Isolation of a human liver fructose-1,6-bisphosphatase cDNA and expression of the protein in Escherichia coli. Role of ASP-118 and ASP-121 in catalysis

Insulin signaling and pharmacology in humans and in corals

Once thought to be a unique capability of the Langerhans islets in the pancreas of mammals, insulin (INS) signaling is now recognized as an evolutionarily ancient function going back to prokaryotes. INS is ubiquitously present not only in humans but also in unicellular eukaryotes, fungi, worms, and Drosophila. Remote homologue identification also supports the presence of INS and INS receptor in corals where the availability of glucose is largely dependent on the photosynthetic activity of the symbiotic algae. The cnidarian animal host of corals operates together with a 20,000-sized microbiome, in direct analogy to the human gut microbiome. In humans, aberrant INS signaling is the hallmark of metabolic disease, and is thought to play a major role in aging, and age-related diseases, such as Alzheimer's disease. We here would like to argue that a broader view of INS beyond its human homeostasis function may help us understand other organisms, and in turn, studying those non-model organisms may enable a novel view of the human INS signaling system. To this end, we here review INS signaling from a new angle, by drawing analogies between humans and corals at the molecular level.

Cloning, purification and characterisation of cytosolic fructose-1,6-bisphosphatase from mung bean (Vigna radiata)

To improve the crop yield and quality, the cytosolic fructose-1,6-bisphosphatase (cFBPase) from mung bean (Vigna radiata), a rate-limiting enzyme in gluconeogenesis, was cloned, purified, and structurally characterised. To function it required Mg²⁺ and Mn²⁺ at 0.01-10 mM. The Michaelis-Menton constant and adenosine monophosphate (AMP) inhibitory constant (K_i) were 7.96 and 111.09 μM, respectively. The functional site residues of AMP binding (Arg³⁰, Asp³², and Phe³³) and the active site residues (Asn²¹⁸ and Met²⁵¹) were tested via site-directed mutagenesis and molecular docking. Asn²¹⁸ and Met²⁵¹ were replaced by Tyr and Leu, respectively. The M251L mutant showed enhanced substrate affinity and activity, resulting from decreased binding energy (-2.58 kcal·mol^-1) and molecular distance (4.2 Å). AMP binding site mutations changed the enzyme activities, indicating a connection between the binding and active sites. Furthermore, K_i and docking analysis revealed that Asp³² plays a key role in maintaining the AMP binding conformation.

Parabacteroides distasonis Alleviates Obesity and Metabolic Dysfunctions via Production of Succinate and Secondary Bile Acids

We demonstrated the metabolic benefits of Parabacteroides distasonis (PD) on decreasing weight gain, hyperglycemia, and hepatic steatosis in ob/ob and high-fat diet (HFD)-fed mice. Treatment with live P. distasonis (LPD) dramatically altered the bile acid profile with elevated lithocholic acid (LCA) and ursodeoxycholic acid (UDCA) and increased the level of succinate in the gut. In vitro cultivation of PD demonstrated its capacity to transform bile acids and production of succinate. Succinate supplementation in the diet decreased hyperglycemia in ob/ob mice via the activation of intestinal gluconeogenesis (IGN). Gavage with a mixture of LCA and UDCA reduced hyperlipidemia by activating the FXR pathway and repairing gut barrier integrity. Co-treatment with succinate and LCA/UDCA mirrored the benefits of LPD. The binding target of succinate was identified as fructose-1,6-bisphosphatase, the rate-limiting enzyme in IGN. The succinate and secondary bile acids produced by P. distasonis played key roles in the modulation of host metabolism.

Cerebral Gluconeogenesis and Diseases

The gluconeogenesis pathway, which has been known to normally present in the liver, kidney, intestine, or muscle, has four irreversible steps catalyzed by the enzymes: pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose 1,6-bisphosphatase, and glucose 6-phosphatase. Studies have also demonstrated evidence that gluconeogenesis exists in brain astrocytes but no convincing data have yet been found in neurons. Astrocytes exhibit significant 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 activity, a key mechanism for regulating glycolysis and gluconeogenesis. Astrocytes are unique in that they use glycolysis to produce lactate, which is then shuttled into neurons and used as gluconeogenic precursors for reduction. This gluconeogenesis pathway found in astrocytes is becoming more recognized as an important alternative glucose source for neurons, specifically in ischemic stroke and brain tumor. Further studies are needed to discover how the gluconeogenesis pathway is controlled in the brain, which may lead to the development of therapeutic targets to control energy levels and cellular survival in ischemic stroke patients, or inhibit gluconeogenesis in brain tumors to promote malignant cell death and tumor regression. While there are extensive studies on the mechanisms of cerebral glycolysis in ischemic stroke and brain tumors, studies on cerebral gluconeogenesis are limited. Here, we review studies done to date regarding gluconeogenesis to evaluate whether this metabolic pathway is beneficial or detrimental to the brain under these pathological conditions.

New insight into the binding modes of TNP-AMP to human liver fructose-1,6-bisphosphatase

Human liver fructose-1,6-bisphosphatase (FBPase) contains two binding sites, a substrate fructose-1,6-bisphosphate (FBP) active site and an adenosine monophosphate (AMP) allosteric site. The FBP active site works by stabilizing the FBPase, and the allosteric site impairs the activity of FBPase through its binding of a nonsubstrate molecule. The fluorescent AMP analogue, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-monophosphate (TNP-AMP) has been used as a fluorescent probe as it is able to competitively inhibit AMP binding to the AMP allosteric site and, therefore, could be used for exploring the binding modes of inhibitors targeted on the allosteric site. In this study, we have re-examined the binding modes of TNP-AMP to FBPase. However, our present enzyme kinetic assays show that AMP and FBP both can reduce the fluorescence from the bound TNP-AMP through competition for FBPase, suggesting that TNP-AMP binds not only to the AMP allosteric site but also to the FBP active site. Mutagenesis assays of K274L (located in the FBP active site) show that the residue K274 is very important for TNP-AMP to bind to the active site of FBPase. The results further prove that TNP-AMP is able to bind individually to the both sites. Our present study provides a new insight into the binding mechanism of TNP-AMP to the FBPase. The TNP-AMP fluorescent probe can be used to exam the binding site of an inhibitor (the active site or the allosteric site) using FBPase saturated by AMP and FBP, respectively, or the K247L mutant FBPase.

A Toxoplasma gondii Gluconeogenic Enzyme Contributes to Robust Central Carbon Metabolism and Is Essential for Replication and Virulence

The expression of gluconeogenic enzymes is typically repressed when glucose is available. The protozoan parasite Toxoplasma gondii utilizes host glucose to sustain high rates of intracellular replication. However, despite their preferential utilization of glucose, intracellular parasites constitutively express two isoforms of the gluconeogenic enzyme fructose 1,6-bisphosphatase (TgFBP1 and TgFBP2). The rationale for constitutive expression of FBPases in T. gondii remains unclear. We find that conditional knockdown of TgFBP2 results in complete loss of intracellular growth in vitro under glucose-replete conditions and loss of acute virulence in mice. TgFBP2 deficiency was rescued by expression of catalytically active FBPase and was associated with altered glycolytic and mitochondrial TCA cycle fluxes, as well as dysregulation of glycolipid, amylopectin, and fatty acid biosynthesis. Futile cycling between gluconeogenic and glycolytic enzymes may constitute a regulatory mechanism that allows T. gondii to rapidly adapt to changes in nutrient availability in different host cells.

Effects of dietary glucose and dextrin on activity and gene expression of glucokinase and fructose-1,6-bisphosphatase in liver of turbot Scophthalmus maximus

Glucokinase (GK) and fructose-1,6-bisphosphatase (FBPase) play crucial role in glucose metabolism. In the present study, the cDNA encoding GK and FBPase was cloned from the liver of turbot Scophthalmus maximus by rapid amplification of cDNA end technique. Effects of dietary glucose and dextrin on the activities and gene expressions of these two enzymes were also studied. Results showed that the full length of GK cDNA was 2226 bp, consisting of an open reading frame (ORF) of 1434 bp. The full-length cDNA coding FBPase was 1314 bp with a 1014 bp ORF encoding 337 amino acids. Analyses of gene expression of GK and FBPase were conducted in gill, liver, the whole intestine, the whole kidney, heart, the dorsal white muscle and brain. The highest expression of GK was found in liver, followed by muscle. The expression of FBPase was found higher in liver than heart and gill. Both hepatic GK activity and mRNA expression were highly induced in turbot after being fed with dietary carbohydrates (p < 0.05). However, the GK activity and mRNA expression in the group with dietary glucose did not significantly differ from those in the group with dietary dextrin (p > 0.05). Compared with the control group, there were no significant differences in FBPase activity and mRNA expression in the glucose as well as dextrin group (p > 0.05). The increased hepatic GK activity and gene expression indicated that the first step of glycolysis was activated in turbot by dietary carbohydrates.

Determination of protein-ligand binding constants of a cooperatively regulated tetrameric enzyme using electrospray mass spectrometry

This study highlights the benefits of nano electrospray ionization mass spectrometry (nanoESI-MS) as a fast and label-free method not only for determination of dissociation constants (KD) of a cooperatively regulated enzyme but also to better understand the mechanism of enzymatic cooperativity of multimeric proteins. We present an approach to investigate the allosteric mechanism in the binding of inhibitors to the homotetrameric enzyme fructose 1,6-bisphosphatase (FBPase), a potential therapeutic target for glucose control in type 2 diabetes. A series of inhibitors binding at an allosteric site of FBPase were investigated to determine their KDs by nanoESI-MS. The KDs determined by ESI-MS correlate very well with IC50 values in solution. The Hill coefficients derived from nanoESI-MS suggest positive cooperativity. From single-point measurements we could obtain information on relative potency, stoichiometry, conformational changes, and mechanism of cooperativity. A new X-ray crystal structure of FBPase tetramer binding ligand 3 in a 4:4 stoichiometry is also reported. NanoESI-MS-based results match the current understanding of the investigated system and are in agreement with the X-ray structural data, but provide additional mechanistic insight on the ligand binding, due to the better dynamic resolution. This method offers a powerful approach for studying other proteins with allosteric binding sites, as well.

Fructose-1, 6-bisphosphatase inhibitors for reducing excessive endogenous glucose production in type 2 diabetes

Fructose-1,6-bisphosphatase (FBPase), a rate-controlling enzyme of gluconeogenesis, has emerged as an important target for the treatment of type 2 diabetes due to the well-recognized role of excessive endogenous glucose production (EGP) in the hyperglycemia characteristic of the disease. Inhibitors of FBPase are expected to fulfill an unmet medical need because the majority of current antidiabetic medications act primarily on insulin resistance or insulin insufficiency and do not reduce gluconeogenesis effectively or in a direct manner. Despite significant challenges, potent and selective inhibitors of FBPase targeting the allosteric site of the enzyme were identified by means of a structure-guided design strategy that used the natural inhibitor, adenosine monophosphate (AMP), as the starting point. Oral delivery of these anionic FBPase inhibitors was enabled by a novel diamide prodrug class. Treatment of diabetic rodents with CS-917, the best characterized of these prodrugs, resulted in a reduced rate of gluconeogenesis and EGP. Of note, inhibition of gluconeogenesis by CS-917 led to the amelioration of both fasting and postprandial hyperglycemia without weight gain, incidence of hypoglycemia, or major perturbation of lactate or lipid homeostasis. Furthermore, the combination of CS-917 with representatives of the insulin sensitizer or insulin secretagogue drug classes provided enhanced glycemic control. Subsequent clinical evaluations of CS-917 revealed a favorable safety profile as well as clinically meaningful reductions in fasting glucose levels in patients with T2DM. Future trials of MB07803, a second generation FBPase inhibitor with improved pharmacokinetics, will address whether this novel class of antidiabetic agents can provide safe and long-term glycemic control.

Determination of enzyme activity inhibition by FTIR spectroscopy on the example of fructose bisphosphatase

A mid-infrared enzymatic assay for label-free monitoring of the enzymatic reaction of fructose-1,6-bisphosphatase with fructose 1,6-bisphosphate has been proposed. The whole procedure was done in an automated way operating in the stopped flow mode by incorporating a temperature-controlled flow cell in a sequential injection manifold. Fourier transform infrared difference spectra were evaluated for kinetic parameters, like the Michaelis-Menten constant (K(M)) of the enzyme and Vmax of the reaction. The obtained K(M) of the reaction was 14 +/- 3 g L(-1) (41 microM). Furthermore, inhibition by adenosine 5'-monophosphate (AMP) was evaluated, and the K(M)(App) value was determined to be 12 +/- 2 g L(-1) (35 microM) for 7.5 and 15 microM AMP, respectively, with Vmax decreasing from 0.1 +/- 0.03 to 0.05 +/- 0.01 g L(-1) min(-1). Therefore, AMP exerted a non-competitive inhibition.

Evolutionary conserved N-terminal region of human muscle fructose 1,6-bisphosphatase regulates its activity and the interaction with aldolase

N-terminal residues of muscle fructose 1,6-bisphosphatase (FBPase) are highly conserved among vertebrates. In this article, we present evidence that the conservation is responsible for the unique properties of the muscle FBPase isozyme: high sensitivity to AMP and Ca(2+) inhibition and the high affinity to muscle aldolase, which is a factor desensitizing muscle FBPase toward AMP and Ca(2+). The first N-terminal residue affecting the affinity of muscle FBPase to aldolase is arginine 3. On the other hand, the first residue significantly influencing the kinetics of muscle FBPase is proline 5. Truncation from 5-7 N-terminal residues of the enzyme not only decreases its affinity to aldolase but also reduces its k-(cat) and activation by Mg(2+), and desensitizes FBPase to inhibition by AMP and calcium ions. Deletion of the first 10 amino acids of muscle FBPase abolishes cooperativity of Mg(2+) activation and results in biphasic inhibition of the enzyme by AMP. Moreover, this truncation lowers affinity of muscle FBPase to aldolase about 14 times, making it resemble the liver isozyme. We suggest that the existence of highly AMP-sensitive muscle-like FBPase, activity of which is regulated by metabolite-dependent interaction with aldolase enables the precise regulation of muscle energy expenditures and might contributed to the evolutionary success of vertebrates.

Design, synthesis, and biological evaluation of substituted 2,3-dihydro-1H-cyclopenta[b]quinolin-9-ylamine related compounds as fructose-1,6-bisphosphatase inhibitors

In a search for structurally new inhibitors of fructose-1,6-bisphosphatase (F16BPase), substituted 2,3-dihydro-1H-cyclopenta[b]quinoline derivatives were synthesized. It has been shown that the 2,3-dihydro-1H-cyclopenta[b]quinoline moiety may represent a suitable scaffold for the synthesis of potent F16BPase inhibitors endowed with significantly lower EGFR tyrosine kinase inhibitory activity.

Separation and quantitation of fructose-6-phosphate and fructose-1,6-diphosphate by LC-ESI-MS for the evaluation of fructose-1,6-biphosphatase activity

An LC-ESI-MS method was developed for the identification and quantification of fructose-1,6-biphosphate (F1,6BP) and fructose-6-phosphate (F6P), respectively the substrate and the product of the enzymatic reaction catalysed by fructose-1,6-bisphosphatase (F1,6BPase). F1,6BPase, expressed predominantly in liver and kidney, is one of the rate-limiting enzymes of hepatic gluconeogenesis and has become a target for the development of new drugs for type 2 diabetes. The two sugar phosphates were separated on a Phenomenex Luna NH2 column (150 mm x 2.0 mm id) using the following mobile phase: 5 mM triethylamine acetate buffer/ACN (80:20) v/v in a linear pH gradient (from pH = 9 to 10 in 15 min) at the flow rate of 0.3 mL/min. The detection was performed with an IT mass spectrometer in negative polarity (full scan 100-450 m/z) and in SIM mode on the generated anions at m/z = 339 (F1,6BP) and m/z = 259 (F6P). Under the optimised final conditions, the method was validated for accuracy, specificity, precision (inter- and intradays RSD comprised between 1.0 and 6.3% over the range of concentrations used), linearity (50-400 microM), LODs (0.44 microM) and LOQs (1.47 microM), and the method was applied to F6P determination in the F1,6BPase catalysed hydrolysis of F1,6BP.

Benzoxazole benzenesulfonamides are novel allosteric inhibitors of fructose-1,6-bisphosphatase with a distinct binding mode

We have identified benzoxazole benzenesulfonamide 1 as a novel allosteric inhibitor of fructose-1,6-bisphosphatase (FBPase-1). X-ray crystallographic and biological studies of 1 indicate a distinct binding mode that recapitulates features of several previously reported FBPase-1 inhibitor classes.

The origin of the high sensitivity of muscle fructose 1,6-bisphosphatase towards AMP

Adenosine 5'-monophosphate (AMP) inhibits muscle fructose 1,6-bisphosphatase (FBPase) about 44 times stronger than the liver isozyme. The key role in strong AMP binding to muscle isozyme play K20, T177 and Q179. Muscle FBPase which has been mutated towards the liver enzyme (K20E/T177M/Q179C) is inhibited by AMP about 26 times weaker than the wild-type muscle enzyme, but it binds the fluorescent AMP analogue, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-monophosphate (TNP-AMP), similarly to the wild-type liver enzyme. The reverse mutation of liver FBPase towards the muscle isozyme significantly increases the affinity of the mutant to TNP-AMP. High affinity to the inhibitor but low sensitivity to AMP of the liver triple mutant suggest differences between the isozymes in the mechanism of allosteric signal transmission.

MB06322 (CS-917): A potent and selective inhibitor of fructose 1,6-bisphosphatase for controlling gluconeogenesis in type 2 diabetes

In type 2 diabetes, the liver produces excessive amounts of glucose through the gluconeogenesis (GNG) pathway and consequently is partly responsible for the elevated glucose levels characteristic of the disease. In an effort to find safe and efficacious GNG inhibitors, we targeted the AMP binding site of fructose 1,6-bisphosphatase (FBPase). The hydrophilic nature of AMP binding sites and their widespread use for allosteric regulation of enzymes in metabolic pathways has historically made discovery of AMP mimetics suitable for drug development difficult. By using a structure-based drug design strategy, we discovered a series of compounds that mimic AMP but bear little structural resemblance. The lead compound, MB05032, exhibited high potency and specificity for human FBPase. Oral delivery of MB05032 was achieved by using the bisamidate prodrug MB06322 (CS-917), which is converted to MB05032 in two steps through the action of an esterase and a phosphoramidase. MB06322 inhibited glucose production from a variety of GNG substrates in rat hepatocytes and from bicarbonate in male Zucker diabetic fatty rats. Analysis of liver GNG pathway intermediates confirmed FBPase as the site of action. Oral administration of MB06322 to Zucker diabetic fatty rats led to a dose-dependent decrease in plasma glucose levels independent of insulin levels and nutritional status. Glucose lowering occurred without signs of hypoglycemia or significant elevations in plasma lactate or triglyceride levels. The findings suggest that potent and specific FBPase inhibitors represent a drug class with potential to treat type 2 diabetes through inhibition of GNG.

Construction of chimeric cytosolic fructose-1,6-bisphosphatases by insertion of a chloroplastic redox regulatory cluster

In order to transform cytosolic fructose-1,6-bisphosphatases (FBPase)(EC 3.1.3.11) into potential reductively-modulated chloroplast-type enzymes, we have constructed four chimeric FBPases, which display structural viability as deduced by previous modelling. In the X1-type BV1 and HL1 chimera the N-half of cytosolic sugar beet (Beta vulgaris L.) and human FBPases was fused with the C-half of the pea (Pisum sativum L.) chloroplast enzyme, which carries the cysteine-rich light regulatory sequence. In the X2-type BV2 and HL2 chimera this regulatory fragment was inserted in the corresponding site of the sugar beet cytosolic and human enzymes. Like the plant cytosolic FBPases, the chimeric enzymes show a low rise of activity by dithiothreitol. Both BV1 and BV2, but not HL1 and HL2, display a negligible activation by Trx f, but neither of them by Trx m. Antibodies raised against the pea chloroplast enzyme showed a positive reaction against the four chimeric FBPases and the human enzyme, but not against the sugar beet one. The four chimera display typical kinetics of cytosolic FBPases, with Km values in the 40-140 microM range. We conclude the existence of a structural capacity of cytosolic FBPases for incorporating the redox regulatory cluster of the chloroplast enzyme. However, the ability of these chimeric FBPases for an in vitro redox regulation seems to be scarce, limiting their use from a biotechnology standpoint in in vivo regulation of sugar metabolism.

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

Fructose-1,6-bisphosphatase requires a divalent metal cation for catalysis, Mg(2+) being its most studied activator. Phosphatase activity increases sigmoidally with the concentration of Mg(2+), but the mechanistic basis for such cooperativity is unknown. Bound magnesium cations can interact within a single subunit or between different subunits of the enzyme tetramer. Mutations of Asp(118), Asp(121), or Glu(97) to alanine inactivate the recombinant porcine enzyme. These residues bind directly to magnesium cations at the active site. Three different hybrid tetramers of fructose-1,6-bisphosphatase, composed of one wild-type subunit and three subunits bearing one of the mutations above, exhibit kinetic parameters (K(m) for fructose-1,6-bisphosphate, 1.1-1.8 microm; K(a) for Mg(2+), 0.34-0.76 mm; K(i) for fructose-2,6-bisphosphate, 0.11-0.61 microm; and IC(50) for AMP, 3.8-7.4 microm) nearly identical to those of the wild-type enzyme. Notwithstanding these similarities, the k(cat) parameter for each hybrid tetramer is approximately one-fourth of that for the wild-type enzyme. Evidently, each subunit in the wild-type tetramer can independently achieve maximum velocity when activated by Mg(2+). Moreover, the activities of the three hybrid tetramers vary sigmoidally with the concentration of Mg(2+) (Hill coefficients of approximately 2). The findings above are fully consistent with a mechanism of cooperativity that arises from within a single subunit of fructose-1,6-bisphosphatase.

3-(2-carboxyethyl)-4,6-dichloro-1H-indole-2-carboxylic acid: an allosteric inhibitor of fructose-1,6-bisphosphatase at the AMP site

3-(2-Carboxyethyl)-4,6-dichloro-1H-indole-2-carboxylic acid (MDL-29951), an antagonist of the glycine site of the NMDA receptor, has been found to be an allosteric inhibitor of the enzyme fructose 1,6-bisphosphatase. The compound binds at the AMP regulatory site by X-ray crystallography. This represents a new approach to inhibition of fructose 1,6-bisphosphatase and serves as a lead for further drug design.

Different sensitivities of mutants and chimeric forms of human muscle and liver fructose-1,6-bisphosphatases towards AMP

AMP is an allosteric inhibitor of human muscle and liver fructose-1,6-bisphosphatase (FBPase). Despite strong similarity of the nucleotide binding domains, the muscle enzyme is inhibited by AMP approximately 35 times stronger than liver FBPase: I0.5 for muscle and for liver FBPase are 0.14 microM and 4.8 microM, respectively. Chimeric human muscle (L50M288) and chimeric human liver enzymes (M50L288), in which the N-terminal residues (1-50) were derived from the human liver and human muscle FBPases, respectively, were inhibited by AMP 2-3 times stronger than the wild-type liver enzyme. An amino acid exchange within the N-terminal region of the muscle enzyme towards liver FBPase (Lys20-->Glu) resulted in 13-fold increased I0.5 values compared to the wild-type muscle enzyme. However, the opposite exchanges in the liver enzyme (Glu20-->Lys and double mutation Glu19-->Asp/Glu20-->Lys) did not change the sensitivity for AMP inhibition of the liver mutant (I0.5 value of 4.9 microM). The decrease of sensitivity for AMP of the muscle mutant Lys20-->Glu, as well as the lack of changes in the inhibition by AMP of liver mutants Glu20-->Lys and Glu19-->Asp/Glu20-->Lys, suggest a different mechanism of AMP binding to the muscle and liver enzyme.

Fructose-1,6-bisphosphatase genes in animals

A comparison of the amino acid sequences of the liver and muscle fructose-1,6-bisphosphatase (FbPase) isoforms in primates and rodents suggested an ancient duplication event leading to the corresponding genes. We investigated the presence of both genes in the rabbit (order lagomorphs) and in species belonging to further distantly related metazoan taxa. By an analysis of the available complete genomes and proteomes of the nematode Caenorhabditis elegans and of Drosophila melanogaster only one sequence homologous to known FbPases was found in each species. The corresponding mRNAs were characterized by cDNA sequencing. We then carried out reverse transcription-polymerase chain reactions to amplify central fragments of the FbPase cDNAs from liver and muscle of Gallus gallus, Xenopus laevis, and Esox lucius, respectively. Their sequencing revealed that (i) the livers of chicken, frog, and fish contain mRNAs which are closely related to mammalian liver FbPase mRNAs, (ii) chicken muscle contains an mRNA which is most homologous to mammalian muscle FbPase mRNAs, (iii) frog muscle contains both a liver-type and a muscle-type FbPase mRNA, while (iv) in fish muscle no FbPase mRNA could be detected by our approach despite the doubtless presence of the enzyme in this organ. An alignment of the partial amino acid sequences of the different FbPases showed that the residues that are thought to be in contact with the substrate, fructose-2,6-bisphosphate, and Mg(2+) are totally conserved, while some amino acids having contact with adenosine monophosphate were found to vary among several species. The question of what might be the advantage of having more than one gene coding for FbPase per haploid genome is discussed.

Characterization of the mouse liver fructose-1,6-bisphosphatase gene

A cDNA encoding fructose-1,6-bisphosphatase (FBPase) was isolated from mouse liver RNA. The cDNA encodes a polypeptide of 338 amino acids (36.9 kDa). The liver and muscle FBPase isoenzymes of the mouse show positional identities of 69% at the cDNA level and 72% at the protein primary structure level. Starting from genomic YAC libraries and based upon the cDNA sequence all functional parts of the mouse liver FBPase gene (including exon-intron boundaries) were PCR-amplified and sequenced. The 5'-flanking regions of the liver and muscle FBPase genes were compared and showed no sequence similarity. Both genes are co-localized at chromosome 13B3-C1. The transcriptional start site was assigned to a guanine 118 bases before the start codon in the liver FBPase gene. An analysis of the steady state mRNA levels of liver and muscle FBPase in various mouse tissues was performed by Northern blotting and RT/PCR.

Allosteric inhibition of fructose-1,6-bisphosphatase by anilinoquinazolines

Anilinoquinazolines currently of interest as inhibitors of tyrosine kinases have been found to be allosteric inhibitors of the enzyme fructose 1,6-bisphosphatase. These represent a new approach to inhibition of F16BPase and serve as leads for further drug design. Enzyme inhibition is achieved by binding at an unidentified allosteric site.

Kinetic properties of pig (Sus scrofa domestica) and bovine (Bos taurus) D-fructose-1,6-bisphosphate 1-phosphohydrolase (F1,6BPase): liver-like isozymes in mammalian lung tissue

F1,6BPases from porcine and bovine lung were isolated and their kinetic properties were determined. Ks, Kis and beta were determined assuming partial-noncompetitive inhibition (simple intersecting hyperbolic noncompetitive inhibition) of the enzyme by the substrate. Values for Ks were 4.1 and 4.4 microM for porcine and bovine F1,6BPase, respectively and values for 1 were close to 0.55 in both cases. Kis were 9 and 15 microM for porcine and bovine F1,6BPase, respectively. I0.5 for AMP were determined as 7 microM for pig enzyme and 14 microM for F1,6BPase from bovine lung. The enzymes were inhibited by F2,6BP with Ki's of 0.19 and 0.21 microM for porcine and bovine enzymes, respectively. In the presence of AMP concentration equal to I0.5, the Ki values for pig and bovine enzymes were 0.07 and 0.09 microM, respectively. The levels of F2,6BP, AMP and antioxidant enzymes activities in pig and bovine lung tissues were also determined. The cDNA coding sequence of pig lung F1,6BPase1 showed a high homology with pig liver enzyme, differing only in four positions (G/C-63, T/A-808, G/C-884 and T/A-1005) resulting in a single amino acid substitution (Gly-295 for Ala-295). It is hypothesized that the lung F1,6BPase participates in gluconeogenesis, surfactant synthesis and antioxidant reactions.

Molecular cloning, expression and purification of muscle fructose-1,6-bisphosphatase from Zaocys dhumnades: the role of the N-terminal sequence in AMP activation at alkaline pH

An open reading frame (ORF) of snake muscle fructose-1,6-bisphosphatase (Fru-1,6-P2ase) was obtained by the RT-PCR method with degenerate primers, followed by RACE-PCR. The cDNA of Fru-1,6-P2ase, encoding 340 amino acids, is highly homologous to that of mammalian species, especially human muscle, with a few exceptions. Kinetic parameters of the purified recombinant enzyme, including inhibition behavior by AMP, were identical to that of the tissue form. Replacement of the N-terminal sequence of this enzyme by the corresponding region of rat liver Fru-1,6-P2ase shows that the activity was fully retained in the chimeric enzyme. The inhibition constant (Ki) of AMP at pH 7.5, however, increases sharply from 0.85 microM (wild-type) to 1.2 mM (chimeric enzyme). AMP binding is mainly located in the N-terminal region, and the allosteric inhibition was shown not to be merely determined by the backbone of this region. The fact that the chimeric enzyme could be activated at alkaline pH by AMP indicated that the AMP activation requires the global structure beyond the area.

Structure and chromosomal localization of the human and mouse muscle fructose-1,6-bisphosphatase genes

Mammalian skeletal muscle contains fructose-1,6-bisphosphatase (Fru-1,6-P(2)ase), a key enzyme of glyconeogenesis. We have shown previously that muscle Fru-1,6-P(2)ase is encoded by a gene different from that coding for the liver isoenzyme. Starting with genomic YAC libraries and based on the cDNA sequences of human and mouse muscle Fru-1,6-P(2)ases together with the known gene structures of two mammalian liver fructose-1,6-bisphosphatases, we have PCR-amplified and sequenced all functional parts of the human and mouse muscle fructose-1,6-bisphosphatase genes and determined their chromosomal localization. The human gene (FBP2), localized at chromosome 1p36.1-2, spans about 30 kb, while the mouse gene (Fbp2) at chromosome 13B3-C1 is more compact (about 21 kb). Intron lengths are only poorly conserved between the two genes, while intron number and positions are identical in all hitherto analyzed mammalian fructose-1,6-bisphosphatase isoenzyme genes. Transcriptional start sites were found to be located 97 and 95bp before the start codon in the human gene and 35 bp before the start codon in the mouse homolog. A comparison of the 5'-flanking sequences of the two genes revealed a 56% homology up to human bp -607 before the first transcriptional start point, while upstream of this region we found no similarity. The data presented in this paper provide a basis for further studies of the mechanism of expression regulation and the elucidation of the physiological role of the enzyme.

Rat muscle fructose-1,6-bisphosphatase: cloning of the cDNA, expression of the recombinant enzyme, and expression analysis in different tissues

The 1282 bp cDNA of an isoenzyme of fructose-1,6-bisphosphatase was cloned from rat muscle. It shows 70% positional identity to the cDNA of rat liver fructose-1,6-bisphosphatase and is clearly the product of a gene different from that coding for the liver enzyme. After cloning of the coding region of the rat muscle fructose-1,6-bisphosphatase cDNA in an expression vector, the recombinant enzyme could be detected in E. coli cell-free extracts by activity determination and Western blotting. Overexpressed fructose-1,6-bisphosphatase was found to be allosterically inhibited by AMP comparably to the enzyme isolated from rat muscle. Analysis of steady-state mRNA levels of various rat tissues with reverse-transcriptase polymerase chain reaction (RT-PCR) and Northern blotting revealed one or the two fructose-1,6-bisphosphatase isoenzyme mRNAs in most tissues tested with significant quantitative differences. Quantitative PCR using a homologous competitor showed that 1 microg of total RNA of rat muscle contains 1.7 x 10(6) molecules of rat muscle fructose-1,6-bisphosphatase mRNA. 3 x 10(4) copies of this message were found per microg total RNA of heart and kidney, respectively.

Novel mutations in patients with fructose-1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal recessive disorder of gluconeogenesis. Mutations have recently been identified in Japanese patients but none has been reported in patients of other ethnic backgrounds. We have undertaken sequence analysis on genomic DNA isolated from leukocytes of four patients with FBPase deficiency. Homozygous mutations were found in all four cases. One patient was homozygous for the common mutation identified in Japanese patients (960-961insG in exon 7). The other three patients were all homozygous for novel mutations (35delA in exon 1,778G-->A in exon and 966delC in exon 7). Normal and mutant FBPases were expressed in prokaryotic (E. coli TG2) and eukaryotic (COS1) cells. In cell-free extracts the mutant proteins were enzymatically inactive, indicating that the mutations are responsible for the disease. In one affected family, molecular genetic analysis allowed the diagnosis to be excluded promptly in a newborn child 3 days after birth.

A hydrogen-bonding triad stabilizes the chemical transition state of a group I ribozyme

BACKGROUND

The group I intron is an RNA enzyme capable of efficiently catalyzing phosphoryl-transfer reactions. Functional groups that stabilize the chemical transition state of the cleavage reaction have been identified, but they are all located within either the 5'-exon (P1) helix or the guanosine cofactor, which are the substrates of the reaction. Functional groups within the ribozyme active site are also expected to assist in transition-state stabilization, and their role must be explored to understand the chemical basis of group I intron catalysis.

RESULTS

Using nucleotide analog interference mapping and site-specific functional group substitution experiments, we demonstrate that the 2'-OH at A207, a highly conserved nucleotide in the ribozyme active site, specifically stabilizes the chemical transition state by approximately 2 kcal mol-1. The A207 2'-OH only makes its contribution when the U(-1) 2'-OH immediately adjacent to the scissile phosphate is present, suggesting that the 2'-OHs of A207 and U(-1) interact during the chemical step.

CONCLUSIONS

These data support a model in which the 3'-oxyanion leaving group of the transesterification reaction is stabilized by a hydrogen-bonding triad consisting of the 2'-OH groups of U(-1) and A207 and the exocyclic amine of G22. Because all three nucleotides occur within highly conserved non-canonical base pairings, this stabilization mechanism is likely to occur throughout group I introns. Although this mechanism utilizes functional groups distinctive of RNA enzymes, it is analogous to the transition states of some protein enzymes that perform similar phosphoryl-transfer reactions.

Isolation and characterization of an allelic cDNA for human muscle fructose-1,6-bisphosphatase

By applying a newly developed method, cDNAs for the human muscle isoform of fructose-1,6-bisphosphatase were isolated from phage- and plasmid-derived libraries. From these cDNAs and an EST clone, a composite sequence (1302 bp) was deduced that contains an open reading frame encoding a polypeptide of 339 amino acids with an estimated molecular weight of 36 755. After overexpression in E. coli, recombinant human muscle fructose 2,6-bisphosphatase was found to be active in cel-free extracts and could be strongly inhibited by AMP and fructose 2,6-bisphosphate. Sequence comparisons revealed that (1) all amino acids thought to be in contact with substrate molecules, regulatory molecules or metal ions in mammalian liver fructose-1,6-bisphosphatases are, with one exception, conserved in the human muscle enzyme and (2) the human muscle isoform is more homologous to the mouse intestine fructose-1,6-bisphosphatase than to the mammalian liver isoform. This is the first report of the cloning and expression of a muscle fructose-1,6-bisphosphatase isoenzyme.

Identification of genetic mutations in Japanese patients with fructose-1,6-bisphosphatase deficiency

Fructose-1,6-bisphosphatase (FBPase) deficiency is an autosomal recessive inherited disorder and may cause sudden unexpected infant death. We reported the first case of molecular diagnosis of FBPase deficiency, using cultured monocytes as a source for FBPase mRNA. In the present study, we confirmed the presence of the same genetic mutation in this patient by amplifying genomic DNA. Molecular analysis was also performed to diagnose another 12 Japanese patients with FBPase deficiency. Four mutations responsible for FBPase deficiency were identified in 10 patients from 8 unrelated families among a total of 13 patients from 11 unrelated families; no mutation was found in the remaining 3 patients from 3 unrelated families. The identified mutations included the mutation reported earlier, with an insertion of one G residue at base 961 in exon 7 (960/961insG) (10 alleles, including 2 alleles in the Japanese family from our previous report [46% of the 22 mutant alleles]), and three novel mutations--a G-->A transition at base 490 in exon 4 (G164S) (3 alleles [14%]), a C-->A transversion at base 530 in exon 4 (A177D) (1 allele [4%]), and a G-->T transversion at base 88 in exon 1 (E30X) (2 alleles [9%]). FBPase proteins with G164S or A177D mutations were enzymatically inactive when purified from E. coli. Another new mutation, a T-->C transition at base 974 in exon 7 (V325A), was found in the same allele with the G164S mutation in one family (one allele) but was not responsible for FBPase deficiency. Our results indicate that the insertion of one G residue at base 961 was associated with a preferential disease-causing alternation in 13 Japanese patients. Our results also indicate accurate carrier detection in eight families (73%) of 11 Japanese patients with FBPase deficiency, in whom mutations in both alleles were identified.

Characterization of the allosteric binding pocket of human liver fructose-1,6-bisphosphatase by protein crystallography and inhibitor activity studies

The structures of three complexes of human fructose-1,6-bisphosphatase (FB) with the allosteric inhibitor AMP and two AMP analogues have been determined and all fully refined. The data used for structure determination were collected at cryogenic temperature (110 K), and with the use of synchrotron radiation. The structures reveal a common mode of binding for AMP and formycine monophosphate (FMP). 5-Amino-4-carboxamido-1 beta-D-5-phosphate-ribofuranosyl-1H-imidazole (AICAR-P) shows an unexpected mode of binding to FB, different from that of the other two ligands. The imidazole ring of AICAR-P is rotated 180 degrees compared to the AMP and FMP bases. This rotation results in a slightly different hydrogen bonding pattern and minor changes in the water structure in the binding pocket. Common features of binding are seen for the ribose and phosphate moieties of all three compounds. Although binding in a different mode, AICAR-P is still capable of making all the important interactions with the residues building the allosteric binding pocket. The IC50 values of AMP, FMP, and AICAR-P were determined to be 1.7, 1.4, and 20.9 microM, respectively. Thus, the approximately 10 times lower potency of AICAR-P is difficult to explain solely from the variations observed in the binding pocket. Only one water molecule in the allosteric binding pocket was found to be conserved in all four subunits in all three structures. This water molecule coordinates to a phosphate oxygen atom and the N7 atom of the AMP molecule, and to similarly situated atoms in the FMP and AICAR-P complexes. This implies an important role of the conserved water molecule in binding of the ligand.

Crystal structures of the active site mutant (Arg-243-->Ala) in the T and R allosteric states of pig kidney fructose-1,6-bisphosphatase expressed in Escherichia coli

The active site of pig kidney fructose-1,6-bisphosphatase (EC 3.1.3.11) is shared between subunits, Arg-243 of one chain interacting with fructose-1,6-bisphosphate or fructose-2,6-bisphosphate in the active site of an adjacent chain. In this study, we present the X-ray structures of the mutant version of the enzyme with Arg-243 replaced by alanine, crystallized in both T and R allosteric states. Kinetic characteristics of the altered enzyme showed the magnesium binding and inhibition by AMP differed slightly; affinity for the substrate fructose-1,6-bisphosphate was reduced 10-fold and affinity for the inhibitor fructose-2,6-bisphosphate was reduced 1,000-fold (Giroux E, Williams MK, Kantrowitz ER, 1994, J Biol Chem 269:31404-31409). The X-ray structures show no major changes in the organization of the active site compared with wild-type enzyme, and the structures confirm predictions of molecular dynamics simulations involving Lys-269 and Lys-274. Comparison of two independent models of the T form structures have revealed small but significant changes in the conformation of the bound AMP molecules and small reorganization of the active site correlated with the presence of the inhibitor. The differences in kinetic properties of the mutant enzyme indicate the key importance of Arg-243 in the function of fructose-1,6-bisphosphatase. Calculations using the X-ray structures of the Arg-243-->Ala enzyme suggest that the role of Arg-243 in the wild-type enzyme is predominantly electrostatic in nature.

Disorders of gluconeogenesis

Gluconeogenesis, or the formation of glucose from mainly lactate/ pyruvate, glycerol and alanine, plays an essential role in the maintenance of normoglycaemia during fasting. Inborn deficiencies are known of each of the four enzymes of the glycolytic-gluconeogenic pathway that ensure a unidirectional flux from pyruvate to glucose: pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. In this paper, the clinical picture, pathophysiology, diagnostic tests, genetics, treatment and prognosis of the deficiencies of fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase are reviewed.

Liver fructose-1,6-bisphosphatase cDNA: trans-complementation of fission yeast and characterization of two human transcripts

The SV40 early promoter is active both in mammalian cells and in the fission yeast Schizosaccharomyces pombe, and is used to drive full-length cDNA in polyvalent pcD-libraries. Two such liver libraries, of human and rat origin, were used to trans-complement a S. pombe mutant deficient in fructose-1,6-bisphosphatase (Fru-1,6-Pase) activity, a key gluconeogenic enzyme restricted to liver, kidney and intestine in mammals. A rat liver Fru-1,6-Pase cDNA was readily cloned and sequenced. Complementary PCR experiments revealed full-length Fru-1,6-Pase cDNA also present in the human liver library, however at a low abundance. Two human liver transcripts were thus characterized. Contrary to expectation, they were not differentially spliced products. They both encoded the same protein and were generated by a polyadenylation choice mechanism. The longest transcript comprised two polyadenylation signals and a consensus GT-rich element for the 3' processing of the upstream site. Rapid amplification of cDNA ends-polymerase chain reaction (RACE-PCR) analysis of 3' ends from hepatic, renal and intestinal mRNA disclosed that both Fru-1,6-Pase transcripts are expressed in the three main gluconeogenic cell types and are subject to insulin differential modulation. On the other hand, overcoming liver cell heterogeneity problems, sequence analysis of 16 independent clones of 3' end-cDNA demonstrated that, in addition to a monocytic type corresponding to a previously described lambda gt11 clone, human liver does not contain a hepatic type Fru-1,6-Pase comprising a liver-specific carboxyl-terminal extension like its rat counterpart. This liver-specific extension is involved in enzyme up-regulation and appears to give a conclusive advantage to the rat hepatic enzyme over the human one when trans-complementing mutant yeast.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycine 122 is essential for cooperativity and binding of Mg2+ to porcine fructose-1,6-bisphosphatase

Site-directed mutagenesis of an amino acid residue in the substrate binding site of porcine fructose-1,6-bisphosphatase was carried out based on the crystal structure of the enzyme (Zhang, Y., Liang, J.-Y., Huang, S., Ke, H., and Lipscomb, W. N. (1993) Biochemistry 32, 1844-1857). A mutant enzyme form of fructose-1,6-bisphosphatase, G122A, was purified and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, circular dichroism spectrometry (CD), and initial rate kinetics. There were no discernible differences between the secondary structures of the wild-type and the mutant enzyme on the basis of CD data. Altering Gly-122 to alanine caused a significant decrease in the enzyme's activity and affinity for Mg2+. The kcat for this mutant enzyme was only about 5% of that of wild-type fructose-1,6-bisphosphatase, and the Ka for Mg2+ was about 20-fold higher than that of the wild-type enzyme. The Ki for AMP was increased 77-fold in the case of the mutant enzyme; however, the Hill coefficient was unaltered. Most importantly, it was observed that replacement of Gly-122 with alanine caused the total loss of cooperativity for Mg2+. It is concluded that Gly-122 is essential for Mg2+ cooperativity and important for binding of Mg2+ and AMP as well as for enzyme activity.