Fructose-1,6-Bisphosphate Is an Allosteric Activator of Pyrophosphate:Fructose-6-Phosphate 1-Phosphotransferase

The activity of highly purified pyrophosphate:fructose-6-phosphate 1-phosphotransferase (PFP) from barley (Hordeum vulgare) leaves was studied under conditions where the catalyzed reaction was allowed to approach equilibrium. The activity of PFP was monitored by determining the changes in the levels of fructose-6-phosphate, orthophosphate, and fructose-1,6-bisphosphate (Fru-1,6-bisP). Under these conditions PFP activity was not dependent on activation by fructose-2,6-bisphosphate (Fru-2,6-bisP). Inclusion of aldolase in the reaction mixture temporarily restored the dependence of PFP on Fru-2,6-bisP. Alternatively, PFP was activated by Fru-1,6-bisP in the presence of aldolase. It is concluded that Fru-1,6-bisP is an allosteric activator of barley PFP, which can substitute for Fru-2,6-bisP as an activator. A significant activation was observed at a concentration of 5 to 25 [mu]M Fru-1,6-bisP, which demonstrates that the allosteric site of barley PFP has a very high affinity for Fru-1,6-bisP. The high affinity for Fru-1,6-bisP at the allosteric site suggests that the observed activation of PFP by Fru-1,6-bisP constitutes a previously unrecognized in vivo regulation mechanism.     

Rat liver pyruvate kinase: influence of ligands on activity and fructose 1,6-bisphosphate binding

The ability for various ligands to modulate the binding of fructose 1,6-bisphosphate (Fru-1,6-P2) with purified rat liver pyruvate kinase was examined. Binding of Fru-1,6-P2 with pyruvate kinase exhibits positive cooperativity, with maximum binding of 4 mol Fru-1,6-P2 per enzyme tetramer. The Hill coefficient (nH), and the concentration of Fru-1,6-P2 giving half-maximal binding [FBP]1/2, are influenced by several factors. In 150 mM Tris-HCl, 70 mM KCl, 11 mM MgSO4 at pH 7.4, [FBP]1/2 is 2.6 microM and nH is 2.7. Phosphoenolpyruvate and pyruvate enhance the binding of Fru-1,6-P2 by decreasing [FBP]1/2. ADP and ATP alone had little influence on Fru-1,6-P2 binding. However, the nucleotides antagonize the response elicited by pyruvate or phosphoenolpyruvate, suggesting that the competent enzyme substrate complex does not favor Fru-1,6-P2 binding. Phosphorylation of pyruvate kinase or the inclusion of alanine in the medium, two actions which inhibit the enzyme activity, result in diminished binding of low concentrations of Fru-1,6-P2 with the enzyme. These effectors do not alter the maximum binding capacity of the enzyme but rather they raise the concentrations of Fru-1,6-P2 needed for maximum binding. Phosphorylation also decreased the nH for Fru-1,6-P2 binding from 2.7 to 1.7. Pyruvate kinase activity is dependent on a divalent metal ion. Substituting Mn2+ for Mg2+ results in a 60% decrease in the maximum catalytic activity for the enzyme and decreases the concentration of phosphoenolpyruvate needed for half-maximal activity from 1 to 0.1 mM. As a consequence, Mn2+ stimulates activity at subsaturating concentrations of phosphoenolpyruvate, but inhibits at saturating concentrations of the substrate or in the presence of Fru-1,6-P2. Both Mg2+ and Mn2+ diminish binding of low concentrations of Fru-1,6-P2; however, the concentrations of the metal ions needed to influence Fru-1,6-P2 binding exceed those needed to support catalytic activity.     

The pH dependence of the allosteric response of human liver pyruvate kinase to fructose-1,6-bisphosphate, ATP, and alanine

The allosteric regulation of human liver pyruvate kinase (hL-PYK) by fructose-1,6-bisphosphate (Fru-1,6-BP; activator), ATP (inhibitor) and alanine (Ala; inhibitor) was monitored over a pH range from 6.5 to 8.0 at 37 °C. As a function of increasing pH, hL-PYK’s affinity for the substrate phosphoenolpyruvate (PEP), and for Fru-1,6-BP decreases, while affinities for ATP and alanine slightly increases. At pH 6.5, Fru-1,6-BP and ATP elicit only small allosteric impacts on PEP affinity. As pH increases, Fru-1,6-BP and ATP elicit greater allosteric responses, but the response to alanine is relatively constant. Since the magnitudes of the allosteric coupling for ATP and for alanine inhibition are different and the pH dependences of these magnitudes are not similar, these inhibitors likely elicit their responses using different molecular mechanisms. In addition, our results fail to support a general correlation between pH dependent changes in effector affinity and pH dependent changes in the corresponding allosteric response.     

Structure refinement of fructose-1,6-bisphosphatase and its fructose 2,6-bisphosphate complex at 2.8 A resolution

The structures of the native fructose-1,6-bisphosphatase (Fru-1,6-Pase), from pig kidney cortex, and its fructose 2,6-bisphosphate (Fru-2,6-P2) complexes have been refined to 2.8 A resolution to R-factors of 0.194 and 0.188, respectively. The root-mean-square deviations from the standard geometry are 0.021 A and 0.016 A for the bond length, and 4.4 degrees and 3.8 degrees for the bond angle. Four sites for Fru-2,6-P2 binding per tetramer have been identified by difference Fourier techniques. The Fru-2,6-P2 site has the shape of an oval cave about 10 A deep, and with other dimensions about 18 A by 12 A. The two Fru-2,6-P2 binding caves of the dimer in the crystallographically asymmetric unit sit next to one another and open in opposite directions. These two binding sites mutually exchange their Arg243 side-chains, indicating the potential for communication between the two sites. The beta, D-fructose 2,6-bisphosphate has been built into the density and refined well. The oxygen atoms of the 6-phosphate group of Fru-2,6-P2 interact with Arg243 from the adjacent monomer and the residues of Lys274, Asn212, Tyr264, Tyr215 and Tyr244 in the same monomer. The sugar ring primarily contacts with the backbone atoms from Gly246 to Met248, as well as the side-chain atoms, Asp121, Glu280 and Lys274. The 2-phosphate group interacts with the side-chain atoms of Ser124 and Lys274. A negatively charged pocket near the 2-phosphate group includes Asp118, Asp121 and Glu280, as well as Glu97 and Glu98. The 2-phosphate group showed a disordered binding perhaps because of the disturbance from the negatively charged pocket. In addition, Asn125 and Lys269 are located within a 5 A radius of Fru-2,6-P2. We argue that Fru-2,6-P2 binds to the active site of the enzyme on the basis of the following observations: (1) the structure similarity between Fru-2,6-P2 and the substrate; (2) sequence conservation of the residues directly interacting with Fru-2,6-P2 or located at the negatively charged pocket; (3) a divalent metal site next to the 2-phosphate group of Fru-2,6-P2; and (4) identification of some active site residues in our structure, e.g. tyrosine and Lys274, consistent with the results of the ultraviolet spectra and the chemical modification. The structures are described in detail including interactions of interchain surfaces, and the chemically modifiable residues are discussed on the basis of the refined structures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fructose-1,6-biphosphatase of the small intestine. Purification and comparison with liver and muscle fructose-1,6-bisphosphatases.

The occurrence of specific fructose-1,6-bisphosphatase [D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11] (Fru-1,6-P2ase) in the small intestine was confirmed. 1. Fru-1,6-P2ase was isolated from mouse small intestine by a simple method. The isolated enzyme preparation was an electrophoretically homogeneous protein. 2. The molecular weight and subunit molecular weight were 140,000 and 38,000, respectively. 3. The intestinal enzyme was electrophoretically distinct from the liver enzyme. 4. The kinetic properties of the purified intestinal enzyme were compared with those of the mouse liver and muscle enzymes. 5. Mouse intestinal and muscle Fru-1,6-P2ases hydrolyzed ribulose-1,5-bisphosphate in addition to fructose-1,6-bisphosphate and sedoheptulose-1,7-bisphosphate.     

Allosteric effect of fructose 1,6-bisphosphate on the conformation of NAD+ as bound to L-lactate dehydrogenase from Thermus caldophilus GK24

The allosteric effect of fructose 1,6-bisphosphate (Fru-1,6-P2) on L-lactate dehydrogenase (L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) from Thermus caldophilus GK24 was studied by means of 1H NMR analyses. The conformation of NAD+ as bound to the T. caldophilus enzyme was elucidated by analyses of the transferred nuclear Overhauser effects (TRNOE), in the presence and the absence of the allosteric effector, Fru-1,6-P2. Upon binding of Fru-1,6-P2 to the enzyme, the ribose ring of the adenosine moiety of NAD+ is converted from the C2'-endo form to the C3'-endo form. This C3'-endo form of the adenosine moiety is similar to that of NAD+ as bound to nonallosteric vertebrate enzymes. However, the anti conformation of the adenine-ribose bond of NAD+ as bound to the T. caldophilus enzyme is not affected by the binding of Fru-1,6-P2. In contrast, the syn conformation of the nicotinamide-ribose bond is converted to the anti form on the binding of Fru-1,6-P2, while the ribose ring remains in the C3'-endo form as found in the case of a nonallosteric enzyme. Such a conformational change of enzyme-bound NAD+ as found on TRNOE analysis is essentially involved in the allosteric regulation of the T. caldophilus enzyme by Fru-1,6-P2.     

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.     

On the biochemical basis of hereditary fructose intolerance.

In hereditary fructose intolerance it was found that in addition to an increased K_m value for Fru-1-P, the K_m of aldolase for Fru-1,6-P₂ was also increased. Furthermore, human phosphorylase $\text{a}$ was found to be inhibited by Fru-1-P in a non-competitive way.     

The effect of monovalent and divalent cations on the activity of Streptococcus lactis C10 pyruvate kinase.

The pyruvate kinase (ATP: pyruvate 2-O-phosphotransferase, EC 2.7.1.40) from Streptococcus lactis C10 had an obligatory requirement for both a monovalent cation and divalent cation. NH+4 and K+ activated the enzyme in a sigmoidal manner (nH =1.55) at similar concentrations, whereas Na+ and Li+ could only weakly activate the enzyme. Of eight divalent cations studied, only three (Co2+, Mg2+ and Mn2+) activated the enzyme. The remaining five divalent cations (Cu2+, Zn2+, Ca2+, Ni2+ and Ba2+) inhibited the Mg2+ activated enzyme to varying degrees. (Cu2+ completely inhibited activity at 0.1 mM while Ba2+, the least potent inhibitor, caused 50% inhibition at 3.2 mM). In the presence of 1 mM fructose 1,6-diphosphate (Fru-1,6-P2) the enzyme showed a different kinetic response to each of the three activating divalent cations. For Co2+, Mn2+ and Mg2+ the Hill interaction coefficients (nH) were 1.6, 1.7 and 2.3 respectively and the respective divalent cation concentrations required for 50% maximum activity were 0.9, 0.46 and 0.9 mM. Only with Mn2+ as the divalent cation was there significatn activity in the absence of Fru-1,6-P2. When Mn2+ replaced Mg2+, the Fru-1,6-P2 activation changed from sigmoidal (nH = 2.0) to hyperbolic (nH = 1.0) kinetics and the Fru-1,6-P2 concentration required for 50% maximum activity decreased from 0.35 to 0.015 mM. The cooperativity of phosphoenolpyruvate binding increased (nH 1.2 to 1.8) and the value of the phosphoenolpyruvate concentration giving half maximal velocity decreased (0.18 to 0.015 mM phosphoenolyruvate) when Mg2+ was replaced by Mn2+ in the presence of 1 mM Fru-1,6-P2. The kinetic response to ADP was not altered significantly when Mn2+ was substituted for Mg2+. The effects of pH on the binding of phosphoenolpyruvate and Fru-1,6-P2 were different depending on whether Mg2+ or Mn2+ was the divalent cation.     

Influence of fructose 2,6-bisphosphate on the aggregation properties of rat liver phosphofructokinase

The influence that fructose 2,6-bisphosphate (Fru-2,6-BP) has on the aggregation properties of rat liver phosphofructokinase has been studied by observing the fluorescence polarization of the enzyme covalently bound to the fluorescent probe pyrenebutyric acid. Fru-2,6-BP dramatically slows the dissociation of the high molecular weight aggregate forms of the enzyme when the enzyme is diluted to 3.2 micrograms/ml (4 X 10(-8) M subunits). Furthermore, Fru-2,6-BP is a strong promoter of reassociation to tetramer and larger forms if the enzyme has been previously allowed to dissociate to the dimer in its absence. Unlike many other positive effectors of liver phosphofructokinase, Fru-2,6-BP is also able to overcome the tendency of MgATP to promote tetramer formation and instead stabilize a very high degree of high molecular weight aggregate formation even in the presence of MgATP. The apparent affinity of liver phosphofructokinase for Fru-2,6-BP was measured by its ability to promote reassociation and compared to that for Fru-1,6-BP. The apparent dissociation constant for Fru-2,6-BP under these conditions is 36 microM, about 40-fold lower than the value of 1.4 mM measured for Fru-1,6-BP. Both ligands demonstrate synergism with the substrate Fru-6-P, which can lower the dissociation constant for Fru-2,6-BP 9-fold to 4 microM and that for Fru-1,6-BP 5-fold to 0.28 mM. These data are interpreted to suggest that influencing the aggregation state of rat liver phosphofructokinase may be one way in which Fru-2,6-BP achieves its effects on the enzyme in vivo.     

Nuclear magnetic resonance studies of fructose 2,6-bisphosphate and adenosine 5'-monophosphate interaction with bovine liver fructose-1,6-biphosphatase

1H and 31P nuclear magnetic resonance was used to investigate the interaction of AMP and fructose 2,6-bisphosphate (Fru-2,6-P2) with bovine liver fructose-1,6-bisphosphatase. Mn2+ bound to fructose-1,6-bisphosphatase was used as a paramagnetic probe to map the active and AMP allosteric sites of fructose-1,6-bisphosphatase. Distances between enzyme-bound Mn2+ and the phosphorus atoms at C-6 of fructose-6-P and alpha-methyl-D-fructofuranoside 1,6-bisphosphate were identical, and the enzyme-Mn to phosphorus distance determined for the C-6 phosphorus atom of Fru-2,6-P2 was very similar to these values. Likewise, the enzyme-Mn to phosphorus distances for Pi, the C-1 phosphorus atom of alpha-methyl-D-fructofuranoside 1,6-bisphosphate, and the C-2 phosphorus atom of Fru-2,6-P2 agreed within 0.5 A. The distance between enzyme-bound Mn2+ and the phosphorus atom of AMP was significantly shorter than the distances obtained for any of the aforementioned ligands, but the presence of Fru-2,6-P2 caused the enzyme-Mn to phosphorus distance for AMP to lengthen markedly. NMR line broadening of AMP protons was studied at various temperatures. The dissociation rate constant was found to be greater than 20 s-1. It was concluded that Fru-2,6-P2 strongly affects the interaction of AMP with fructose-1,6-bisphosphatase and that the sugar most likely acts at the active site of the enzyme.     

Phosphorylation and inactivation of yeast fructose-1,6-bisphosphatase by cyclic AMP-dependent protein kinase from yeast

Purified fructose-1,6-bisphosphatase from Saccharomyces cerevisiae was phosphorylated in vitro by purified yeast cAMP-dependent protein kinase. Maximal phosphorylation was accompanied by an inactivation of the enzyme by about 60%. In vitro phosphorylation caused changes in the kinetic properties of fructose-1,6-bisphosphatase: 1) the ratio R(Mg2+/Mn2+) of the enzyme activities measured at 10 mM Mg2+ and 2 mM Mn2+, respectively, decreased from 2.6 to 1.2; 2) the ratio R(pH 7/9) of the activities measured at pH 7.0 and pH 9.0, respectively, decreased from 0.62 to 0.38, indicating a shift of the pH optimum to the alkaline range. However, the affinity of the enzyme for its inhibitors fructose-2,6-bisphosphate (Fru-2,6-P2) and AMP, expressed as the concentration required for 50% inhibition, was not changed. The maximum amount of phosphate incorporated into fructose-1,6-bisphosphatase was 0.6-0.75 mol/mol of the 40-kDa subunit. Serine was identified as the phosphate-labeled amino acid. The initial rate of in vitro phosphorylation of fructose-1,6-bisphosphatase, obtained with a maximally cAMP-activated protein kinase, increased when Fru-2,6-P2 and AMP, both potent inhibitors of the enzyme, were added. As Fru-2,6-P2 and AMP did not affect the phosphorylation of histone by cAMP-dependent protein kinase, the inhibitors must bind to fructose-1,6-bisphosphatase in such a way that the enzyme becomes a better substrate for phosphorylation. Nevertheless, Fru-2,6-P2 and AMP did not increase the maximum amount of phosphate incorporated into fructose-1,6-bisphosphatase beyond that observed in the presence of cAMP alone.     

Amino-acids imitate the EDTA activation on the fructose-1,6-bisphosphatase of mantle tissue from the sea mussel (Mytilus galloprovincialis Lmk).

In the absence of AMP and Fru-2,6-P2, several amino-acids such as histidine, lysine, alanine, aspartic acid, and other molecules, as reduced glutathione or citrate, activate FBPase-1 from Mytilus galloprovincialis mantle. AMP decreases Vmax and Km for Fru-1,6-P2 both in the absence and in the presence of activators; but the addition of Fru-2,6-P2 decreases the affinity of the enzyme by its substrate. Na+ acts as a inhibitor reducing both Vmax and Km. The Km value is lower than the physiological level of Fru-1,6-P2, suggesting that the enzyme is operative but its activity is very reduced.     

Metabolism of exogenous N-acetylglucosamine in extracts of rat kidney, liver and hepatoma.

1. The metabolism of exogenous N-acetylglucosamine (GlcNAc) in rat kidney extracts was greatly stimulated by fructose 1,6-diphosphate (Fru-1,6-P2) and to a lesser extent by phosphoenolpyruvate. They served as a generator of ATP. Under these conditions, the majority of metabolized GlcNAc was recovered in the form of glycolytic intermediates. 2. The metabolism of exogenous GlcNAc in rat liver extracts was stimulated by phosphoenolpyruvate but not by Fru-1,6P2. With phosphoenolpyruvate present, most of the metabolized GlcNAc was recovered as sialic acid. 3. The metabolism of exogenous GlcNAc in rat hepatoma (AH-130) extracts was stimulated by Fru-1,6-P2 and to a lesser extent by phosphoenolpyruvate. Even with phosphoenolpyruvate present, the synthesis of sialic acid was extremely small. In these respects, hepatoma extracts resemble kidney extracts rather than those of liver.     

On the mechanism of inhibition of fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate

The inhibition of rabbit liver fructose 1,6-bisphosphatase (EC 3.1.3.11) by fructose 2,6-bisphosphate (Fru-2,6-P₂) is shown to be competitive with the substrate, fructose 1,6-bisphosphate (Fru-1,6-P₂), with K_i for Fru-2,6-P₂ of approximately 0.5 μm. Binding of Fru-2,6-P₂ to the catalytic site is confirmed by the fact that it protects this site against modification by pyridoxal phosphate. Inhibition by Fru-2,6-P₂ is enhanced in the presence of a noninhibitory concentration (5 μm) of the allosteric inhibitor AMP and decreased by modification of the enzyme by limited proteolysis with subtilisin. Fru-2,6-P_2, unlike the substrate Fru-1,6-P₂, protects the enzyme against proteolysis by subtilisin or lysosomal proteinases.     

Activation of mammalian phosphofructokinases by ribose 1,5-bisphosphate

Ribose 1,5-bisphosphate (Rib-1,5-P2), a newly discovered activator of rat brain phosphofructokinase, forms rapidly during the initiation of glycolytic flux and disappears within 20 s (Ogushi, S., Lawson, J.W. R., Dobson, G.P., Veech, R.L., and Uyeda, K. (1990) J. Biol. Chem. 265, 10943-10949). Activation of various mammalian phosphofructokinases and plant pyrophosphate-dependent phosphofructokinases by Rib-1,5-P2 was investigated. The order of decreasing potency for activation of rabbit muscle phosphofructokinase was: fructose (Fru) 2,6-P2, Rib-1,5-P2, Fru-1,6-P2, Glc-1,6-P2, phosphoribosylpyrophosphate, ribulose-1,5-P2, sedoheptulose-1,7-P2, and myoinositol-1,4-P2. The K0.5 values for activation by Rib-1,5-P2 of rat brain, rat liver, and rabbit muscle phosphofructokinases and potato and mung bean pyrophosphate-dependent phosphofructokinases were 64 nM, 230 nM, 82 nM, 710 nM, and 80 microM, respectively. The corresponding K0.5 values for Fru-2,6-P2 were 9, 8.6, 10, 7, and 65 nM, respectively. Rib-1,5-P2 was a competitive inhibitor of Fru-2,6-P2, binding to the muscle enzyme with Ki of 26 microM. Citrate increased the K0.5 for Rib-1,5-P2 without affecting the maximum activation, and AMP lowered the K0.5 for Rib-1,5-P2 without affecting the maximum activation. These effects of citrate and AMP were similar to those observed with Fru-2,6-P2 and different from those with Fru-1,6-P2. Rib-1,5-P2 is the second most potent activator of phosphofructokinase thus far discovered. The Rib-1,5-P2-activated conformation of the enzyme seems to be similar to that induced by Fru-2,6-P2, but different from that induced by Fru-1,6-P2.     

Altered glucose 1,6-bisphosphate and fructose 2,6-biphosphate levels in low-frequency stimulated rabbit fast-twitch muscle

Glucose 1,6-bisphosphate (Glc-1,6-P2) and fructose 2,6-bisphosphate (Fru-2,6-P2) concentrations display pronounced increases in rabbit fast-twitch muscle during chronic low-frequency stimulation. These increases are first seen after stimulation periods exceeding 3 h and reach maxima after 12-24 h of stimulation (approximately 3-fold for Glc-1,6-P2 and 5-fold for Fru-2,6-P2). Both metabolites regress to normal values after stimulation periods longer than 4 days. The fact that their increases coincide with the replenishment of glycogen after its initial depletion, could point to a role of Glc-1,6-P2 and Fru-2,6-P2 in glycogen metabolism.     

In vivo regulation of monomer-tetramer conversion of pyruvate kinase subtype M2 by glucose is mediated via fructose 1,6-bisphosphate

The activity of pyruvate kinase, subtype M2 (PKM2), is known to be increased by fructose 1,6-bisphosphate (Fru-1,6-P2), one of the metabolites in the glycolytic pathway. Recently, we have shown that in vitro, Fru-1,6-P2 activated the association of monomer to form the tetrameric PKM2. To ascertain whether this mode of regulation also occurs in vivo, we prepared monomer-specific monoclonal antibody and quantified the monomer formation in situ in cultured cells by immunocytochemistry. The intracellular Fru-1,6-P2 was manipulated by the glucose concentration in the media. At the physiological concentration of glucose (4-6 mM), 30-35% of PK existed as a monomer. However, PKM2 was dissociated into monomer within minutes after cells were deprived of glucose. The maximal level of monomer was detected after 1 h at 37 degrees C. Monomer was rapidly (within minutes) converted to tetramer after addition of glucose. Furthermore, when cells cultured in 10 mM of glucose were treated with cytochalasin B, an inhibitor of the glucose transporter, a maximal level of monomer was detected within 20-30 min. Determination of Fru-1,6-P2 indicated that its intracellular concentration decreased concomitantly with the reduction in glucose concentration in the medium. These results indicate that monomer-tetramer inter-conversion is a major in vivo cellular regulatory mechanism in response to changes in the extracellular glucose concentration via Fru-1,6-P2.     

Evidence that fructose 1,6-bisphosphate specifically protects the alpha-subunit of pyrophosphate-dependent 6-phosphofructo-1-phosphotransferase against proteolytic degradation.

Pyrophosphate-dependent 6-phosphofructo-1-phosphotransferase (PFP) consists of alpha (regulatory) and beta (catalytic) subunits. The alpha-subunit was previously reported to be much more susceptible to tryptic digestion than the beta-subunit. In this study, ligand-induced protection of PFP subunits against proteolysis by subtilisin was investigated in vitro and the data obtained demonstrated that fructose 1,6-bisphosphate (Fru-1,6-P(2)), while exerting negligible effect on the beta-subunit, remarkably protected the alpha-subunit against proteolytic degradation. Western blot analysis revealed a good correlation between the Fru-1,6-P(2) concentration and the degree of corresponding protection on the alpha-subunit against proteolysis. In contrast, none of other examined ligands including fructose 2,6-bisphosphate, fructose 6-phosphate and pyrophosphate had such protection on the alpha-subunit. This finding (1) indicates that the stability of the alpha-subunit can be selectively increased by Fru-1,6-P(2), and (2) suggests that Fru-1,6-P(2) is likely a special effector of the alpha-subunit.