchanger Mono-Q and blue Trisacryl. A low-Km fructose-2,6-bisphosphatase copurified with phosphofructokinase 2 and its constitutive subunits could be easily identified by sodium dodecyl sulphate gel electrophoresis thanks to the formation of a [32P]phosphoenzyme intermediate upon short-time incubation in the presence of 1 microM fructose 2,6-[2-32P]bisphosphate. On anion-exchange chromatography, two peaks of phosphofructokinase 2/fructose-2,6-bisphosphatase were resolved. The first one, called L (light), represented about 10% of the phosphofructokinase 2 activity and was characterized by a phosphofructokinase 2/fructose-2,6-bisphosphatase activity ratio close to 1, by an Mr of 132,000 as measured by gel filtration, and by a series of subunits of Mr comprised between 44,000 and 70,000. The second and major peak of phosphofructokinase 2, called H (heavy), had a phosphofructokinase 2/fructose-2,6-bisphosphatase ratio close to 8, an Mr of 390,000 and was made of 90,000-Mr subunits. The H form of phosphofructokinase 2 had a lower Km for fructose 6-phosphate than the L form and a higher Ki for a series of physiological inhibitors. By contrast, the kinetics of fructose-2,6-bisphosphatase was the same for the two forms of the enzyme. Upon incubation in the presence of papain or of a crude spinach leaf extract, the purified H form gave rise to products made of subunits of Mr comprised between 70,000 and 44,000 but also of lower values which maintained their fructose-2,6-bisphosphatase activity. The H and L forms of phosphofructokinase 2/fructose-2,6-bisphosphatase were also detected in crude homogenates of castor bean endosperm and of Jerusalem artichoke tubers.

Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: phosphate dependence and effects of other oxyanions

The effects of various oxyanions on the activities of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (EC 2.7.1.105/3.1.3.46) were examined. No evidence could be found for an absolute dependence of the kinase activity on inorganic phosphate as was recently reported by M. Laloux, E. Van Schaftingen, and H.-G. Hers ((1985) Eur. J. Biochem. 148, 155-159). Rather, phosphate and arsenate activated the kinase by decreasing the enzyme's Km for fructose 6-phosphate without affecting its Km for ATP or Vmax. The Km of the kinase for fructose 6-phosphate in the presence of inorganic phosphate was found to be significantly lower (6 microM) than previously reported (30 microM) when the hydrolysis of fructose 2,6-bisphosphate by the concomitant bisphosphatase activity at low Fru 6-P concentrations was taken into account. The KA's for phosphate and arsenate activation of the kinase were 0.2 and 0.3 mM, respectively. A number of other oxyanions, including pyrophosphate, sulfate, tungstate, selenate, and molybdate all inhibited the kinase by increasing the Km for fructose 6-phosphate. The apparent Ki's for inhibition of the kinase were in the 0.5-1 mM range. In contrast, all of these oxyanions activated the bisphosphatase, with half-maximal effects requiring millimolar concentrations. Inorganic phosphate was the most potent activator with a KA of 1 mM. In contrast to the other oxyanions, vanadate and meta-periodate inhibited the kinase but had no effect on the bisphosphatase. Vanadate appeared to be a noncompetitive inhibitor since its effects were not overcome by Pi, ATP, or fructose 6-phosphate, and the species responsible was shown to be decavanadate. Like vanadate, meta-periodate had no effect on the bisphosphatase, though it was a potent inhibitor (I0.5 = 30 microM) of the kinase. Its effects were shown to be time-dependent and reversed by dithiothreitol, suggesting that it acted by an oxidative mechanism. These results augment the mounting body of evidence that the enzyme's two reactions are catalyzed at discrete active sites.

Purification and properties of phosphofructokinase 2/fructose 2,6-bisphosphatase from chicken liver and from pigeon muscle

Phosphofructokinase 2 and fructose 2,6-bisphosphatase extracted from either chicken liver or pigeon muscle co-purified up to homogeneity. The two homogeneous proteins were found to be dimers of relative molecular mass (Mr) close to 110,000 with subunits of Mr 54,000 for the chicken liver enzyme and 53,000 for the pigeon muscle enzyme. The latter also contained a minor constituent of Mr 54,000. Incubation of the chicken liver enzyme with the catalytic subunit of cyclic-AMP-dependent protein kinase in the presence of [gamma-32P]ATP resulted in the incorporation of about 0.8 mol phosphate/mol enzyme. Under similar conditions, the pigeon muscle enzyme was phosphorylated to an extent of only 0.05 mol phosphate/mol enzyme and all the incorporated phosphate was found in the minor 54,000-Mr constituent. The maximal activity of the native avian liver phosphofructokinase 2 was little affected by changes of pH between 6 and 10. Its phosphorylation by cyclic-AMP-dependent protein kinase resulted in a more than 90% inactivation at pH values below 7.5 and in no or little change in activity at pH 10. Intermediary values of inactivation were observed at pH values between 8 and 10. Muscle phosphofructokinase 2 had little activity at pH below 7 and was maximally active at pH 10. Its partial phosphorylation resulted in a further 25% decrease of its already low activity measured at pH 7.1 and in a negligible inactivation at pH 8.5. Phosphoenolpyruvate and citrate inhibited phosphofructokinase 2 from both origins non-competitively. The muscle enzyme and the phosphorylated liver enzyme displayed much more affinity for these inhibitors than the native liver enzyme. Fructose 2,6-bisphosphatase from both sources had about the same specific activity but only the chicken liver enzyme was activated about twofold upon incubation with ATP and cyclic-AMP-dependent protein kinase. All enzyme forms were inhibited by fructose 6-phosphate and this inhibition was released by inorganic phosphate and by glycerol 3-phosphate. Both liver and muscle fructose 2,6-bisphosphatases formed a 32P-labeled enzyme intermediate when incubated in the presence of fructose 2,6-[2-32P]bisphosphate.

The effect of arabinose 1,5-bisphosphate on rat hepatic 6-phosphofructo-1-kinase and fructose-1,6-bisphosphatase

The alpha- and beta-anomers of arabinose 1,5-bisphosphate and ribose 1,5-bisphosphate were tested as effectors of rat liver 6-phosphofructo-1-kinase and fructose-1,6-bisphosphatase. Both anomers of arabinose 1,5-bisphosphate activated the kinase and inhibited the bisphosphatase. The alpha-anomer was the more effective kinase activator while the beta-anomer was the more potent inhibitor of the bisphosphatase. Inhibition of the bisphosphatase by both anomers was competitive, and both potentiated allosteric inhibition by AMP. beta-Arabinose 1,5-bisphosphate was also more effective in decreasing fructose 2,6-bisphosphate binding to the enzyme. Neither anomer of ribose 1,5-bisphosphate affected 6-phosphofructo-1-kinase or fructose-1,6-bisphosphatase, indicating that the configuration of the C-2 (C-3 in Fru 2,6-P2) hydroxyl group is important for biological activity. These results are also consistent with arabinose 1,5-bisphosphate binding to the active site and thereby enhancing the interaction of AMP with the allosteric site.