Characterization of fructose 1,6-bisphosphatase from bakers' yeast

Active nonphosphorylated fructose bisphosphatase (EC 3.1.3.11) was purified from bakers' yeast. After chromatography on phosphocellulose, the enzyme appeared as a homogeneous protein as deduced from polyacrylamide gel electrophoresis, gel filtration, and isoelectric focusing. A Stokes radius of 44.5 A and molecular weight of 116,000 was calculated from gel filtration. Polyacrylamide gel electrophoresis of the purified enzyme in the presence of sodium dodecyl sulfate resulted in three protein bands of Mr = 57,000, 40,000, and 31,000. Only one band of Mr = 57,000 was observed, when the single band of the enzyme obtained after polyacrylamide gel electrophoresis in the absence of sodium dodecyl sulfate was eluted and then resubmitted to electrophoresis in the presence of sodium dodecyl sulfate. Amino acid analysis indicated 1030 residues/mol of enzyme including 12 cysteine moieties. The isoelectric point of the enzyme was estimated by gel electrofocusing to be around pH 5.5. The catalytic activity showed a maximum at pH 8.0; the specific activity at the standard pH of 7.0 was 46 units/mg of protein. Fructose 1,6-bisphosphatase b, the less active phosphorylated form of the enzyme, was purified from glucose inactivated yeast. This enzyme exhibited maximal activity at pH greater than or equal to 9.5; the specific activity measured at pH 7.0 was 25 units/mg of protein. The activity ratio, with 10 mM Mg2+ relative to 2 mM Mn2+, was 4.3 and 1.8 for fructose 1,6-bisphosphatase a and fructose 1,6-bisphosphatase b, respectively. Activity of fructose 1,6-bisphosphatase a was 50% inhibited by 0.2 microM fructose 2,6-bisphosphate or 50 microM AMP. Inhibition by fructose 2,6-bisphosphate as well as by AMP decreased with a more alkaline pH in a range between pH 6.5 and 9.0. The inhibition exerted by combinations of the two metabolites at pH 7.0 was synergistic.     

Specific, reversible inactivation of phosphofructokinase by fructose-1,6-bisphosphatase. Involvement of adenosine 5'-triphosphate, oleate, and 3-phosphoglycerate.

Optimal conditions necessary for the reversible inactivation of crystalline rabbit muscle phosphofructokinase by homogeneous rabbit liver fructose-1,6-bisphosphatase have been studied. At higher enzyme levels (to 530 mug/ml of phosphofructokinase) the two proteins were mixed and incubated in a pH 7.5 buffer composed of 50 mM Tris-HC1, 2 mM potassium phosphate, and 0.2 mM dithiothreitol. Aliquots were removed at various times and assayed for enzyme activity. A time dependent inactivation of phosphofructokinase caused by 1-2.3 times its weight of fructose-1,6-bisphosphatase was observed at 30, 23, and 0 degree C. This inactivation did not require the presence of adenosine 5'-triphosphate or Mg2+ in the incubation mixture, but an adenosine 5'-triphosphate concentration of 2.7 mM or greater was required in the assay to keep phosphofructokinase in an inactive form. A mixture of activators (inorganic phosphate, (NH4)2SO4, and adenosine 5'-monophosphate), when added to the assay cuvette, restored nearly all of the expected enzyme activity. Incubations with other proteins, including aldolase, at concentrations equal to or greater than the effective quantity of fructose-1,6-bisphosphatase had no inhibitory effect on phosphofructokinase activity. Removal of tightly bound fructose 1,6-bisphosphate from phosphofructokinase could not explain this inactivation, since several analyses of crystalline phosphofructokinase averaged less than 0.1 mol of fructose 1,6-bisphosphate/320 000 g of enzyme. Furthermore, the inactivation occurred in the absence of Mg2+ where the complete lack of fructose-1-6-bisphosphatase activity was confirmed directly. At lower phosphofructokinase concentrations (0.2-2 mug/ml) the inactivation was studied directly in the assay cuvette. Higher ratios of fructose-1,6-bisphosphatase to phosphofructokinase were necessary in these cases, but oleate and 3-phosphoglycerate acted synergistically with lower amounts of fructose-1,6-bisphosphatase to cause inactivation. The inactivation did not occur when high concentrations of fructose 6-phosphate were present in the assay, or when the level of adenosine 5'-triphosphate was decreased. However, the inactivation was found at pH 8, where the effects of allosteric regulators on phosphofructokinase are greatly reduced. Experiments with rat liver phosphofructokinase showed that this enzyme was also subject to inhibition by rabbit liver fructose 1,6-bisphosphatase under conditions similar to those used in the muscle enzyme studies. Attempts to demonstrate direct interaction between phosphofructokinase and fructose-1,6-bisphosphate by physical methods were unsuccessful. Nevertheless, our results suggest that, under conditions which approximate the physiological state, the presence of fructose-1,6bisphosphatase can cause phosphofructokinase to assume an inactive conformation. This interaction may have a significant role in vivo in controlling the interrelationship between glycolysis and gluconeogenesis.     

A binding study of the interaction of beta-D-fructose 2,6-bisphosphate with phosphofructokinase and fructose-1,6-bisphosphatase

The binding of beta-D-fructose 2,6-bisphosphate to rabbit muscle phosphofructokinase and rabbit liver fructose-1,6-bisphosphatase was studied using the column centrifugation procedure (Penefsky, H. S., (1977) J. Biol. Chem. 252, 2891-2899). Phosphofructokinase binds 1 mol of fructose 2,6-bisphosphate/mol of protomer (Mr = 80,000). The Scatchard plots of the binding of fructose 2,6-bisphosphate to phosphofructokinase are nonlinear in the presence of three different buffer systems and appear to exhibit negative cooperativity. Fructose 1,6-bisphosphate and glucose 1,6-bisphosphate inhibit the binding of fructose-2,6-P2 with Ki values of 15 and 280 microM, respectively. Sedoheptulose 1,7-bisphosphate, ATP, and high concentrations of phosphate also inhibit the binding. Other metabolites including fructose-6-P, AMP, and citrate show little effect. Fructose-1,6-bisphosphatase binds 1 mol of fructose 2,6-bisphosphate/mol of subunit (Mr = 35,000) with an affinity constant of 1.5 X 10(6) M-1. Fructose 1,6-bisphosphate, fructose-6-P, and phosphate are competitive inhibitors with Ki values of 4, 2.7, and 230 microM, respectively. Sedoheptulose 1,7-bisphosphate (1 mM) inhibits approximately 50% of the binding of fructose 1,6-bisphosphate to fructose bisphosphatase, but AMP has no effect. Mn2+, Co2+, and a high concentration of Mg2+ inhibit the binding. Thus, we may conclude that fructose 2,6-bisphosphate binds to phosphofructokinase at the same allosteric site for fructose 1,6-bisphosphate while it binds to the catalytic site of fructose-1,6-bisphosphatase.     

Fructose bisphosphatase from Escherichia coli. Purification and characterization

Escherichia coli fructose-1,6-bisphosphatase has been purified for the first time, using a clone containing an approximately 50-fold increased amount of the enzyme. The procedure includes chromatography in phosphocellulose followed by substrate elution and gel filtration. The enzyme has a subunit molecular weight of approximately 40,000 and in nondenaturing conditions is present in several aggregated forms in which the tetramer seems to predominate at low enzyme concentrations. Fructose bisphosphatase activity is specific for fructose 1,6-bisphosphate (Km of approximately 5 microM), shows inhibition by substrate above 0.05 mM, requires Mg2+ for catalysis, and has a maximum of activity around pH 7.5. The enzyme is susceptible to strong inhibition by AMP (50% inhibition around 15 microM). Phosphoenolpyruvate is a moderate inhibitor but was able to block the inhibition by AMP and may play an important role in the regulation of fructose bisphosphatase activity in vivo. Fructose 2,6-bisphosphate did not affect the rate of reaction.     

Regulation of rabbit liver fructose-1,6-bisphosphatase by metals, nucleotides, and fructose 2,6-bisphosphate as determined from fluorescence studies

The fluorescent nucleotide analogue formycin 5'-monophosphate (FMP) inhibits rabbit liver fructose-1,6-bisphosphatase (I50 = 17 microM, Hill coefficient = 1.2), as does the natural regulator AMP (I50 = 13 microM, Hill coefficient = 2.3), but exhibits little or no cooperativity of inhibition. Binding of FMP to fructose-1,6-bisphosphatase can be monitored by the increased fluorescence emission intensity (a 2.7-fold enhancement) or the increased fluorescence polarization of the probe. A single dissociation constant for FMP binding of 6.6 microM (4 sites per tetramer) was determined by monitoring fluorescence intensity. AMP displaces FMP from the enzyme as evidenced by a decrease in FMP fluorescence and polarization. The substrates, fructose 6-phosphate and fructose 1,6-bisphosphate, and inhibitors, methyl alpha-D-fructofuranoside 1,6-bisphosphate and fructose 2,6-bisphosphate, all increase the maximal fluorescence of enzyme-bound FMP but have little or no effect on FMP binding. Weak metal binding sites on rabbit liver fructose-1,6-bisphosphatase have been detected by the effect of Zn2+, Mn2+, and Mg2+ in displacing FMP from the enzyme. This is observed as a decrease in FMP fluorescence intensity and polarization in the presence of enzyme as a function of divalent cation concentration. The order of binding by divalent cations is Zn2+ = Mn2+ greater than Mg2+, and the Kd for Mn2+ displacement of FMP is 91 microM. Methyl alpha-D-fructofuranoside 1,6-bisphosphate, as well as fructose 6-phosphate and inorganic phosphate, enhances metal-mediated FMP displacement from rabbit liver fructose-1,6-bisphosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of fructose-1,6-bisphosphatase by fructose 2,6-bisphosphate

Rat liver fructose-1,6-bisphosphatase, which was assayed by measuring the release of 32P from fructose 1,6-[1-32P]bisphosphate at pH 7.5, exhibited hyperbolic kinetics with regard to its substrate. beta-D-Fructose 2,6-bisphosphate, an activator of hepatic phosphofructokinase, was found to be a potent inhibitor of the enzyme. The inhibition was competitive in nature and the Ki was estimated to be 0.5 microM. The Hill coefficient for the reaction was 1.0 in the presence and absence of fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate also enhanced inhibition of the enzyme by the allosteric inhibitor AMP. The possible role of fructose 2,6-bisphosphate in the regulation of substrate cycling at the fructose-1,6-bisphosphatase step is discussed.     

Studies on the hysteretic properties of chloroplast fructose-1,6-bisphosphatase

Chloroplast fructose-1,6-bisphosphatase hysteresis in response to modifiers was uncovered by carrying out the enzyme assays in two consecutive steps. The activity of chloroplast fructose-1,6-bisphosphatase, assayed at low concentrations of both fructose-1,6-bisphosphatase and Mg2+, was enhanced by preincubating the enzyme with dithiothreitol, thioredoxin f, fructose 1,6-bisphosphate, and Ca2+. In the time-dependent activation process, fructose 1,6-bisphosphate and Ca2+ could be replaced by other sugar biphosphates and Mn2+, respectively. Once activated, chloroplast fructose-1,6-bisphosphatase hydrolyzed fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate in the presence of Mg2+, Mn2+, or Fe2+. The A0.5 for fructose 1,6-bisphosphate (activator) was lowered by reduced thioredoxin f and remained unchanged when Mg2+ was varied during the assay of activity. On the contrary, the S0.5 for fructose 1,6-bisphosphate (substrate) was unaffected by reduced thioredoxin f and depended on the concentration of Mg2+. Ca2+ played a dual role on the activity of chloroplast fructose-1,6-bisphosphatase; it was a component of the concerted activation and an inhibitor in the catalytic step. Provided dithiothreitol was present, the activating effectors were not required to maintain the enzyme in the active form. Considered together these results strongly suggest that the regulation of fructose-1,6-bisphosphatase in chloroplast occurs at two different levels, the activation of the enzyme and the catalysis.     

Characterization of rat muscle fructose 1,6-bisphosphatase

Fructose 1,6-bisphosphatase has been purified from rat muscle. Although the specific activity of the enzyme in the crude extract of rat muscle was extremely low, purification by the present procedure is highly reproducible. The purified enzyme showed a single band in SDS-polyacrylamide gel electrophoresis. The subunit molecular weight of the muscle enzyme was 37,500 in contrast to 43,000 in the case of the liver enzyme. Immunoreactivity of the muscle enzyme to anti-muscle and anti-liver fructose 1,6-bisphosphatase sera was clearly distinct from that of the liver enzyme. All one-dimensional peptide mappings of the muscle enzyme with staphylococcal V8 protease, chymotrypsin, and papain showed different patterns from those of the liver enzyme. When incubated with subtilisin, the extent of activation of muscle fructose 1,6-bisphosphatase at pH 9.1 was smaller than that of the liver enzyme. The subtilisin digestion pattern of the muscle enzyme on SDS-polyacrylamide gel electrophoresis was distinct from that of the liver enzyme. The AMP-concentration giving 50% inhibition of the muscle enzyme was 0.54 microM, whereas that of the liver enzyme was 85 microM. The concentrations of fructose 2,6-bisphosphate that gave 50% inhibition of rat muscle and liver enzymes were 6.3 and 1.5 microM, respectively. Fructose 1,6-bisphosphatase protein was not detected in soleus muscle by immunoelectroblotting with anti-muscle fructose 1,6-bisphosphatase serum.     

Selective thiol group modification renders fructose-1,6-bisphosphatase insensitive to fructose 2,6-bisphosphate inhibition

Limited treatment of native pig kidney fructose-1,6-bisphosphatase (50 microM enzyme subunit) with [14C]N-ethylmaleimide (100 microM) at 30 degrees C, pH 7.5, in the presence of AMP (200 microM) results in the modification of 1 reactive cysteine residue/enzyme subunit. The N-ethylmaleimide-modified fructose-1,6-bisphosphatase has a functional catalytic site but is no longer inhibited by fructose 2,6-bisphosphate. The enzyme derivative also exhibits decreased affinity toward Mg2+. The presence of fructose 2,6-bisphosphate during the modification protects the enzyme against the loss of fructose 2,6-bisphosphate inhibition. Moreover, the modified enzyme is inhibited by monovalent cations, as previously reported (Reyes, A., Hubert, E., and Slebe, J.C. (1985) Biochem. Biophys. Res. Commun. 127, 373-379), and does not show inhibition by high substrate concentrations. A comparison of the kinetic properties of native and N-ethylmaleimide-modified fructose-1,6-bisphosphatase reveals differences in some properties but none is so striking as the complete loss of fructose 2,6-bisphosphate sensitivity. The results demonstrate that fructose 2,6-bisphosphate interacts with a specific allosteric site on fructose-1,6-bisphosphatase, and they also indicate that high levels of fructose 1,6-bisphosphate inhibit the enzyme by binding to this fructose 2,6-bisphosphate allosteric site.     

Purification and properties of spinach leaf cytoplasmic fructose-1,6-bisphosphatase.

Cytoplasmic fructose-1,6-bisphosphatase has been purified from spinach leaves to apparent homogeneity. The enzyme is a tetramer of molecular weight about 130,000. At pH 7.5, the Km for fructose 1.6-bisphosphate was 2.5 micron, and for MgCl2 0.13 mM; the enzyme was specific for fructose 1,6-bisphosphate. Saturation with Mg2+ was achieved with lower concentrations at pH 8 than at pH 7. AMP and high concentrations of fructose 1,6-bisphosphate inhibited enzyme activity. Ammonium sulfate relieved the latter inhibition but was itself inhibitory when substrate concentrations were low. Acetylation studies demonstrated that the AMP regulatory site was distinct from the catalytic site. Cytoplasmic fructose-1,6-bisphosphatase may contribute to the regulation of sucrose biosynthesis in plant leaves.     

Concerted action of cosolvents, chaotropic anions and thioredoxin on chloroplast fructose-1,6-bisphosphatase. Reactivity to iodoacetamide

The incubation of chloroplast fructose-1,6-bisphosphatase with both dithiothreitol and protein denaturants made sulfhydryl groups available for reaction with [1-14C]iodoacetamide (10-12 mol iodoacetamide incorporated/mol enzyme). Digestion of S-carboxyamidomethylated enzyme with trypsin and polyacrylamide gel electrophoresis, in the presence of sodium dodecylsulfate, yielded two 14C-labeled fragments whose apparent molecular mass were 10 kDa and 16 kDa. In the absence of either dithiothreitol or protein denaturants the incorporation of iodoacetamide to the enzyme was lower than 4 mol. When chloroplast fructose-1,6-bisphosphatase was initially incubated with dithiothreitol (2.5 mM) and (a) high concentrations of both fructose 1,6-bisphosphate (4 mM) and Ca2+ (0.3 mM) or (b) low concentrations of both fructose 1,6-bisphosphate (0.8 mM) and Ca2+ (0.05 mM) in the presence of either 2-propanol (15%, by vol.), trichloroacetate (0.15 M) or chloroplast thioredoxin-f (0.5 microM) and subsequently subjected to proteolysis and electrophoresis, S-carboxyamidomethylated tryptic fragments had similar molecular masses. Thus, conditions that stimulated the specific activity of chloroplast fructose-1,6-bisphosphatase caused conformational changes which favoured both the reduction of disulfide bridges and the exposure of sulfhydryl groups. In this aspect, thioredoxin exerted structural and kinetic effects similar to compounds not involved in redox reactions (organic solvents, chaotropic anions). These results indicated that the modification of hydrophobic (intramolecular) interactions in chloroplast fructose-1,6-bisphosphatase constituted the underlying mechanism in light-activation by the ferredoxin-thioredoxin system.     

Relationship between thiol group modification and the binding site for fructose 2,6-bisphosphate on rabbit liver fructose-1,6-bisphosphatase

A thiol group present in rabbit liver fructose-1,6-bisphosphatase is capable of reacting rapidly with N-ethylmaleimide (NEM) with a stoichiometry of one per monomer. Either fructose 1,6-bisphosphate or fructose 2,6-bisphosphate at 500 microM protected against the loss of fructose 2,6-bisphosphate inhibition potential when fructose-1,6-bisphosphatase was treated with NEM in the presence of AMP for up to 20 min. Fructose 2,6-bisphosphate proved more effective than fructose 1,6-bisphosphate when fructose-1,6-bisphosphatase was treated with NEM for 90-120 min. The NEM-modified enzyme exhibited a significant loss of catalytic activity. Fructose 2,6-bisphosphate was more effective than the substrate in protecting against the thiol group modification when the ligands are present with the enzyme and NEM. 100 microM fructose 2,6-bisphosphate, a level that should almost saturate the inhibitory binding site of the enzyme under our experimental conditions, affords only partial protection against the loss of activity of the enzyme caused by the NEM modification. In addition, the inhibition pattern for fructose 2,6-bisphosphate of the NEM-derivatized enzyme was found to be linear competitive, identical to the type of inhibition observed with the native enzyme. The KD for the modified enzyme was significantly greater than that of untreated fructose-1,6-bisphosphatase. Examination of space-filling models of the two bisphosphates suggest that they are very similar in conformation. On the basis of these observations, we suggest that fructose 1,6-bisphosphate and fructose 2,6-bisphosphate occupy overlapping sites within the active site domain of fructose-1,6-bisphosphatase. Fructose 2,6-bisphosphate affords better shielding against thiol-NEM modification than fructose 1,6-bisphosphate; however, the difference between the two ligands is quantitative rather than qualitative.     

The interaction of fructose 2,6-bisphosphate and AMP with rat hepatic fructose 1,6-bisphosphatase

The binding of the inhibitory ligands fructose 2,6-bisphosphate and AMP to rat liver fructose 1,6-bisphosphatase has been investigated. 4 mol of fructose-2,6-P2 and 4 mol of AMP bind per mol of tetrameric enzyme at pH 7.4. Fructose 2,6-bisphosphate exhibits negative cooperatively as indicated by K'1 greater than K'2 greater than K'3 greater than or equal to K'4 and a Hill plot, the curvature of which indicates K'2/K'1 less than 1, K'3/K'2 less than 1, and K'4/K'3 = 1. AMP binding, on the other hand, exhibits positive cooperativity as indicated by K'1 less than K'2 less than K'3 less than K'4 and an nH of 2.05. Fructose 2,6- and fructose 1,6-bisphosphates enhance the binding of AMP as indicated by an increase in the intrinsic association constants. At pH 9.2, where fructose 2,6-bisphosphate and AMP inhibition of the enzyme are diminished, fructose 2,6-bisphosphate binds with a lower affinity but in a positively cooperative manner, whereas AMP exhibits half-sites reactivity with only 2 mol of AMP bound per mol of tetramer. Ultraviolet difference spectroscopy confirmed the results of these binding studies. The site at which fructose 2,6-bisphosphate binds to fructose 1,6-bisphosphatase has been identified as the catalytic site on the basis of the following. 1) Fructose 2,6-bisphosphate binds with a stoichiometry of 1 mol/mol of monomer; 2) covalent modification of the active site with acetylimidazole inhibits fructose 2,6-bisphosphate binding; and 3) alpha-methyl D-fructofuranoside-1,6-P2 and beta-methyl D-fructofuranoside-1,6-P2, substrate analogs, block fructose 2,6-bisphosphate binding. We propose that fructose 2,6-bisphosphate enhances AMP affinity by binding to the active site of the enzyme and bringing about a conformational change which may be similar to that induced by AMP interaction at the allosteric site.     

Fructose-1,6-bisphosphatase from rat liver. A comparison of the kinetics of the unphosphorylated enzyme and the enzyme phosphorylated by cyclic AMP-dependent protein kinase

A purification procedure for rat hepatic fructose-1,6-bisphosphatase, described earlier, has been improved, resulting in an enzyme preparation with a neutral pH optimum and with both phosphorylatable serine residues present. The subunit Mr was 40,000. Phosphorylation in vitro with cyclic AMP-dependent protein kinase resulted in the incorporation of 1.4 mol of phosphate/mol of subunit and led to an almost 2-fold decrease in apparent Km for fructose-1,6-bisphosphate. In contrast to yeast fructose-1,6-bisphosphatase, fructose-2,6-bisphosphate had no effect on the rate of phosphorylation or dephosphorylation of the intact enzyme. The effects of the composition of the assay medium, with regard to buffering substance and Mg2+ concentration, on the apparent Km values of phosphorylated and unphosphorylated enzyme were investigated. The kinetics of phosphorylated and unphosphorylated fructose-1,6-bisphosphatase were studied with special reference to the inhibitory effects of adenine nucleotides and fructose-2,6-bisphosphate. Unphosphorylated fructose-1,6-bisphosphatase was more susceptible to inhibition by both AMP and fructose 2,6-bisphosphate than phosphorylated enzyme, at high and low substrate concentrations. Both ATP and ADP had a similar effect on the two enzyme forms, ADP being the more potent inhibitor. Finally, the combined effect of several inhibitors at physiological concentrations was studied. Under conditions resembling the gluconeogenic state, phosphorylated fructose-1,6-bisphosphatase was found to have twice the activity of the unphosphorylated enzyme.     

Purification and characterization of fructose-1,6-bisphosphatase from bovine brain

Fructose-1,6-bisphosphatase from bovine brain tissue has been purified to near homogeneity. This enzyme is similar to other mammalian fructose-1,6-bisphosphatases in many respects, and its properties are distinctly different from those reported for the enzyme from rat brain [A. L. Majumder and F. Eisenberg (1977) Proc. Natl. Acad. Sci. USA 74, 3222-3225; S. Chattoraj and A. L. Majumder (1986) Biochem. Biophys. Res. Commun. 139, 571-580]. The bovine enzyme (sp act 4, pH ratio (7.5/9.6) = 3.6) has a pH optimum of 7.5. The Km is 2 microM. Divalent metal ion is required for activity, and Vmax is obtained at either 4 mM Mg2+ or 0.3 mM Mn2+. Fructose 2,6-bisphosphate is a competitive inhibitor (Ki = 0.07 microM), and AMP a noncompetitive inhibitor (kis = 24 microM, Kii = 10 microM) of bovine brain fructose-1,6-bisphosphatase. The enzyme activity is enhanced by small amounts of EDTA relative to metal, and AMP inhibits fructose-1,6-bisphosphatase in either the presence or absence of the metal chelator; however, AMP is more effective in the absence of EDTA.     

Variation of (2''-5'')oligo(adenylate) synthetase activity during rat-liver regeneration

The inhibitory effect of fructose 2,6-biphosphate on fructose 1,6-bisphosphatase was reinvestigated in order to solve the apparent contradiction between competition with the substrate and the synergism with AMP, a strictly noncompetitive inhibitor. The effect of fructose 2,6-bisphosphate was compared to that of other ligands of the enzyme, which, like the substrate and methyl (alpha + beta)fructofuranoside 1,6-bisphosphate bind to the active site or which, like AMP, bind to an allosteric site. An increase in temperature or pH, or the presence of sulfosalicylate, lithium or higher concentrations of magnesium as well as partial proteolysis by subtilisin increased [I]0.5 for fructose 2,6-bisphosphate and AMP without affecting Km. With the exception of the pH change, all these conditions were also without effect on the affinity of the enzyme for the competitive inhibitor, methyl (alpha + beta)fructofuranoside 1,6-bisphosphate. These observations can be explained by assuming that fructose 2,6-bisphosphate has no affinity for the active site of fructose 1,6-bisphosphatase but binds to an allosteric site which is different from the AMP site. Fructose 2,6-bisphosphate is therefore classified as an allosteric competitive inhibitor and a model is proposed which explains its synergism with AMP as well as the various cooperative effects.     

Fructose 2,6-bisphosphate activates the cAMP-dependent phosphorylation of yeast fructose-1,6-bisphosphatase in vitro

Fructose-1,6-bisphosphatase purified from Saccharomyces cerevisiae is phosphorylated in vitro by a cAMP-dependent protein kinase. The phosphorylation reaction incorporates 1 mol of phosphate/mol of enzyme and is greatly stimulated by fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate acts upon fructose-1,6-bisphosphatase, not on the protein kinase. The phosphorylation of fructose 1,6-bisphosphatase lowers its activity by about 50%. The characteristics of the phosphorylation reaction in vitro show that this modification is responsible for the inactivation of fructose-1,6-bisphosphatase observed in vivo.     

Purification and properties of fructose-1,6-bisphosphatase of Bacillus subtilis.

Fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrase, EC 3.1.3.11) of Bacillus subtilis is a constitutive enzyme that was purified 1000-fold (30% yield) to 80% purity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis where it exhibits a band corresponding to 72,000 daltons. It sediments at 15 S in sucrose density gradients indicating a molecular weight of 380,000, but apparently is very asymmetric. Its activity is irreversibly inactivated in the absence of Mn2+. The enzyme specifically catalyzes dephosphorylation of D-fructose 1,6-bisphosphate with a pH optimum of 8.0. It has 40 to 60% of full activity in the absence of P-enolpyruvate; 20 microM P-enolpyruvate activates it maximally. High concentrations of monovalent cations also activate, NH4+ being most effective. Inhibitors fall into two groups. 1) Nucleoside monophosphates, phosphorylated coenzymes, and polynucleotides inhibit competitively with P-enolpyruvate (AMP (Ki = 2 microM) and dAMP are most effective). 2) The inhibition by nucleoside di- and triphosphates, PPi, and highly phosphorylated nucleotides (guanosine 5'-triphosphate 3'-diphosphate (pppGpp) and adenosine 5'-triphosphate 3'-diphosphate are most effective) is not competed by P-enolpyruvate but is partially overcome by fructose 1,6-bisphosphate (2 microM). Therefore, highly phosphorylated nucleotides (pppGpp and others), produced in over 0.2 mM concentrations upon step down from fast to slow growth rates (Gallant, J., and Lazzarini, R.A. (1976) in Protein Synthesis (McConkey, E.H., ed) Vol. 2, pp. 309-349, Marcel Dekker, Inc., New York), can reduce the conversion rate of fructose 1,6-bisphosphate to fructose 6-phosphate during gluconeogenesis. Comparing glycolytic growth on D-glucose and gluconeogenic growth on L-malate, the intracellular concentrations of fructose 1,6-bisphosphate differ but are both above the Km (13 microM) of the enzyme, those of AMP are similar, whereas those of P-enolpyruvate (0.18 mM versus 1.3 mM) indicate that the enzyme has only 40% of its full activity during glycolysis; nucleotides other than AMP may inhibit additionally. Thus, the futile cycle of fructose 1,6-bisphosphate synthesis and degradation during glycolysis is partially avoided, but the cells are poised for rapid adaptation upon change to gluconeogenic growth conditions.     

Biochemical characterization of cytosolic fructose-1,6-bisphosphatase from apple (Malus domestica) leaves

Cytosolic fructose-1,6-bisphosphatase was purified to apparent homogeneity from the leaves of apple, a sorbitol synthesizing species. The enzyme was a homotetramer with a subunit mass of 37 kDa, and was highly specific for fructose 1,6-bisphosphate (F1,6BP) with a Km of 3.1 micro M and a Vmax of 48 units (mg protein)(-1). Either Mg2+ or Mn2+ was required for its activity with a Km of 0.59 mM and 62 micro M, respectively. Li+, Ca2+, Zn2+, Cu2+ and Hg2+ inhibited whereas Mn2+ enhanced the Mg2+ activated enzyme activity. Fructose 6-phosphate (F6P) was found to be a mixed type inhibitor with a Ki of 0.47 mM. Fructose 2,6-bisphosphate (F2,6BP) competitively inhibited the enzyme activity and changed the substrate saturation curve from hyperbolic to sigmoidal. AMP was a non-competitive inhibitor for the enzyme. F6P interacted with F2,6BP and AMP in a synergistic way to inhibit the enzyme activity. Dihydroxyacetone phosphate slightly inhibited the enzyme activity in the presence or absence of F2,6BP. Sorbitol increased the susceptibility of the enzyme to the inhibition by high concentrations of F1,6BP. High concentrations of sorbitol in the reaction mixture led to a reduction in the enzyme activity.     

The effect of Zn2+ on the inhibition of Fru-P2ase by AMP.

In the absence of chelating agents, the sensitivity of rabbit liver fructose 1,6-bisphosphatase to inhibition by AMP is increased by approximately 10-fold, apparently related to the presence of bound Zn²⁺. At pH 7.5, 10 μM AMP causes virtually complete inhibition and nearly full activity is restored by the addition of chelating agents such as EDTA or histidine. These properties provide a mechanism for the induction of Fru-P₂ase activity under gluconeogenic conditions.