Binding of hexose bisphosphates to muscle phosphofructokinase

Selective activation of PFKL suppresses the phagocytic oxidative burst

In neutrophils, nicotinamide adenine dinucleotide phosphate (NADPH) generated via the pentose phosphate pathway fuels NADPH oxidase NOX2 to produce reactive oxygen species for killing invading pathogens. However, excessive NOX2 activity can exacerbate inflammation, as in acute respiratory distress syndrome (ARDS). Here, we use two unbiased chemical proteomic strategies to show that small-molecule LDC7559, or a more potent designed analog NA-11, inhibits the NOX2-dependent oxidative burst in neutrophils by activating the glycolytic enzyme phosphofructokinase-1 liver type (PFKL) and dampening flux through the pentose phosphate pathway. Accordingly, neutrophils treated with NA-11 had reduced NOX2-dependent outputs, including neutrophil cell death (NETosis) and tissue damage. A high-resolution structure of PFKL confirmed binding of NA-11 to the AMP/ADP allosteric activation site and explained why NA-11 failed to agonize phosphofructokinase-1 platelet type (PFKP) or muscle type (PFKM). Thus, NA-11 represents a tool for selective activation of PFKL, the main phosphofructokinase-1 isoform expressed in immune cells.

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase modulates oscillations of pancreatic islet metabolism

Pulses of insulin from pancreatic beta-cells help maintain blood glucose in a narrow range, although the source of these pulses is unclear. It has been proposed that a positive feedback circuit exists within the glycolytic pathway, the autocatalytic activation of phosphofructokinase-1 (PFK1), which endows pancreatic beta-cells with the ability to generate oscillations in metabolism. Flux through PFK1 is controlled by the bifunctional enzyme PFK2/FBPase2 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase) in two ways: via (1) production/degradation of fructose-2,6-bisphosphate (Fru2,6-BP), a potent allosteric activator of PFK1, as well as (2) direct activation of glucokinase due to a protein-protein interaction. In this study, we used a combination of live-cell imaging and mathematical modeling to examine the effects of inducibly-expressed PFK2/FBPase2 mutants on glucose-induced Ca²⁺ pulsatility in mouse islets. Irrespective of the ability to bind glucokinase, mutants of PFK2/FBPase2 that increased the kinase:phosphatase ratio reduced the period and amplitude of Ca²⁺ oscillations. Mutants which reduced the kinase:phosphatase ratio had the opposite effect. These results indicate that the main effect of the bifunctional enzyme on islet pulsatility is due to Fru2,6-BP alteration of the threshold for autocatalytic activation of PFK1 by Fru1,6-BP. Using computational models based on PFK1-generated islet oscillations, we then illustrated how moderate elevation of Fru-2,6-BP can increase the frequency of glycolytic oscillations while reducing their amplitude, with sufficiently high activation resulting in termination of slow oscillations. The concordance we observed between PFK2/FBPase2-induced modulation of islet oscillations and the models of PFK1-driven oscillations furthermore suggests that metabolic oscillations, like those found in yeast and skeletal muscle, are shaped early in glycolysis.

The kinetics and regulation of phosphofructokinase from Teladorsagia circumcincta

Phosphofructokinase (PFK-1) activity was examined in L(3) and adult Teladorsagia circumcincta, both of which exhibit oxygen consumption. Although activities were higher in the adult stage, the kinetic properties of the enzyme were similar in both life cycle stages. T. circumcincta PFK-1 was subject to allosteric inhibition by high ATP concentration, which increased both the Hill coefficient (from 1.4±0.2 to 1.7±0.2 in L(3)s and 2.0±0.3 to 2.4±0.4 in adults) and the K(½) for fructose 6 phosphate (from 0.35±0.02 to 0.75±0.05mM in L(3)s and 0.40±0.03 to 0.65±0.05mM in adults). The inhibitory effects of high ATP concentration could be reversed by fructose 2,6 bisphosphate and AMP, but glucose 1,6 bisphosphate had no effect on activity. Similarly, phosphoenolpyruvate had no effect on activity, while citrate, isocitrate and malate exerted mild inhibitory effects, but only at concentrations exceeding 2mM. The observed kinetic properties for T. circumcincta PFK-1 were very similar to those reported for purified Ascaris suum PFK-1, though slight differences in sensitivity to ATP concentration suggests there may be subtle variations at the active site. These results are consistent with the conservation of properties of PFK-1 amongst nematode species, despite between species variation in the ability to utilise oxygen.

Regulation of pea seed pyrophosphate-dependent phosphofructokinase: Evidence for interconversion of two molecular forms as a glycolytic regulatory mechanism

Two molecular forms of pyrophosphate-dependent phosphofructokinase (PP(i)-PFK; pyrophosphate:D-fructose-6-phosphate 1-phosphotransferase; EC 2.7.1.90) have been found whose activity depends upon association and dissociation characteristics regulated by fructose 2,6-bisphosphate (Fru-2,6-P(2)). PP(i)-PFK was purified 200-fold from cotyledons of germinating pea seeds and found to exist in two interconvertible molecular forms. The two forms of PP(i)-PFK have sedimentation coefficients of 6.3 and 12.7 S during ultracentrifugation in sucrose density gradients and also differ both in sensitivity to the activator Fru-2,6-P(2) and in affinity for the substrate fructose 6-phosphate. The major component of enzyme activity is in the large form (12.7 S), but the small, less-active, form (6.3 S) predominates when the enzyme preparation is extracted and stored in buffer without Fru-2,6-P(2) and glycerol. Urea (1 M) or pyrophosphate (20 mM) treatment results in at least a 50% loss of activity in the glycolytic direction, whereas these treatments had much less influence on the gluconeogenic direction activity. Concomitant with the loss of glycolytic activity the enzyme dissociates into the small form. Fru-2,6-P(2) stabilizes the large form of the enzyme against the dissociating effects of pyrophosphate and prevents the inactivation in the glycolytic direction during either urea or pyrophosphate treatment. The small molecular form of the enzyme is converted into the large form in the presence of Fru-2,6-P(2). We propose that glycolytic and gluconeogenic hexose metabolism in plants includes a regulatory mechanism induced by Fru-2,6-P(2) that involves the interconversion of two molecular forms of PP(i)-PFK.

A model of phosphofructokinase and glycolytic oscillations in the pancreatic beta-cell

We have constructed a model of the upper part of the glycolysis in the pancreatic beta-cell. The model comprises the enzymatic reactions from glucokinase to glyceraldehyde-3-phosphate dehydrogenase (GAPD). Our results show, for a substantial part of the parameter space, an oscillatory behavior of the glycolysis for a large range of glucose concentrations. We show how the occurrence of oscillations depends on glucokinase, aldolase and/or GAPD activities, and how the oscillation period depends on the phosphofructokinase activity. We propose that the ratio of glucokinase and aldolase and/or GAPD activities are adequate as characteristics of the glucose responsiveness, rather than only the glucokinase activity. We also propose that the rapid equilibrium between different oligomeric forms of phosphofructokinase may reduce the oscillation period sensitivity to phosphofructokinase activity. Methodologically, we show that a satisfying description of phosphofructokinase kinetics can be achieved using the irreversible Hill equation with allosteric modifiers. We emphasize the use of parameter ranges rather than fixed values, and the use of operationally well-defined parameters in order for this methodology to be feasible. The theoretical results presented in this study apply to the study of insulin secretion mechanisms, since glycolytic oscillations have been proposed as a cause of oscillations in the ATP/ADP ratio which is linked to insulin secretion.

Analysis of phosphofructokinase subunits and isozymes in ascites tumor cells and its original tissue, murine mammary gland

Phosphofructokinase (PFK) subunits and isozymes have been examined in ascites tumor cells and murine mammary gland, the tissue from where this tumor originated. Ascites tumor was found to contain predominantly the C-type subunit, whereas the L-type subunit was more abundant in mammary gland. An altered M-type subunit of lower electrophoretic mobility was found in both cell types and no M4 homotetramers were detected in either of them. Characteristic regulatory properties of ascites tumor PFK, i.e. insensitivity to fructose-1,6-P2 activation and inhibition by P-enolpyruvate, were also observed in the mammary gland isozyme. The nature of these properties and the contribution of the distinct subunit types to fructose-1,6-P2 activation are discussed.

Fructose 2,6-bisphosphate and glycolytic flux in skeletal muscle of swimming frog

Glycolytic flux in skeletal muscle is controlled by 6-phosphofructokinase but how this is achieved is controversial. Brief exercise (swimming) in frogs caused a dramatic increase in the phosphofructokinase activator, fructose 2,6-bisphosphate, in working muscle. The kinetics of phosphofructokinase suggest that in resting muscle, the enzyme is inhibited by ATP plus citrate and that the increase in fructose 2,6-bisphosphate is part of the mechanism to activate phosphofructokinase when exercise begins. When exercise was sustained, fructose 2,6-bisphosphate in muscle was decreased as was the rate of lactate accumulation. Glycolytic flux and the content of fructose 2,6-bisphosphate appear to be closely correlated in working frog muscle in vivo.

Role of phosphofructokinase during trout haemopoiesis: physiological regulation of glycolysis

1. The regulatory properties of phosphofructokinase (PFK) has been investigated in two cellular population representatives of trout haemopoiesis; haemopoietic cells (capable of replication and differentiation) and erythrocytes (highly specialized cells). 2. The intracellular levels of substrates and effectors have been quantified and their effect on PFK activity determined. 3. Fructose 1,6-bisphosphate anc cyclic AMP show a higher activation of the PFK from haemopoietic cells than the enzyme from erythrocytes. 4. AMP and phosphoenolpyruvate act as activators of the haemopoietic cell PFK while for erythrocytes PFK, AMP is an inhibitor and phosphoenolpyruvate does not display any effect. 5. Citrate inhibits PFK activity from haemopoietic cells but was not assayed in erythrocytes since it was not detected in these cells. 6. The differences in PFK regulation in both cellular populations may be attributed to the intracellular levels of the effectors and/or different isoenzymatic patterns. 7. The different regulation of PFK together with the higher enzymatic activity of PFK and pyruvate kinase from haemopoietic cells are related to the higher glycolytic flux that exhibits the haemopoietic cells. 8. The results shown in this investigation allow us to conclude that PFK has a specific role depending on the energetic requirements of the cellular population in which the enzyme is present. 9. The requirements are related to the physiological function of each type of cell.

Stability and conformation of porcine phosphofructokinase M and L

1. The inactivation of porcine liver enzyme in the presence of urea proceeded more rapidly than that of porcine heart muscle enzyme. 2. The inactivation of both enzymes by urea was protected by allosteric activators, but inhibitors had no effect. 3. The circular dichroism spectrum of liver enzyme in the near ultraviolet region was markedly affected by urea, whereas that of heart muscle enzyme was not, except for the band at 255 nm.

Phosphofructokinase in alveolar type II cells isolated from fetal and adult rat lung

The glycolytic enzyme 6-phosphofructokinase (EC 2.7.1.11) was studied in adult and fetal type II pneumocytes which had been isolated from rat lung at different days of development. In addition, the activities of the enzymes hexokinase (EC 2.7.1.1), enolase (EC 4.2.1.11) and pyruvate kinase (EC 2.7.1.40) were assayed. The specific activities of the latter enzymes decrease during perinatal development and reach about adult values shortly after birth. In contrast, 6-phosphofructokinase activity increases slightly until 2 days before birth, and drops sharply afterwards. The 6-phosphofructokinase subunit composition was determined in fetal and adult type II cells. The ratio of the three subunits of 6-phosphofructokinase in type II cells isolated on fetal days 19 and 21 (term is at day 22) and in adult type II cells was identical: the three subunits were present in a ratio of 68: 14: 18 for types L, M and C, respectively. In addition, we investigated some regulatory properties of 6-phosphofructokinase from alveolar type II cells. 6-Phosphofructokinase from alveolar type II cells is strongly inhibited by increasing MgATP concentrations. This inhibition is reflected by an increase in the S0.5 for fructose 6-phosphate. Fructose 2,6-bisphosphate stimulates alveolar type II 6-phosphofructokinase. Half-maximal stimulation occurs at 1.6 and 2.0 microM fructose 2,6-bisphosphate for fetal and adult type II cells, respectively. The level of the most potent positive effector of 6-phosphofructokinase, fructose 2,6-bisphosphate, was also determined. The level of the hexose bisphosphate decreases during prenatal development; however, the level in the adult type II cells is considerably lower. The concentration of fructose 2,6-bisphosphate appears to be sufficient to fully activate 6-phosphofructokinase both in fetal and adult type II cells.

Inositol 1,4-bisphosphate is an allosteric activator of muscle-type 6-phosphofructo-1-kinase

The allosteric effects of various inositol biphosphate (InsP2) isomers and other inositol phosphates, of glycerophosphoinositol phosphates (GroPInsPx) and of phosphoinositides (PtdInsPx) on muscle-type 6-phosphofructo-1-kinase (PFK) were investigated. The binding of these substances to PFK was indirectly estimated by their ability to stabilize the tetrameric enzyme. At near-physiological concentrations of other allosteric effectors, muscle PFK was activated AMP-dependently by Ins(1,4)P2 (Ka = 43 microM), Ins(2,4)P2 (Ka = 70 microM) and GroPIns4P (Ka = 20 microM). These compounds activated PFK by a mechanism similar to that established for activating hexose bisphosphates. Indirect binding experiments indicated minimal Kd,app. values of about 5 microM for the binding of Ins(1,4)P2 in the presence of 0.1 mM-AMP at pH 7.4. This apparent affinity was comparable with that of fructose 1,6-bisphosphate and glucose 1,6-bisphosphate at identical conditions. The enzyme was also found to interact specifically with PtdIns4P (Kd,app. = 37 microM), the inositol phospholipid carrying Ins(1,4)P2 as its head group. The regulatory behaviour of muscle-type PFK in vitro and the concentrations of Ins(1,4)P2 in vivo (between 4 and greater than 50 nmol/g wet wt. of tissue) are consistent with the hypothesis that there is a functional interaction in vivo. Furthermore, a role of PtdIns4P in membrane compartmentation of PFK is suggested. Comparative experiments with liver PFK indicate that these regulatory properties may be relatively specific for the muscle isoform. Unlike muscle PFK, the liver isoform was slightly activated by sub-micromolar concentrations of Ins(1,4,5)P3.

Activation of muscle phosphofructokinase by alpha-glucose 1,6-bisphosphate and fructose 2,6-bisphosphate is differently affected by other allosteric effectors and by pH

Citrate, ATP and AMP affect similarly the activation of muscle phosphofructokinase by alpha-glucose 1,6-bisphosphate and fructose 1,6-bisphosphate, but they affect differently its activation by fructose 2,6-bisphosphate. Activation by alpha-glucose 1,6-bisphosphate and fructose 2,6-bisphosphate is also differently affected by pH. This suggest that both alpha-glucose 1,6-bisphosphate and fructose 1,6-bisphosphate induce the same conformational change on muscle phosphofructokinase, distinct from that produced by fructose 2,6-bisphosphate.

Modulation of phosphofructokinase action by macromolecular interactions. Quantitative analysis of the phosphofructokinase-aldolase-calmodulin system

The simultaneous effect of calmodulin and aldolase (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphate-lyase, EC 4.1.2.13) on the concentration-dependent behaviour of muscle phosphofructokinase (ATP: D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) has been analysed by means of a covalently attached fluorescent probe, gel penetration experiments, and using a kinetic approach. We found that calmodulin-induced inactivation of phosphofructokinase is suspended by addition of an equimolar amount of aldolase. This effect was attributed to an apparent competition of calmodulin and aldolase for the dimeric forms of kinase. Moreover, the direct binding of aldolase to calmodulin has also been demonstrated, which resulted in a significant decrease in the kcat value of the enzyme. The quantitative analysis of these interactions in the system phosphofructokinase-calmodulin-aldolase is presented. A possible molecular model for the modulation of phosphofructokinase action by macromolecular interactions is envisaged.

Changes in glucose 1,6-bisphosphate content in rat skeletal muscle during contraction

Glucose 1,6-bisphosphate, fructose 2,6-bisphosphate, glycogen, lactate and other glycolytic metabolites were measured in rat gastrocnemius muscle, which was electrically stimulated in situ via the sciatic nerve. Both the frequency and the duration of stimulation were varied to obtain different rates of glycolysis. There was no apparent relationship between fructose 2,6-bisphosphate content and lactate accumulation in contracting muscle. In contrast, glucose 1,6-bisphosphate content increased with lactate concentration during contraction. It is suggested that the increase in glucose 1,6-bisphosphate could play a role in phosphofructokinase stimulation and in the activation of the glycolytic flux during muscle contraction.

A competitive binding assay for fructose 2,6-bisphosphate

A new direct assay method for fructose 2,6-bisphosphate has been developed based on competitive binding of labeled and unlabeled fructose 2,6-P2 to phosphofructokinase. Phosphofructokinase (0.5-1.3 pmol protomer) is incubated with saturating concentrations (5.0-5.5 pmol) of fructose 2,6-[2-32P]P2 and samples containing varying concentrations of fructose 2,6-P2. The resulting stable binary complex is retained on nitrocellulose filters with a binding efficiency of up to 70%. Standard curves obtained with this assay show strict linearity with varying fructose 2,6-P2 in the range of 0.5 to 45 pmol, which exceeds the sensitivity of most of the previously described assay methods. Fructose 2,6-P2, ATP, and high concentrations of phosphate interfere with this assay. However, the extent of this inhibition is negligible since their tissue contents are one-half to one-tenth that examined. This new assay is simple, direct, rapid, and does not require pretreatment of tissue extracts.

Stereospecificity of the fructose 2,6-bisphosphate site of muscle 6-phosphofructo-1-kinase

We explored the stereospecificity of the fructose 2,6-bisphosphate site of rabbit muscle 6-phosphofructo-1-kinase by determination of the activation constants (Ka) of several structurally locked analogues of this potent metabolic regulator. Under the assay conditions used, the Ka of fructose 2,6-bisphosphate was 0.12 microM. The most effective synthetic analogues and their Ka's were 2,5-anhydro-D-mannitol 1,6-bisphosphate (2.9 microM), 1,4-butanediol bisphosphate (6.6 microM), hexitol 1,6-bisphosphate (40 microM), and 2,5-anhydro-D-glucitol 1,6-bisphosphate (47 microM). Ten other bisphosphate compounds were much less effective as activators of the enzyme. These findings indicate that, unlike its active site, this allosteric site of 6-phosphofructo-1-kinase does not require the furanose ring. Its basic requirement seems to be a compound with two phosphate groups approximately 9 A apart. Although the free hydroxy groups of the activator do not seem to be essential, their presence enhances appreciably the affinity of the ligand for this regulatory site.

On the ontogeny and interactions of phosphofructokinase in mouse tissues

The distribution and interactions of phosphofructokinase isozymes with cellular structure have been studied in the major tissues of the mouse during development. The ontogenic patterns of isozymes which were obtained were consistent with those observed for other species and are interpreted in terms of the presence of three genes and three homotetrameric forms of the enzyme (A4, B4 and C4) in the tissues of the mouse. In addition, the data provides a clear indication that interactions between the enzyme and cellular structure are appreciable in all major tissues and at all stages of development, with all isozyme types exhibiting such interactions. The significance of the study of subcellular interactions of these isozymes in contributing to a comprehensive physiological rationale for this mammalian enzyme and its multiple forms is discussed.

Effect of glutamine on fructose 2,6-bisphosphate and on glucose metabolism in HeLa cells and in chick-embryo fibroblasts

Glutamine caused a dose-dependent decrease in fructose 2,6-bisphosphate concentration in both HeLa cells and chick-embryo fibroblasts. The effect was complete within 15 min in HeLa cells, but required more than 9 h in the fibroblasts. Half-maximal effects were obtained with 0.1-0.3 mM-glutamine. In chick-embryo fibroblasts, but not in HeLa cells, glutamine induced a time-dependent decrease in the activity of phosphofructokinase-2, which correlated with the decrease in fructose 2,6-bisphosphate. Glutamine decreased the glycolytic flux by about 25% only in chick-embryo fibroblasts. The difference in glycolytic response between the two types of cells might correspond to a difference in the sensitivity of phosphofructokinase-1 for fructose 2,6-bisphosphate. In HeLa cells, glutamine caused a 2-3-fold stimulation of the synthesis of glycogen, a 50% decrease in the concentration of fructose 1,6-bisphosphate and a more than 80% decrease in the concentration of 5-phosphoribosyl pyrophosphate; the concentrations of hexose 6-phosphates and ATP were not affected.

Fructose 2,6-bisphosphate in rat skeletal muscle during contraction

Fructose 2,6-bisphosphate and several glycolytic intermediates were measured in two rat muscles, extensor digitorum longus and gastrocnemius, which were electrically stimulated in situ. Both the duration and the frequency of stimulation were varied to obtain different rates of glycolysis. There was no relationship between fructose 2,6-bisphosphate content and the increase in tissue lactate in contracting muscle. However, in gastrocnemius stimulated at low frequencies (less than or equal to 5 Hz), there was a 2-fold increase in fructose 2,6-bisphosphate at 10s, followed by a return to basal values, whereas lactate increased only after 1 min of contraction. The concentrations of hexose 6-phosphates, fructose 1,6-bisphosphate and triose phosphates were all increased during the 3 min stimulation. During tetanus (frequencies greater than or equal to 10 Hz) fructose 2,6-bisphosphate was not increased, whereas glycolysis was maximally stimulated and resulted in an accumulation of tissue lactate, mostly from glycogen. The concentrations of hexose 6-phosphate increased continuously during the 1 min tetanus, whereas fructose 1,6-bisphosphate was increased at 10s and then decreased progressively. It therefore appears that fructose 2,6-bisphosphate does not play a role in the stimulation of glycolysis during tetanus; it may, however, be involved in the control of glycolysis when the muscles are stimulated at low frequencies for short periods of time.

Interaction of calmodulin with muscle phosphofructokinase. Interplay with metabolic effectors of the enzyme under physiological conditions

The hysteretic calmodulin-induced inactivation of muscle phosphofructokinase and the calmodulin-mediated reactivation are essentially dependent on environmental conditions. The interplay of calmodulin during these reactions and at allosteric conditions with Mg . ATP, fructose 6-phosphate, adenosine 5'-[beta, gamma-imido]triphosphate and with the allosteric effectors AMP, ADP, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate and glucose 1,6-bisphosphate was studied by two techniques. (a) A two-step technique with a preincubation of enzyme, calmodulin and effectors in close to physiological concentrations before dilution into an optimal activity assay. It reveals aggregation and slowly reversible conformation changes. (b) A direct assay of dilute enzyme at allosteric conditions. Dominating in the interplay of calmodulin with metabolic effectors is the competitive-like action of calmodulin on Mg . ATP binding to the regulatory sites of the enzyme. At high enzyme concentrations in the absence of hexose phosphates, i.e. at noncatalytic conditions calmodulin counteracts the stabilization of the highly active tetrameric form caused by Mg . ATP. In the allosteric assay it counteracts the ATP-induced allosteric inhibition. In both cases calmodulin acts synergistic with AMP and ADP. To a minor degree calmodulin also counteracts the stabilization of the tetrameric form caused by fructose 6-phosphate and hexose bisphosphate, now however antagonistically to AMP and ADP. By the demonstrated interactions the enzyme can be slowly and hysteretically shifted between an active tetrameric and an inactive dimeric state under control metabolic conditions and of Ca2+ and calmodulin. Resting conditions will inactivate and high contractile activity reactivate available enzyme.

Interaction of calmodulin with muscle phosphofructokinase. Changes of the aggregation state, conformation and catalytic activity of the enzyme

Phosphofructokinase from muscle has been shown to be a calmodulin-binding protein [Mayr, G.W. and Heilmeyer, L.M.G., Jr (1983) FEBS Lett. 159, 51-57]. Details of the influence of calmodulin on the aggregation state, the conformation and the catalytic properties of phosphofructokinase have been studied by enzymatic and light-scattering analyses. Calmodulin acts as a Ca2+-dependent hysteretic inhibitor of the highly active enzyme. At least one mole of calmodulin binds to each protomer of the enzyme, induces a shift from the highly active tetrameric towards an inactive dimeric state and slowly changes the conformation of the dimers. Dissociation of calmodulin from conformationally changed dimers by removal of Ca2+ stops the inactivation. Without a significant regain of catalytic activity large polymers are rapidly formed. For a reactivation of the inactivated enzyme, calmodulin has to remain associated and the incubation conditions must be changed in a way to allow for a back isomerization and reassociation of dimers. The isomerization reaction is promoted by Mg . ATP, the reassociation reaction most effectively by fructose bisphosphate. A model for the calmodulin-phosphofructokinase interaction is proposed.

Evolution of phosphofructokinase--gene duplication and creation of new effector sites

Phosphofructokinases (PFK; EC 2.7.1.11) are tetrameric enzymes that have a key role in the regulation of glycolysis; as such, they are subject to allosteric activation and inhibition by various metabolites. Eukaryotic PFKs are about twice the size of prokaryotic enzymes and are regulated by a wider repertoire of effectors: for example, the subunit molecular weights of rabbit muscle (RM) PFK and Bacillus stearothermophilus (Bs) PFK are 82,000 and 36,000, respectively. Both enzymes are activated by ADP (or AMP), but RM-PFK is also activated by fructose bisphosphates (FBP) and inhibited by ATP and citrate. This, together with other evidence, has led to speculation that mammalian PFKs have evolved by duplication of a prokaryotic gene, although previous peptide analysis failed to reveal internal homology in RM-PFK. Here we demonstrate clear homology among the N- and C-halves of RM-PFK and Bs-PFK, thus establishing an evolutionary relationship by series gene duplication and divergence. Furthermore, detailed knowledge of the Bs-PFK structure provides the basis for inferences concerning the structural organization of RM-PFK and the evolution of new effector sites in the enzyme tetramer.

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

Fructose 2,6-bisphosphate, a potent inhibitor of fructose-1,6-bisphosphatases, was found to be an inhibitor of the Escherichia coli enzyme. The substrate saturation curves in the presence of inhibitor were sigmoidal and the inhibition was much stronger at low than at high substrate concentrations. At a substrate concentration of 20 microM, 50% inhibition was observed at 4.8 microM fructose 2,6-bisphosphate. Escherichia coli fructose-1,6-bisphosphatase was inhibited by AMP (Ki = 16 microM) and phosphoenolpyruvate caused release of AMP inhibition. However, neither AMP inhibition nor its release by phosphoenolpyruvate was affected by the presence of fructose 2,6-bisphosphate. The results obtained, together with previous observations, provide further evidence for the fructose 2,6-bisphosphate - fructose-1,6-bisphosphatase active site interaction.