Isolation and sequence analysis of the cDNA for pig kidney fructose 1,6-bisphosphatase

Characterization of recombinant fructose-1,6-bisphosphatase gene mutations: evidence of inhibition/activation of FBPase protein by gene mutation

Specific residues of the highly regulated fructose-1,6-bisphosphatase (FBPase) enzyme serve as important contributors to the catalytic activity of the enzyme. Previous clinical studies exploring the genetic basis of hypoglycemia revealed two significant mutations in the coding region of the FBPase gene in patients with hypoglycemia, linking the AMP-binding site to the active site of the enzyme. In the present study, a full kinetic analysis of similar mutants was performed. Kinetic results of mutants Y164A and M177A revealed an approximate two to three-fold decrease in inhibitory constants (K _i's) for natural inhibitors AMP and fructose-2,6-bisphosphate (F2,6-BP) compared with the Wild-type enzyme (WT). A separate mutation (M248D) was performed in the active site of the enzyme to investigate whether the enzyme could be activated. This mutant displayed an approximate seven-fold increase in K _i for F2,6-BP. Interfacial mutants L56A and L73A exhibited an increase in K _i for F2,6-BP by approximately five-fold. Mutations in the AMP-binding site (K112A and Y113A) demonstrated an eight to nine-fold decrease in AMP inhibition. Additionally, mutant M248D displayed a four-fold decrease in its apparent Michelis constant (K _m), and a six-fold increase in catalytic efficiency (CE). The importance-and medical relevance-of specific residues for FBPase structural/functional relationships in both the catalytic site and AMP-binding site is discussed.

Structural and biochemical characterization of fructose-1,6/sedoheptulose-1,7-bisphosphatase from the cyanobacterium Synechocystis strain 6803

Cyanobacterial fructose-1,6/sedoheptulose-1,7-bisphosphatase (cy-FBP/SBPase) plays a vital role in gluconeogenesis and in the photosynthetic carbon reduction pathway, and is thus a potential enzymatic target for inhibition of harmful cyanobacterial blooms. Here, we describe the crystal structure of cy-FBP/SBPase in complex with AMP and fructose-1,6-bisphosphate (FBP). The allosteric inhibitor AMP and the substrate FBP exhibit an unusual binding mode when in complex with cy-FBP/SBPase. Binding mode analysis suggested that AMP bound to the allosteric sites near the interface across the up/down subunit pairs C1C4 and C2C3 in the center of the tetramer, while FBP binds opposite to the interface between the horizontal subunit pairs C1C2 or C3C4. We identified a series of residues important for FBP and AMP binding, and suggest formation of a disulfide linkage between Cys75 and Cys99. Further analysis indicates that cy-FBP/SBPase may be regulated through ligand binding and alteration of the structure of the enzyme complex. The interactions between ligands and cy-FBP/SBPase are different from those of ligand-bound structures of other FBPase family members, and thus provide new insight into the molecular mechanisms of structure and catalysis of cy-FBP/SBPase. Our studies provide insight into the evolution of this enzyme family, and may help in the design of inhibitors aimed at preventing toxic cyanobacterial blooms.

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.

Purification, kinetic studies, and homology model of Escherichia coli fructose-1,6-bisphosphatase

Previous kinetic characterization of Escherichia coli fructose 1,6-bisphosphatase (FBPase) was performed on enzyme with an estimated purity of only 50%. Contradictory kinetic properties of the partially purified E. coli FBPase have been reported in regard to AMP cooperativity and inactivation by fructose-2,6-bisphosphate. In this investigation, a new purification for E. coli FBPase has been devised yielding enzyme with purity levels as high as 98%. This highly purified E. coli FBPase was characterized and the data compared to that for the pig kidney enzyme. Also, a homology model was created based upon the known three-dimensional structure of the pig kidney enzyme. The kcat of the E. coli FBPase was 14.6 s(-1) as compared to 21 s(-1) for the pig kidney enzyme, while the K(m) of the E. coli enzyme was approximately 10-fold higher than that of the pig kidney enzyme. The concentration of Mg2+ required to bring E. coli FBPase to half maximal activity was estimated to be 0.62 mM Mg2+, which is twice that required for the pig kidney enzyme. Unlike the pig kidney enzyme, the Mg2+ activation of the E. coli FBPase is not cooperative. AMP inhibition of mammalian FBPases is cooperative with a Hill coefficient of 2; however, the E. coli FBPase displays no cooperativity. Although cooperativity is not observed, the E. coli and pig kidney enzymes show similar AMP affinity. The quaternary structure of the E. coli enzyme is tetrameric, although higher molecular mass aggregates were also observed. The homology model of the E. coli enzyme indicated slight variations in the ligand-binding pockets compared to the pig kidney enzyme. The homology model of the E. coli enzyme also identified significant changes in the interfaces between the subunits, indicating possible changes in the path of communication of the allosteric signal.

AMP inhibition of pig kidney fructose-1,6-bisphosphatase

Lys-112 and Tyr-113 in pig kidney fructose-1,6-bisphosphatase (FBPase) make direct interactions with AMP in the allosteric binding site. Both residues interact with the phosphate moiety of AMP while Tyr-113 also interacts with the 3'-hydroxyl of the ribose ring. The role of these two residues in AMP binding and allosteric inhibition was investigated. Site-specific mutagenesis was used to convert Lys-112 to glutamine (K112Q) and Tyr-113 to phenylalanine (Y113F). These amino acid substitutions result in small alterations in k(cat) and increases in K(m). However, both the K112Q and Y113F enzymes show alterations in Mg(2+) affinity and dramatic reductions in AMP affinity. For both mutant enzymes, the AMP concentration required to reduced the enzyme activity by one-half, [AMP](0.5), was increased more than a 1000-fold as compared to the wild-type enzyme. The K112Q enzyme also showed a 10-fold reduction in affinity for Mg(2+). Although the allosteric site is approximately 28 A from the metal binding sites, which comprise part of the active site, these site-specific mutations in the AMP site influence metal binding and suggest a direct connection between the allosteric and the active sites.

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.

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

To understand the mechanism of signal propagation involved in the cooperative AMP inhibition of the homotetrameric enzyme pig-kidney fructose-1,6-bisphosphatase, Arg49 and Lys50 residues located at the C1-C2 interface of this enzyme were replaced using site-directed mutagenesis. The mutant enzymes Lys50Ala, Lys50Gln, Arg49Ala and Arg49Gln were expressed in Escherichia coli, purified to homogeneity and the initial rate kinetics were compared with the wild-type recombinant enzyme. The mutants exhibited kcat, Km and I50 values for fructose-2,6-bisphosphate that were similar to those of the wild-type enzyme. The kinetic mechanism of AMP inhibition with respect to Mg2+ was changed from competitive (wild-type) to noncompetitive in the mutant enzymes. The Lys50Ala and Lys50Gln mutants showed a biphasic behavior towards AMP, with total loss of cooperativity. In addition, in these mutants the mechanism of AMP inhibition with respect to fructose-1,6-bisphosphate changed from noncompetitive (wild-type) to uncompetitive. In contrast, AMP inhibition was strongly altered in Arg49Ala and Arg49Gln enzymes; the mutants had > 1000-fold lower AMP affinity relative to the wild-type enzyme and exhibited no AMP cooperativity. These studies strongly indicate that the C1-C2 interface is critical for propagation of the cooperative signal between the AMP sites on the different subunits and also in the mechanism of allosteric inhibition of the enzyme by AMP.

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.

Importance of the dimer-dimer interface for allosteric signal transduction and AMP cooperativity of pig kidney fructose-1,6-bisphosphatase. Site-specific mutagenesis studies of Glu-192 and Asp-187 residues on the 190's loop

The role of the 190's loop of fructose-1,6-bisphosphatase (Fru-1, 6-P2ase) in the allosteric regulation of Fru-1,6-P2ase has been investigated through kinetic studies on three mutant enzymes, Glu-192 --> Ala, Glu-192 --> Gln, and Asp-187 --> Ala. AMP is an allosteric inhibitor, which binds to the regulatory sites and induces the R- to T-state transition; for wild-type Fru-1,6-P2ase AMP inhibition is cooperative with a Hill coefficient of 2.0. The replacement of Asp-187, which forms an interaction across the C1:C2 monomer-monomer interface, with alanine did not change the catalytic efficiency, and it had no effect on the cooperativity of AMP inhibition; however, the apparent dissociation constant for AMP increased more than 4-fold as compared to the value for the wild-type enzyme. The replacement of Glu-192, which forms interactions across the C1:C4 dimer-dimer interface, with Ala and Gln lowered kcat from 21 s-1 for wild-type enzyme to 15 s-1 and 13 s-1, respectively, for the mutant enzymes, while their respective Km values were not changed. However, these replacements did have dramatic effects on AMP inhibition; first, cooperative AMP inhibition was lost; second, the AMP inhibition was biphasic, which can be interpreted as due to AMP binding to two classes of binding sites. The high affinity class of sites corresponds to the regulatory sites, while the low affinity class of sites may be the active sites. The results reported here, combined with the structural and kinetic results from the Lys-42 --> Ala enzyme, strongly suggest that the C1:C4 dimer-dimer interface, rather than the C1:C2 monomer-monomer interface, is critical for the propagation of the allosteric signal between the AMP sites on different subunits; in addition, cooperative AMP inhibition is essential for the enzyme to be fully inhibited by the binding of AMP to the allosteric site.

Evidence for an active T-state pig kidney fructose 1,6-bisphosphatase: interface residue Lys-42 is important for allosteric inhibition and AMP cooperativity

During the R-->T transition in the tetrameric pig kidney fructose-1,6-bisphosphatase (Fru-1,6-P2ase, EC 3.1.3.11) a major change in the quaternary structure of the enzyme occurs that is induced by the binding of the allosteric inhibitor AMP (Ke HM, Liang JY, Zhang Y, Lipscomb WN, 1991, Biochemistry 30:4412-4420). The change in quaternary structure involving the rotation of the upper dimer by 17 degrees relative to the lower dimer is coupled to a series of structural changes on the secondary and tertiary levels. The structural data indicate that Lys-42 is involved in a complex set of intersubunit interactions across the dimer-dimer interface with residues of the 190's loop, a loop located at the pivot of the allosteric rotation. In order to test the function of Lys-42, we have replaced it with alanine using site-specific mutagenesis. The kcat and K(m) values for Lys-42-->Ala Fru-1,6-P2ase were 11 s-1 and 3.3 microM, respectively, resulting in a mutant enzyme that was slightly less efficient catalytically than the normal pig kidney enzyme. Although the Lys-42-->Ala Fru-1,6-P2ase was similar kinetically in terms of K(m) and kcat, the response to inhibition by AMP was significantly different than that of the normal pig kidney enzyme. Not only was AMP inhibition no longer cooperative, but also it occurred in two stages, corresponding to high- and low-affinity binding sites. Saturation of the high-affinity sites only reduced the activity by 30%, compared to 100% for the wild-type enzyme. In order to determine in what structural state the enzyme was after saturation of the high-affinity sites, the Lys-42-->Ala enzyme was crystallized in the presence of Mn2+, fructose-6-phosphate (Fru-6-P), and 100 microM AMP and the data collected to 2.3 A resolution. The X-ray structure showed the T state with AMP binding with full occupancy to the four regulatory sites and the inhibitor Fru-6-P bound at the active sites. The results reported here suggest that, in the normal pig kidney enzyme, the interactions between Lys-42 and residues of the 190's loop, are important for propagation of AMP cooperativity to the adjacent subunit across the dimer-dimer interface as opposed to the monomer-monomer interface, and suggest that AMP cooperativity is necessary for full allosteric inhibition by AMP.

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.

Cytosolic fructose-1,6-bisphosphatase: A key enzyme in the sucrose biosynthetic pathway

Fructose-1,6 bisphosphatase (FBPase) is a ubiquitous enzyme controlling a key reaction. In non-photosynthetic tissues, it regulates the rate of gluconeogenesis. In photosynthetic tissues, two FBPase isozymes (chloroplastic and cytosolic) play key roles in carbon assimilation and metabolism. The cytosolic FBPase is one of the regulatory enzymes in the sucrose biosynthetic pathway - its activity is regulated by both fine and coarse control mechanisms. Kinetic and allosteric properties of the plant cytosolic FBPase are remarkably similar to the mammalian and yeast FBPase, but differ greatly from those of the chloroplastic FBPase. Cytosolic FBPase is relatively conserved among various organisms both at amino acid and nucleotide sequence levels. There is slightly higher similarity between mammalian FBPase and plant cytosolic FBPase than there is between the two plant FBPases. Expression of plant cytosolic FBPase gene is developmentally regulated and appears to be coordinated with the expression of Rubisco and other carbon metabolism enzymes. Similar to the gluconeogenic FBPase, relatively rapid end product repression of FBPase gene occurs in plant. However, unlike the gluconeogenic FBPase, a concurrent decline in plant FBPase activity does not occur in response to increased end product levels. The physiological significance of FBPase gene repression, therefore, remains unclear in plants. Both expression and activity of the cytosolic FBPase are regulated by environmental factors such as light and drought conditions. Light-dependent modulation of FBPase activity in plants appears to involve some type of posttranslational modification. In addition to elucidating the exact nature of the presumed posttranslational modification, cloning of genomic and upstream sequences is needed before we fully understand the molecular regulation of the cytosolic FBPase in plants. Use of transgenic plants with altered rates of FBPase activity offers potential for enhanced crop productivity.