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.

Site-directed mutagenesis of the substrate binding site of porcine fructose-1,6-bisphosphatase

Asn 212, Arg 243, Tyr 244, Tyr 264, and Lys 274, which are conserved in all known primary sequences of fructose-1, 6-bisphosphatase, are located in the substrate binding domain on the basis of the crystal structure of the enzyme. Mutations of the five residues of porcine liver fructose-1,6-bisphosphatase (Asn212Ala, Arg243-Met, Tyr244Phe, Tyr264Phe, and Lys274Leu) were carried out by site-directed mutagenesis. The wild-type and mutant forms of the enzyme were purified to homogeneity and characterized by initial rate kinetics and circular dichroism spectrometry. The mutants exhibited kcat values that are similar to those of the wild-type enzyme. The Km values for fructose 1,6-bisphosphate of the mutants are 6- to 44-fold higher than that of the wild-type enzyme. The Ki values for fructose 2,6-bisphosphate and AMP of the mutants increased from 56- to 1950-fold and 12- to 27-fold, respectively, relative to the wild-type enzyme. The alteration of inhibition constants for both inhibitors suggest that these five active site residues are involved in the inhibition by fructose 2,6-bisphosphate and AMP. No apparent differences in secondary structure of the wild-type and mutant forms of fructose-1,6-bisphosphatase were observed as measured by circular dichroism spectrometry. This report demonstrates that Asn 212, Arg 243, Tyr 244, Tyr 264, and Lys 274 not only are the sites for substrate binding, but also play an important role in the binding affinity of inhibitors fructose 2,6-bisphosphate and AMP.

Structural aspects of the allosteric inhibition of fructose-1,6-bisphosphatase by AMP: the binding of both the substrate analogue 2,5-anhydro-D-glucitol 1,6-bisphosphate and catalytic metal ions monitored by X-ray crystallography

The crystal structures of the T form pig kidney fructose-1,6-bisphosphatase (EC 3.1.3.11) complexed with AMP, the substrate analogue 2,5-anhydro-D-glucitol 1,6-bisphosphate (AhG-1,6-P2), and Mn2+ at concentrations of 5, 15, 100, and 300 microM have been determined and refined at resolutions of 2.1-2.3 A to R factors which range from 0.180 to 0.195, respectively. Two metal ions per active site have been identified, one at a binding site of high affinity (metal site 1'), the second in a low affinity site (metal site 2'). The 1-phosphate group of the substrate analogue coordinates to the metal ion at site 1', but not at site 2'. In these four complexes, the distances between the two metal ions are all within 0.2 A of 4.3 A. In the previously determined R form structure of Fru-1,6-Pase complexed with AhG-1,6-P2 and Mn2+, there are also two metal ions in the active site at metal sites 1 and 2. The metal ion at site 1 is only 0.6 A displaced from the metal ion at site 1' in the T form and is also coordinated to the 1-phosphate group of AhG-1,6-P2. However, the second metal ion is located in two distinct sites which are 1.4 A apart in the T and R form structures. In the R form the Mn2+ at site 2 is coordinated to the 1-phosphate group of the substrate analogue. This metal ion is apparently required to orient the phosphate group for nucleophilic attack at the phosphorus center.(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.

Crystal structure of fructose-1,6-bisphosphatase complexed with fructose 2,6-bisphosphate, AMP, and Zn2+ at 2.0-A resolution: aspects of synergism between inhibitors

The crystal structure of fructose-1,6-bisphosphatase (Fru-1,6-Pase; EC 3.1.3.11) complexed with Zn2+ and two allosteric regulators, AMP and fructose 2,6-bisphosphate (Fru-2,6-P2) has been determined at 2.0-A resolution. In the refined model, the crystallographic R factor is 0.189 with rms deviations of 0.014 A and 2.8 degrees from ideal geometries for bond lengths and bond angles, respectively. A 15 degrees rotation is observed between the upper dimer C1C2 and the lower dimer C3C4 relative to the R-form structure (fructose 6-phosphate complex), consistent with that expected from a T-form structure. The major difference between the structure of the previously determined Fru-2,6-P2 complex (R form) and that of the current quaternary T-form complex lies in the active site domain. A zinc binding site distinct from the three binding sites established earlier was identified within each monomer. Helix H4 (residues 123-127) was found to be better defined than in previously studied ligated Fru-1,6-Pase structures. Interactions between monomers in the active site domain were found involving H4 residues from one monomer and residues Tyr-258 and Arg-243 from the adjacent monomer. Cooperativity between AMP and Fru-2,6-P2 in signal transmission probably involves the following features: an AMP site, the adjacent B3 strand (residues 113-118), the metal site, the immediate active site, the short helix H4 (residues 123-127), and Tyr-258 and Arg-243 from the adjacent monomer within the upper (or lower) dimer. The closest distance between the immediate active site and that on the adjacent monomer is only 5 A. Thus, the involvement of H4 in signal transmission adds another important pathway to the scheme of the allosteric mechanism of Fru-1,6-Pase.

Shared active sites of fructose-1,6-bisphosphatase. Arginine 243 mediates substrate binding and fructose 2,6-bisphosphate inhibition

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, Arg-243 was replaced by alanine using techniques of site-specific mutagenesis and the cloned pig kidney enzyme expressed in Escherichia coli. Compared with wild-type enzyme, kinetic parameters of the altered enzyme characterizing catalytic efficiency, magnesium binding, and inhibition by AMP differed but by less than an order of magnitude; affinity for substrate fructose 1,6-bisphosphate was 10-fold poorer, and affinity for inhibitor fructose 2,6-bisphosphate was 1000-fold poorer. Molecular dynamics simulations were undertaken to determine possible alterations in active sites of the enzyme due to replacement of Arg-243 by Ala and suggested that in the mutant enzyme loss of one cationic group leads to reorganization of the active site especially involving lysine residues 269 and 274. The differences in properties of the mutant enzyme indicate the key importance of Arg-243 in the function of fructose-1,6-bisphosphatase and confirm on a functional basis the shared active site in this important metabolic enzyme.

The allosteric site of human liver fructose-1,6-bisphosphatase. Analysis of six AMP site mutants based on the crystal structure

The molecular structure of human liver fructose-1,6-bisphosphatase complexed with AMP was determined by x-ray diffraction using molecular replacement, starting from the pig kidney enzyme AMP complex. Of the 34 amino acid residues which differ between these two sequences, only one interacts with AMP; Met30 in pig kidney is Leu30 in human liver. From this analysis, six sites in which side chains of amino acid residues are in contact with AMP, Ala24, Leu30, Thr31, Tyr113, Arg140, and Met177, were mutated by polymerase chain reaction. The wild-type and mutant forms were expressed in Escherichia coli, purified, and their kinetic properties determined. Circular dichroism spectra of the mutants were indistinguishable from that of the wild-type enzyme. Kinetic analyses revealed that all forms had similar turnover numbers, Km values for fructose 2,6-bisphosphate, and inhibition constants for fructose 2,6-bisphosphate. Apparent Ki values for AMP inhibition of the Leu30 --> Phe and Met177 --> Ala mutants were similar to those of the wild-type enzyme, but the apparent Ki values for the Arg140 --> Ala and Ala24 --> Phe mutants were 7-to 20-fold higher, respectively. The Thr31 --> Ser mutant exhibited a 5-fold increase in apparent Ki for AMP, while mutation of Thr31 to Ala increased the apparent Ki 120-fold. AMP inhibition of the Tyr113 --> Phe mutant was undetectable even at millimolar AMP concentrations. Fructose 2,6-bisphosphate potentiated AMP inhibition of the mutants to the same extent as for the wild-type enzyme, except in the case of the Thr31 --> Ala and Tyr113 --> Phe mutants. Thus, the Met177 --> Ala mutant suggests that the side chain beyond C alpha is not needed for AMP binding, and that the Leu30 --> Phe mutant preserves the AMP contacts with these side chains. Thr31, Tyr113, and Arg140 form key hydrogen bonds to AMP consistent with strong side chain interactions in the wild-type enzyme. Finally, the absence of any effect of fructose 2,6-bisphosphate on AMP inhibition observed in the Thr31 --> Ala mutant may be an important clue relating to the mechanism of synergism of these two inhibitors.

Inactivation precedes changes in allosteric properties and conformation of D-glyceraldehyde-3-phosphate dehydrogenase and fructose-1,6-bisphosphatase during denaturation by guanidinium chloride

It has been shown that inactivation of several enzymes precedes overall conformational changes of the enzyme molecules as a whole during denaturation [Tsou (1993) Science, 262, 380-381]. However, the relation between inactivation, loss of allosteric properties of oligomeric enzymes and unfolding of the enzyme molecule during denaturation remain little explored. These have now been compared for D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and fructose-1,6-bisphosphatase (FruP2ase) during denaturation by guanidinium chloride (GdmCl). GAPDH is completely inactivated at 0.3 M GdmCl but at this GdmCl concentration it still binds NAD+ with negative co-operativity. At 0.4 M GdmCl, inactivation of FruP2ase reaches completion whereas its allosteric properties, including the heterotropic effect of AMP inhibition and K+ activation with positive co-operativity, are only partially affected. Much higher GdmCl concentrations are required to bring about unfolding of the overall structures of both enzymes.

Catabolite inactivation of fructose-1,6-bisphosphatase in yeast is mediated by the proteasome

Fructose-1,6-bisphosphatase, a key enzyme in gluconeogenesis, undergoes catabolite inactivation when glucose is added to gluconeogenetically active cells of the yeast Saccharomyces cerevisiae. Phosphorylation of the enzyme is followed by rapid degradation. To elucidate the cellular proteolytic system involved in catabolite-triggered degradation of fructose-1,6-bisphosphatase this event was followed in different protease-deficient yeast mutants. In a mutant defective in the proteolytic function of the vacuole the degradation rate of the enzyme is not diminished. In contrast mutants defective in the proteolytic activity of the proteasome exhibit a strongly reduced glucose-induced degradation of fructose-1,6-bisphosphatase as compared to their isogenic wild-type counterparts. Our studies suggest that catabolite inactivation of fructose-1,6-bisphosphatase occurs in the cytosol, the degradation event being mediated by the proteasome. An explanation is presented which tries to resolve the formerly conflicting results, which suggested glucose-triggered uptake of fructose-1,6-bisphosphatase into the vacuole followed by vacuolar proteolysis.

Catabolite inactivation of heterologous fructose-1,6-bisphosphatases and fructose-1,6-bisphosphatase-beta-galactosidase fusion proteins in Saccharomyces cerevisiae

Fructose-1,6-bisphosphatase (FruP2ase) from Saccharomyces cerevisiae is rapidly inactivated upon addition of glucose to a culture growing on non-sugar carbon sources. Under the same conditions the FruP2ases from Schizosaccharomyces pombe or Escherichia coli expressed in S. cerevisiae were not affected. A chimaeric protein containing the first 178 amino acids from the N-terminal half of S. cerevisiae FruP2ase fused to E. coli beta-galactosidase was susceptible to catabolite inactivation. Elimination of a putative destruction box, RAELVNLVG ... KK .... K., beginning at amino acid 60 did not prevent catabolite inactivation. Similarly a change of the vacuole-targeting sequence QKKLD, amino acids 80-84, to QKNSD did not affect significantly the course of inactivation of beta-galactosidase. A fusion protein carrying only the first 138 amino acids from FruP2ase was inactivated at a higher rate than the one carrying the first 178, suggesting the existence of a protective region between amino acids 138 and 178. A fusion protein carrying the first 81 amino acids from FruP2ase was inactivated by glucose at a similar rate to the one carrying the 178 amino acids, but one with only the first 18 amino acids was resistant to catabolite inactivation. Inactivation of FruP2ase in mutants ubr1 that lack a protein required for ubiquitin-dependent proteolysis, or pra1 that lack vacuolar protease A, proceeded as in a wild type. Our results suggest that at least two domains of FruP2ase may mark beta-galactosidase for catabolite inactivation and that FruP2ase can be inactivated by a mechanism independent of transfer to the vacuole.

Enhancement of the reductive activation of chloroplast fructose-1,6-bisphosphatase by modulators and protein perturbants

To characterize the mechanism of chloroplast fructose-1,6-bisphosphatase activation, we have examined kinetic and structural changes elicited by protein perturbants and reductants. At variance with its well-known capacity for enzyme inactivation, 150 mM sodium trichloroacetate yielded an activatable chloroplast fructose-1,6-bisphosphatase in the presence of 1.0 mM fructose 1,6-bisphosphate and 0.1 mM Ca2+. Other sugar bisphosphates did not replace fructose 1,6-bisphosphate whereas Mg2+ and Mn2+ were functional in place of Ca2+. Variations of the emission fluorescence of intrinsic fluorophores and a noncovalently bound extrinsic probe [2-(p-toluidinyl)naphthalene-6-sulfonate] indicated the presence of conformations different from the native form. A similar conclusion was drawn from the analysis of absorption spectra by means of fourth-derivative spectrophotometry. The effect of these conformational changes on the reductive process was studied by subsequently incubating the enzyme with dithiothreitol. The reaction of chloroplast fructose-1,6-bisphosphatase with dithiothreitol was accelerated 13-fold by the chaotropic anion: second-order rate constants were 48.1 M-1.min-1 and 3.7 M-1.min-1 in the presence and in the absence of trichloroacetate, respectively. Thus, the enhancement of the reductive activation by compounds devoid of redox activity illustrated that the modification of intramolecular noncovalent interactions of chloroplast fructose-1,6-bisphosphatase plays an essential role in the conversion of enzyme disulfide bonds to sulfhydryl groups. In consequence, a conformational change would operate concertedly with the reduction of disulfide bridges in the light-dependent activation mediated by the ferredoxin-thioredoxin system.

Replacement of glutamic acid 29 with glutamine leads to a loss of cooperativity for AMP with porcine fructose-1,6-bisphosphatase

Mutations in the AMP binding site of porcine fructose-1,6-bisphosphatase were carried out by site-specific mutagenesis based on the crystal structure of the enzyme (Ke, H., Zhang, Y., and Lipscomb, W.L. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 5243-5247). The mutant and wild-type enzymes were characterized by SDS-polyacrylamide gel electrophoresis, circular dichroism spectrometry, and initial rate kinetics. One of the mutant forms of fructose-1,6-bisphosphatase, Glu-29-->Gln, is ligated to the phosphoryl moiety of AMP, a potent inhibitor of the reaction, whereas the other mutant, Thr-31-->Val, is associated with the purine base of AMP. No discernible alteration in structure as measured by circular dichroism spectrometry was noted for the mutants relative to the wild-type enzyme. As expected, major changes in kinetic parameters between the mutants and the wild-type enzyme were associated with inhibition by AMP. AMP, a competitive inhibitor with respect to Mg2+ in the fructose-1,6-bisphosphatase reaction, exhibits cooperativity in the case of the wild-type and the mutant Thr-31-->Val enzymes with a Hill coefficient of 2.0. On the other hand, cooperativity is completely lost in the case of Glu-29-->Gln fructose-1,6-bisphosphatase.

Antigenic relationships between chloroplast and cytosolic fructose-1,6-bisphosphatases

Cytosolic fructose-1,6-biphosphatases (FBPase, EC 3.1.3.11) from pea (Pisum sativum L. cv Lincoln) and spinach (Spinacia oleracea L. cv Winter Giant) did not cross-react by double immunodiffusion and western blotting with either of the antisera raised against the chloroplast enzyme of both species; similarly, pea and spinach chloroplast FBPases did not react with the spinach cytosolic FBPase antiserum. On the other hand, spinach and pea chloroplast FBPases showed strong cross-reactions against the antisera to chloroplast FBPases, in the same way that the pea and spinach cytosolic enzymes displayed good cross-reactions against the antiserum to spinach cytosolic FBPase. Crude extracts from spinach and pea leaves, as well as the corresponding purified chloroplast enzymes, showed by western blotting only one band (44 and 43 kD, respectively) in reaction with either of the antisera against the chloroplast enzymes. A unique fraction of molecular mass 38 kD appeared when either of the crude extracts or the purified spinach cytosolic FBPase were analyzed against the spinach cytosolic FBPase antiserum. These molecular sizes are in accordance with those reported for the subunits of the photosynthetic and gluconeogenic FBPases. Chloroplast and cytosolic FBPases underwent increasing inactivation when increasing concentrations of chloroplast or cytosolic anti-FBPase immunoglobulin G (IgG), respectively, were added to the reaction mixture. However, inactivations were not observed when the photosynthetic enzyme was incubated with the IgG to cytosolic FBPase, or vice versa. Quantitative results obtained by enzyme-linked immunosorbent assays (ELISA) showed 77% common antigenic determinants between the two chloroplast enzymes when tested against the spinach photosynthetic FBPase antiserum, which shifted to 64% when assayed against the pea antiserum. In contrast, common antigenic determinats between the spinach cytosolic FBPase and the two chloroplast enzymes were less than 10% when the ELISA test was carried out with either of the photosynthetic FBPase antisera, and only 5% when the assay was performed with the antiserum to the spinach cytosolic FBPase. These results were supported by sequencing data: the deduced amino acid sequence of a chloroplast FBPase clone isolated from a pea cDNA library indicated a 39,253 molecular weight protein, with a homology of 85% with the spinach chloroplast FBPase but only 48.5% with the cytosolic enzyme from spinach.

Regulation of rat-kidney cortex fructose-1,6-bisphosphatase activity. II. Effects of adenine nucleotides

1. The native rat-kidney cortex Fructose-1,6-bisphosphatase is differentially regulated by adenine nucleotides in the presence of divalent cations. 2. Binding of AMP and ADP to the enzyme is co-operative. The inhibition by both nucleotides show an uncompetitive mechanism AMP being the most efficient inhibitor. 3. Mg2+ decreases the inhibition produced by AMP and ADP by enhancing their I0.5 and completely annulates the inhibitory effect of ATP. 4. In the presence of Mn2+ ADP behaves as an inhibitor but no inhibition is evident with AMP, suggesting the existence of different allosteric sites for each nucleotide.

Regulation of rat-kidney cortex fructose-1,6-bisphosphatase activity. I. Effects of fructose-2,6-bisphosphate and divalent cations

1. The native rat-kidney cortex Fructose-1,6-BPase is differentially regulated by Mg2+ and Mn2+. 2. Mg2+ binding to the enzyme is hyperbolic and large concentrations of the cation are non-inhibitory. 3. Mn2+ produces a 10-fold rise in Vmax higher than Mg2+. [Mn2+]0.5 is much larger than [Mg2+]0.5. At elevated [Mn2+] inhibition is observed. 4. Mg2+ and Mn2+ produce antagonistic effects on the inhibition of the enzyme by high substrate. 5. Fru-2,6-P2 inhibits the enzyme by rising the S0.5 and favouring a sigmoidal kinetics. 6. The inhibition by Fru-2,6-P2 is released by Mg2+ and more powerfully by Mn2+ increasing the I0.5.

Glycolysis revisited

Glycolysis is usually considered as a paradigm metabolic pathway, due to the fact that it is present in most organisms, and also because it is the pathway by which an important nutrient, glucose, is consumed. Far from being completely understood, the regulation of this pathway witnessed several important progresses during the last few years. One of these is the discovery of fructose 2,6-bisphosphate, a potent stimulator of phosphofructokinase and inhibitor of fructose-1,6-bisphosphatase. Originally found in the liver during the course of a study on the mechanism by which glucagon acts on gluconeogenesis, this compound is now recognized as a major element in the control of glycolysis and/or gluconeogenesis in many cell types and in various organisms. The other finding is that of a regulatory protein that modulates the activity of glucokinase, the enzyme that phosphorylates glucose in the liver and in the beta cells of pancreatic islets.

Kinetic parameters of human and rabbit liver D-fructose 1,6-diphosphate 1-phosphohydrolase determined at 25 and 37 degrees C

1. Kinetic parameters of human and rabbit liver D-fructose 1,6-diphosphate 1-phosphohydrolase (EC 3.1.3.11) (FDP-ase) at 25 and 37 degrees C have been determined. 2. Km determined at 25 degrees C were 1.4 microM for human and 1.6 microM for rabbit enzyme; at 37 degrees C, corresponding values were 1.7 and 1.8 microM. 3. Both enzymes are allosterically inhibited by AMP. Respective values of I0.5 were 7.2 microM for human and 13.2 microM for rabbit at 25 degrees C, and 16.6 microM for human and 27.3 microM for rabbit at 37 degrees C. 4. Fructose 2,6-diphosphate, a potent regulator of gluconeogenesis, is more effective at 25 than at 37 degrees C. Ki determined at 25 degrees C was 0.07 microM for human and 0.035 microM for rabbit in comparison with 0.17 microM for human and 0.09 microM for rabbit at 37 degrees C. 5. Affinity of FDP-ase for magnesium is also dependent on temperature. For the human enzyme, Km at 25 degrees C was 226 microM and at 37 degrees C, 176 microM. For the rabbit enzyme, corresponding values were 256 and 240 microM. 6. Both enzymes are activated by KCl. Determined values of A0.5 were 91 mM for human, and 50 mM for rabbit enzyme at 25 degrees C, and 129 mM for human and 100 mM for rabbit enzyme at 37 degrees C.

Low activity of the yeast cAMP-dependent protein kinase catalytic subunit Tpk3 is due to the poor expression of the TPK3 gene

Three genes TPK1, TPK2 and TPK3 encode in Saccharomyces cerevisiae distinct catalytic subunits of cAMP-dependent protein kinase (cAPK). We have measured cAPK activity in vitro and, indirectly, in vivo in yeast strains carrying only one of the three TPK genes. The strain containing TPK3 as the only intact TPK gene showed nearly undetectable phosphorylating activity and no TPK3 mRNA could be detected, although the cells grow normally. Overexpression of TPK3 in a high copy vector or under the control of the inducible GAL1 promoter did not by itself result in a corresponding increase in activity but coexpression of BCY1, the gene coding for the regulatory subunit, was necessary in both cases to achieve high levels of phosphorylating activity. Moreover, BCY1 overexpression not only increased Tpk3 catalytic activity but also increased the amount of TPK3 mRNA detected in Northern blots.

Glucose-induced degradation of the MDH2 isozyme of malate dehydrogenase in yeast

MDH2, the nonmitochondrial isozyme of malate dehydrogenase in Saccharomyces cerevisiae, was determined to be a target of glucose-induced proteolytic degradation. Shifting a yeast culture growing with acetate to medium containing glucose as a carbon source resulted in a 25-fold increase in turnover of MDH2. A truncated form of MDH2 lacking amino acid residues 1-12 was constructed by mutagenesis of the MDH2 gene and expressed in a haploid yeast strain containing a deletion disruption of the corresponding chromosomal gene. Measurements of malate dehydrogenase specific activity and determination of growth rates with diagnostic carbon sources indicated that the truncated form of MDH2 was expressed at authentic MDH2 levels and was fully active. However, the truncated enzyme proved to be less susceptible to glucose-induced proteolysis, exhibiting a 3.75-fold reduction in turnover rate following a shift to glucose medium. Rates of loss of activity for other cellular enzymes known to be subject to glucose inactivation were similarly reduced. An extended lag in attaining wild type rates of growth on glucose measured for strains expressing the truncated MDH2 enzyme represents the first evidence of a selective advantage for the phenomenon of glucose-induced proteolysis in yeast.

An inositol phosphate glycan derived from a Trypanosoma brucei glycosyl-phosphatidylinositol mimics some of the metabolic actions of insulin

Some of the acute actions of insulin may be mediated by an enzyme-modulating inositol phosphate glycan, produced by the insulin-sensitive hydrolysis of glycosyl-phosphatidylinositol (GPI) that is structurally similar to a membrane protein anchor. An inositol glycan fragment from the structurally characterized Trypanosoma brucei variant surface glycoprotein GPI anchor is evaluated for insulin-mimetic antilipolytic activity. The fragment specifically and dose-dependently inhibits isoproterenol-stimulated lipolysis. Like the effect of insulin, glycan-induced antilipolysis is blocked by the low Km cAMP phosphodiesterase inhibitor imazodan (CI-914) and the serine/threonine phosphatase inhibitor, okadaic acid, suggesting that the activation of both cAMP phosphodiesterase and serine/threonine protein phosphatases are necessary. Moreover, this fragment causes a specific and dose-dependent inhibition of both microsomal glucose-6-phosphatase (EC 3.1.3.9) and cytosolic fructose-1,6-bisphosphatase (EC 3.1.3.11) activity. Additionally, direct addition of the glycan to hepatocytes caused marked inhibition of glucose production from pyruvate. These results suggest that the direct modification of the activities of these two gluconeogenic enzymes by an inositol glycan may play a role in the inhibition of glucose output by insulin and provide the first evidence for the insulin-mimetic properties of a chemically characterized inositol glycan.

Lysine 274 is essential for fructose 2,6-bisphosphate inhibition of fructose-1,6-bisphosphatase

Lysine 274 is conserved in all known fructose-1,6-bisphosphatase sequences. It has been implicated in substrate binding and/or catalysis on the basis of reactivity with pyridoxal phosphate as well as by x-ray crystallographic analysis. Lys274 of rat liver fructose-1,6-bisphosphatase was mutated to alanine by the polymerase chain reaction, and the T7-RNA polymerase-transcribed construct containing the mutant sequence was expressed in Escherichia coli. The mutant and wild-type forms of the enzyme were purified to homogeneity, and their specific activity, substrate dependence, and inhibition by fructose 2,6-bisphosphate and AMP were compared. While the mutant exhibited no change in maximal velocity, its Km for fructose 1,6-bisphosphate was 20-fold higher than that of the wild-type, and its Ki for fructose 2,6-bisphosphate was increased 1000-fold. Consistent with the unaltered maximal velocity, there were no apparent difference between the secondary structure of the wild-type and mutant enzyme forms, as measured by circular dichroism and ultraviolet difference spectroscopy. The Ki for the allosteric inhibitor AMP was only slightly increased, indicating that Lys274 is not directly involved in AMP inhibition. Fructose 2,6-bisphosphate potentiated AMP inhibition of both forms, but 500-fold higher concentrations of fructose 2,6-bisphosphate were needed to reduce the Ki for AMP for the mutant compared to the wild-type. However, potentiation of AMP inhibition of the Lys274----Ala mutant was evident at fructose 2,6-bisphosphate concentrations (approximately 100 microM) well below those that inhibited the enzyme, which suggests that fructose 2,6-bisphosphate interacts either with the AMP site directly or with other residues involved in the active site-AMP synergy. The results also demonstrate that although Lys274 is an important binding site determinant for sugar bisphosphates, it plays a more significant role in binding fructose 2,6-bisphosphate than fructose 1,6-bisphosphate, probably because it binds the 2-phospho group of the former while other residues bind the 1-phospho group of the substrate. It is concluded that the enzyme utilizes Lys274 to discriminate between its substrate and fructose 2,6-bisphosphate.

Crystal structure of the neutral form of fructose 1,6-bisphosphatase complexed with regulatory inhibitor fructose 2,6-bisphosphate at 2.6-A resolution

The three-dimensional structure of the complex between fructose 1,6-bisphosphatase (EC 3.1.3.11) and the physiological inhibitor beta-D-fructose 2,6-bisphosphate (Fru-2,6-P2), an analogue of the substrate (fructose 1,6-bisphosphate), has been refined at 2.6-A resolution to a residual error (R) factor of 0.171. The rms deviations are 0.012 A and 2.88 degrees from ideal geometries of bond lengths and angles, respectively. The Fru-2,6-P2 occupies the active sites of both polypeptides C1 and C2 in the crystallographic asymmetric unit in the space group P3(2)21. The furanose and 6-phosphate of Fru-2,6-P2 are located at the fructose 6-phosphate site established earlier, and the 2-phosphate binds to the OH of Ser-124, the NH3+ of Lys-274, and the backbone NH of Gly-122 and Ser-123. Backbone displacements of 1 A occur for residues from Asp-121 to Asn-125. Model building of substrate alpha-D-Fru-1,6-P2 based on the binding structure of Fru-2,6-P2 in the active site shows interactions of the 1-phosphate with the backbone NH of Ser-123 and Ser-124. In the AMP sites, density peaks attributed to Fru-2,6-P2 are seen in C1 (and C4) but not in C2 (and C3). This minor binding of Fru-2,6-P2 to AMP sites partially explains the synergistic interaction between AMP and Fru-2,6-P2 and the protection of the AMP site from acetylation in the presence of Fru-2,6-P2. In the synergistic interaction between AMP and Fru-2,6-P2, inhibition of catalytic metal binding by the presence of Fru-2,6-P2 at the active site, and propagation of structural changes over some 28 A along beta-strand B3 from residues 121 to 125 in the active site to Lys-112 and Tyr-113 in the AMP site, as well as movement of helices across the interdimeric interfaces, may affect AMP binding and the subsequent R-to-T transition. In addition, occupancy of Fru-2,6-P2 at the AMP sites of C1 and C4 may favor binding of AMP to the remaining unoccupied AMP sites and thus promote the accompanying quaternary conformational changes.

Molecular cloning of CIF1, a yeast gene necessary for growth on glucose

The cif1 mutation of Saccharomyces cerevisiae (Navon et al., Biochemistry 18, 4487-4499, 1979) causes inability to grow on glucose and absence of catabolite inactivation. We have cloned the CIF1 gene by complementation of function and located it in a 2.75 kb SphI-BstEII fragment situated at ca. 18 kb centromere distal of LYS2 and ca. 80 kb centromere proximal of TYR1 on chromosome II. Southern analysis demonstrated that CIF1 is present in a single copy in the yeast genome. Northern analysis revealed that the corresponding mRNA of 1.8 kb is more abundant in cells grown on galactose than in those grown on glucose. A protein of ca. 54 kDa was predicted from the open reading frame in the sequenced fragment. In strains carrying the cif1 mutation the intracellular concentration of ATP decreased immediately after addition of glucose while the intracellular concentration of cAMP did not increase. cAMP concentration increased in response to galactose or 2,4-dinitrophenol. Disruption of BCY1 or overexpression of CDC25 in a cif1 background did not restore growth on glucose, suggesting that the absence of cAMP signal is not the primary cause of lack of growth on glucose. Complementation tests showed that cif1 is not allelic to fdp1 although the two genes seem to be functionally related.