Phosphofructokinase

Kinetic studies on the reaction catalysed by phosphofructokinase from Trypanosoma brucei

The steady-state kinetics of the reaction catalysed by the bloodstream form of Trypanosoma brucei were studied at pH 6.7. In the presence of 50 mM-potassium phosphate buffer, the apparent co-operativity with respect to fructose 6-phosphate and the non-linear relationship between initial velocity and enzyme concentration, which were found when the enzyme was assayed in 50 mM-imidazole buffer [Cronin & Tipton (1985) Biochem. J. 227, 113-124], are not evident. Studies on the variations of the initial rate with changing concentrations of MgATP and fructose 6-phosphate, the product inhibition by fructose 1,6-bisphosphate and the effects of the alternative substrate ITP were consistent with an ordered reaction pathway, in which MgATP binds to the enzyme before fructose 6-phosphate, and fructose 1,6-bisphosphate is the first product to dissociate from the ternary complex.

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.

Antagonistic effects of hexose 1,6-bisphosphates and fructose 2,6-bisphosphate on the activity of 6-phosphofructokinase purified from honey-bee flight muscle

6-Phosphofructokinase purified from honey-bee flight muscle is inhibited by ATP and, unusually, by glucose 1,6-bisphosphate and fructose 1,6-bisphosphate. The inhibition by either of the bisphosphates is not relieved by AMP, but is relieved by fructose 6-phosphate and especially by fructose 2,6-bisphosphate. Lack of effect by AMP is consistent with a low activity of adenylate kinase in this muscle.

PPi-dependent phosphofructotransferase (phosphofructokinase) activity in the mollicutes (mycoplasma) Acholeplasma laidlawii

A PPi-dependent phosphofructotransferase (PPi-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.90) which catalyzes the conversion of fructose 6 phosphate (F-6-P) to fructose 1,6-bisphosphate (F-1, 6-P2) was isolated from a cytoplasmic fraction of Acholeplasma laidlawii B-PG9 and partially purified (430-fold). PPi was required as the phosphate donor. ATP, dATP, CTP, dCTP, GTP, dGTP, UTP, dUTP, ITP, TTP, ADP, or Pi could not substitute for PPi. The PPi-dependent reaction (2.0 mM PPi) was not altered in the presence of any of these nucleotides (2.0 mM) or in the presence of smaller (less than or equal to 300 microM) amounts of fructose 2,6-bisphosphate, (NH4)2SO4, AMP, citrate, GDP, or phosphoenolpyruvate. Mg2+ and a pH of 7.4 were required for maximum activity. The partially purified enzyme in sucrose density gradient experiments had an approximate molecular weight of 74,000 and a sedimentation coefficient of 6.7. A second form of the enzyme (molecular weight, 37,000) was detected, although in relatively smaller amounts, by using Blue Sepharose matrix when performing electrophoresis experiments. The back reaction, F-1, 6-P2 to F-6-P, required Pi; arsenate could substitute for Pi, but not PPi or any other nucleotide tested. The computer-derived kinetic constants (+/- standard deviation) for the reaction in the PPi-driven direction of F-1, 6-P2 were as follows: v, 38.9 +/- 0.48 mM min-1; Ka(PPi), 0.11 +/- 0.04 mM; Kb(F-6-P), 0.65 +/- 0.15 mM; and Kia(PPi), 0.39 +/- 0.11 mM. A. laidlawii B-PG9 required PPi not only for the PPi-phosphofructotransferase reaction which we describe but also for purine nucleoside kinase activity. a dependency unknown in any other organism. In A. laidlawii B-PG9, the PPi requirement may be met by reactions in this organism already known to synthesize PPi (e.g., dUTPase and purine nucleobase phosphoribosyltransferases). In almost all other cells, the conversion of F-6-P to F-1,6-P2 is ATP dependent, and the reaction is generally considered to be the rate-limiting step of glycolysis. The ability of A. laidlawii B-PG9 and one other acholeplasma to use PPi instead of ATP as an energy source may offer these cytochrome-deficient organisms some metabolic advantage and may represent a conserved metabolic remnant of an earlier evolutionary process.

Adipose-tissue phosphofructokinase. Rapid purification and regulation by phosphorylation in vitro

A new procedure for the purification of phosphofructokinase using Blue Dextran-Sepharose is described. This allowed an approx. 1000-fold purification of phosphofructokinase from rat white and brown adipose tissue to be achieved in essentially a single step. The purified enzymes from both tissues were found to exhibit hyperbolic kinetics with fructose 6-phosphate, to be inhibited by ATP and citrate, and to be activated by 5'-AMP, phosphate and fructose 2,6-bisphosphate. The enzymes were phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase, and phosphorylation was found to be associated with increases in activity when the enzymes were assayed under appropriate sub-optimal conditions. In particular, the phosphorylated enzymes exhibited less inhibition by ATP and the white-adipose-tissue enzyme was more sensitive to activation by fructose 2,6-bisphosphate. It is suggested that an increase in the cytoplasmic concentration of cyclic AMP in tissues other than liver may result in an increase in glycolysis through the phosphorylation of phosphofructokinase by cyclic AMP-dependent protein kinase.

Characterization of the enzymatic lesion in inherited phosphofructokinase deficiency in the dog: an animal analogue of human glycogen storage disease type VII

Mammalian phosphofructokinase (PFK; ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) exists in multimolecular forms, which result from random tetramerization of three distinct subunits, M (muscle-type), L (liver-type), and P (platelet-type), each under a separate genetic control. Human muscle and liver contain homotetramers M4 and L4, respectively, whereas erythrocytes contain a mixture of M4, M3L, M2L2, ML3, and L4 isozymes. Homozygous deficiency of the M subunit in man results in glycogen storage disease (GSD) type VII, which is characterized by exertional muscle weakness and compensated hemolysis; the residual erythrocyte PFK consists of isolated L4 isozyme. Recently, PFK deficiency associated with isolated hemolytic anemia has been identified among English springer spaniel dogs. We investigated the genetic control of the dog PFK system and the nature of the enzymatic defect in two PFK-deficient animals, using chromatographic and immunological techniques. Our studies indicate the existence of a trilocus isozyme system for the dog, as is the case with other mammals. Muscle PFK consists of M4 isozyme, whereas the predominant species of liver and platelet consists, respectively, of the L4 and P4 isozyme; erythrocyte PFK consists of a three- or four-membered set composed of M and P subunits. PFK deficiency in the dogs was found to result from a total and universal lack of the M subunit, as is the case in man. However, the probands consistently exhibited L4 isozyme in their muscle; P4, L4, and hybrids thereof in their erythrocytes; and an increase in the L-containing isozymes in their platelets, indicating a generalized anomalous presence of the L subunit. The apparent absence of muscle disease in these animals is most likely accounted for by both the well-known high oxidative potential of the canine muscle in general and the presence of liver PFK in the M-deficient muscle in particular. In contrast, presence of hemolysis despite residual P4 and hybrids of P and L in the erythrocytes may be inferred to result in severe glycolytic handicap under existing intraerythrocytic conditions.

Isoenzymes of phosphofructokinase in the rat. Demonstration of the three non-identical subunits by biochemical, immunochemical and kinetic studies

In man and the rabbit, 6-phosphofructokinase (PFK, EC 2.7.1.11) exists in tetrameric isoenzymic forms composed of muscle (M or A), liver (L or B) and platelet or brain (P or C) subunits, which are under separate genetic control. In contrast, the genetic control of the rat PFK has not yet been conclusively established; it is unclear whether the P-type or C-type subunit exists in this species. To resolve this question, we investigated the enzyme from the skeletal muscle, liver and brain of rats of Wag/Rij strain. Our studies demonstrate that the rat PFK is also under the control of three structural loci and that the homotetramers M4, P4 and L4 exhibit unique chromatographic, immunological and kinetic-regulatory properties. Skeletal-muscle and brain PFKs consist of isolated M4 and P4 homotetramers respectively. Although liver PFK consists predominantly of L4 homotetramer, it also contains small amounts of PL3 and P2L2 species. All three PFKs exhibit allosteric properties: co-operativity with fructose 6-phosphate and inhibition by ATP decrease in the order P4 greater than L4 greater than M4. P4 and M4 tetramers are the most sensitive to citrate inhibition, whereas L4 tetramer is the least sensitive. More importantly, P4 and L4 isoenzymes are the most sensitive to activation by fructose 2,6-bisphosphate, whereas M4 isoenzyme is the least sensitive. These results indicate that the brain PFK in this strain of rat is a unique tetramer, P4, which also exhibits allosteric kinetics, as do the well-studied M4 and L4 isoenzymes. The reported differences in the number and nature of isoenzymes present in the rat brain and liver most probably reflect the differences in the strains studied by previous investigators. Since the nature of the rat PFK isoenzymes and nomenclatures reported by previous investigators have been now reconciled, it is proposed that, for the sake of uniformity, only well-established nomenclatures used for the rabbit or human PFK isoenzymes be used for the rat isoenzymes.

Purification and regulatory properties of phosphofructokinase from Trypanosoma (Trypanozoon) brucei brucei

Phosphofructokinase (EC 2.7.1.11) from Trypanosoma (Trypanozoon) brucei brucei was purified to homogeneity by using a three-step procedure that may be performed within 1 day. Proteolysis, which removes a fragment of Mr approx. 2000, may occur during the purification, but this can be prevented by including antipain, an inhibitor of cysteine proteinases, in the buffers during the purification. The subunits of the enzyme appear to be identical in size, with an Mr of 49 000. The Mr of the native enzyme was estimated to be approx. 220 000, suggesting a tetrameric structure. Kinetic studies showed the activity to depend hyperbolically on the concentration of ATP but sigmoidally on the concentration of fructose 6-phosphate. Although cyclic AMP, AMP and ADP stimulated the enzyme activity at low concentrations of fructose 6-phosphate, the last two nucleotides were inhibitory at high concentrations of this substrate. Phosphoenolpyruvate behaved as an allosteric inhibitor of the phosphofructokinase. Citrate, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate and Pi did not influence significantly the activity of the enzyme.

The mechanism of rabbit muscle phosphofructokinase at pH8

The mechanism of rabbit muscle phosphofructokinase was investigated by measurement of fluxes, isotope trapping and steady-state velocities at pH8 in triethanolamine/HCl buffer with 4 mM free Mg2+. Most observations were made at I0.2. The ratio Flux of fructose 1,6-bisphosphate----fructose 6-phosphate/Flux of fructose 1,6-bisphosphate----ATP at zero ATP concentration increased hyperbolically from unity to about 3.2 as the concentration of fructose 6-phosphate was increased. Similarly, the ratio Flux of fructose 1,6-bisphosphate----ATP/Flux of fructose 1,6-bisphosphate----fructose 6-phosphate at zero fructose 6-phosphate concentration increased from unity to about 1.4 as the concentration of ATP was increased. The addition of substrates must therefore be random, whatever the other aspects of the reaction. Further, from the plateau values of the ratios, it follows that the substrates dissociate very infrequently from the ternary complex and that at a low substrate concentration 72% of the reaction follows the pathway in which ATP adds first to the enzyme. Isotope-trapping studies with [32P]ATP confirmed that ATP can bind first to the enzyme in rate-limiting step and that dissociation of ATP from the ternary complex is slow in relation to the forward reaction. No isotope trapping of [U-14C]-fructose 6-phosphate could be demonstrated. The ratios Flux of ATP----fructose 1,6-bisphosphate/Flux of ATP----ADP measured at zero ADP concentration and the reciprocal of the ratio measured at zero fructose 1,6-bisphosphate concentration did not differ significantly from unity. Calculated values for these ratios based on the kinetics of the reverse reaction and assuming ordered dissociations of products or a ping-pong mechanism gave values very significantly greater than unity. These findings exclude an ordered dissociation or a substantial contribution from a ping-pong mechanism, and it is concluded that the reaction is sequential and that dissociation of products is random. Rate constants were calculated for the steps in the enzyme reaction. The results indicate a considerable degree of co-operativity in the binding between the two substrates. The observations on phosphofructokinase are discussed in relation to methods of measurement and interpretation of flux ratios and in relation to the mechanism of other kinase enzymes.

Quantitative analysis of intermediary metabolism in hepatocytes incubated in the presence and absence of glucagon with a substrate mixture containing glucose, ribose, fructose, alanine and acetate

Hepatocytes were isolated from the livers of fed rats and incubated, in the presence and absence of 100 nM-glucagon, with a substrate mixture containing glucose (10 mM), fructose (4 mM), alanine (3.5 mM), acetate (1.25 mM), and ribose (1 mM). In any given incubation one substrate was labelled with 14C. Incorporation of 14C into glucose, glycogen, CO2, lactate, alanine, glutamate, lipid glycerol and fatty acids was measured after 20 and 40 min of incubation under quasi-steady-state conditions [Borowitz, Stein & Blum (1977) J. Biol. Chem. 252, 1589-1605]. These data and the measured O2 consumption were analysed with the aid of a structural metabolic model incorporating all reactions of the glycolytic, gluconeogenic, and pentose phosphate pathways, and associated mitochondrial and cytosolic reactions. A considerable excess of experimental measurements over independent flux parameters and a number of independent measurements of changes in metabolite concentrations allowed for a stringent test of the model. A satisfactory fit to the data was obtained for each condition. Significant findings included: control cells were glycogenic and glucagon-treated cells glycogenolytic during the second interval; an ordered (last in, first out) model of glycogen degradation [Devos & Hers (1979) Eur. J. Biochem. 99, 161-167] was required in order to fit the experimental data; the pentose shunt contributed approx. 15% of the carbon for gluconeogenesis in both control and glucagon-treated cells; net flux through the lower Embden-Meyerhof pathway was in the glycolytic direction except during the 20-40 min interval in glucagon-treated cells; the increased gluconeogenesis in response to glucagon was correlated with a decreased pyruvate kinase flux and lactate output; fluxes through pyruvate kinase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase were not coordinately controlled; Krebs cycle activity did not change with glucagon treatment; flux through the malic enzyme was towards pyruvate formation except for control cells during interval II; and 'futile' cycling at each of the five substrate cycles examined (including a previously undescribed cycle at acetate/acetyl-CoA) consumed about 26% of cellular ATP production in control hepatocytes and 21% in glucagon-treated cells.

Quantification of liver and kidney phosphofructokinase by radioimmunoassay in fed, starved and alloxan-diabetic rats

A newly developed specific radioimmunoassay was used to quantify phosphofructokinase protein directly and independently of assayable activity in liver and kidney cytosol of normal fed, starved and alloxan-diabetic rats. In the fed state, liver phosphofructokinase concentration was 0.096 microM and the kidney enzyme was 0.086 microM (mumol/kg of tissue). In the starved state (24h), liver and kidney phosphofructokinase concentrations decreased by 30%. Prolonged starvation up to 72h did not further decrease enzyme concentration. In liver, total enzyme content during starvation declined by more than 50%, secondary also to a decrease in liver weight. In the alloxan-diabetic rats, there was a 22% decrease in enzyme protein concentration in liver and kidney. Total enzyme content per liver actually decreased much more (46%), because diabetes also resulted in a decrease in liver size. In conjunction with assayable activity measurements, the results of the radioimmunoassay allowed us to calculate the apparent specific activity of the enzyme. The specific activity of the kidney enzyme was 2-3 times that of the liver. Little or no change in specific activity of the liver or kidney enzyme occurred as a result of starvation or chemically induced diabetes. Tissue enzyme concentrations of phosphofructokinase unequivocally reconcile the ultimate results of changing rates of synthesis and degradation and are useful data in the design of spectrophotometric, kinetic, aggregation-disaggregation and other studies.

Binding and regulatory properties of phosphofructokinase from swine kidney

The influence of fructose 2,6-bisphosphate on the activation of purified swine kidney phosphofructokinase as a function of the concentration of fructose 6P, ATP and citrate was investigated. The purified enzyme was nearly completely inhibited in the presence of 2 mM ATP. The addition of 20 nM fructose 2,6-P2 reversed the inhibition and restored more than 80% of the activity. In the absence of fructose 2,6-P2 the reaction showed a sigmoidal dependence on fructose-6-phosphate. The addition of 10 nM fructose 2,6-bisphosphate decreased the K0.5 for fructose 6-phosphate from 3 mM to 0.4 mM in the presence of 1.5 mM ATP. These results clearly show that fructose 2,6-bisphosphate increases the affinity of the enzyme for fructose 6-phosphate and decreases the inhibitory effect of ATP. The extent of inhibition by citrate was also significantly decreased in the presence of fructose 2,6-phosphate. The influence of various effectors of phosphofructokinase on the binding of ATP and fructose 6-P to the enzyme was examined in gel filtration studies. It was found that kidney phosphofructokinase binds 5.6 moles of fructose 6-P per mole of enzyme, which corresponds to about one site per subunit of tetrameric enzyme. The KD for fructose 6-P was 13 microM and in the presence of 0.5 mM ATP it increased to 27 microM. The addition of 0.3 mM citrate also increased the KD for fructose 6-P to about 40 microM. AMP, 10 microM, decreased the KD to 5 microM and the addition of fructose 2,6-phosphate decreased the KD for fructose 6-P to 0.9 microM. The addition of these compounds did not effect the maximal amount of fructose 6-P bound to the enzyme, which indicated that the binding site for these compounds might be near, but was not identical to the fructose 6-P binding site. The enzyme bound a maximum of about 12.5 moles of ATP per mole, which corresponds to 3 moles per subunit. The KD of the site with the highest affinity for ATP was 4 microM, and it increased to 15 microM in the presence of fructose 2,6-bisphosphate. The addition of 50 microM fructose 1,6-bisphosphate increased the KD for ATP to 5.9 microM. AMP increased the KD to 5.9 microM whereas 0.3 mM citrate decreased the KD for ATP to about 2 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a specific phosphoryl binding site in swine kidney phosphofructokinase

Phosphofructokinase (PFK) from swine kidney was purified by a procedure which included affinity chromatography on Cibacron blue F3GA-Sepharose 4B and ATP-Sepharose 4B columns in order to examine its binding properties. The homogeneous enzyme was purified more than 3000-fold with a yield of 30% and it had a specific activity of 39.8 mumol/min/mg of protein at 25 degrees C. The molecular weight of the native enzyme was 360 000 and it contained 4 identical subunits of molecular weight 88 000. The principal catalytically reacting form of the enzyme had a S20,w of 13.7 S which corresponds to a molecular weight of 360 000 +/- 6 000. The initial velocity patterns in the forward and reverse directions suggested a sequential mechanism for the reaction. The Km values for fructose 6-phosphate, ATP, fructose, 1,6-bisP and ADP were 33 microM, 8.3 microM, 460 microM and 110 microM, respectively. The homogeneous native enzyme binds specifically to phosphoryl groups immobilized in cellulose phosphate columns. ATP and fructose 6-phosphate interacted with the enzyme and decreased its affinity for phosphoryl binding sites. Other metabolites including fructose 1,6-bisP, glucose 6-phosphate and various nucleotides, alone or in various combinations, were ineffective in promoting the dissociation of the enzyme. Allosteric effectors of the enzyme, such as citrate and AMP were also inactive. However, the cooperatively altered the concentration of ATP required to dissociate the enzyme from phosphoryl groups. The bound enzyme was enzymatically inactive. The enzyme was also inactivated when it was treated with pyridoxal 5'-phosphate and reduced with sodium borohydride and the inactive enzyme no longer bound to cellulose phosphate. These effects were not observed when treatment with pyridoxal 5'-phosphate was carried out in the presence of fructose 6-phosphate. These observations and the results of similar studies with swine kidney fructose 1,6-bisphosphatase (FBPase) show that both enzymes share the unique property of binding specifically to phosphoryl groups. FBPase interacts through its allosteric AMP binding site and PFK binds through its fructose 6-P binding site. This specific binding of both enzymes through these sites result in the inactivation of PFK and the desensitization of FBPase to allosteric inhibition by AMP. In the unbound state PFK may be active and FBPase can be inhibited by AMP. Taken collectively, these binding effects could play a role in the reciprocal regulation of these enzymes during gluconeogenesis in kidney.

Kinetic properties of erythrocyte phosphofructokinase in patients with type VII glycogenosis from two families--close similarity to liver type phosphofructokinase

The kinetic properties of phosphofructokinases (PFKs) from normal human liver, muscle and erythrocytes, and from erythrocytes of two unrelated patients with type VII glycogenosis (muscle PFK deficiency, McKusick 23280) were analysed in this study. Sensitivity to inhibition by ATP and to inhibition by 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate and citrate were quite different for muscle and liver PFKs. The kinetic characteristics of normal erythrocyte PFK were intermediate between those of muscle and liver PFKs. The kinetic constants of erythrocyte PFK of a patient in one family were indistinguishable from those in the other family. In addition, kinetic behaviour of residual PFK activity in erythrocytes from patients in the two families were quite similar to those of normal liver PFK. These results of kinetic analyses provide convincing evidence for the concept that normal erythrocyte PFK consists of muscle and liver type subunits. Residual erythrocyte PFK activity in type VII glycogenosis is thus concluded to reflect the activity of liver type PFK existing in patient's erythrocytes.

Allosteric regulatory properties of muscle phosphofructokinase

We have reviewed the allosteric regulatory properties of skeletal muscle phosphofructokinase and recent results on the phosphorylation of this enzyme. The number and affinities of various ligand binding sites are described, and a simple three state model is presented to explain the kinetic and ligand-binding properties of the enzyme. Data describing a lack of fit to a concerted transition model are presented. The widespread occurrence of partial phosphorylation of phosphofructokinase at a specific site near the carboxyl terminus is documented, as well as the lack of significant kinetic consequences of such phosphorylation.

The isolation and characterization of phosphofructokinase from the epithelial cells of rat small intestine

1. Only a single phosphofructokinase isoenzyme is present in the mucosa of rat small intestine. 2. Mucosal phosphofructokinase was purified to yield a homogeneous preparation of specific activity 175 units/mg of protein. 3. The native enzyme is a tetramer, with monomer Mr 84 500 +/- 5000. 4. The native enzyme may be degraded by the action of endogenous proteinases to give two products with the same specific activity as the native enzyme: degradation occurs in the order native enzyme leads to proteolytic product 1 leads to proteolytic product 2. 5. Proteolytic product 1 has a greater mobility in cellulose acetate electrophoresis at pH8 and binds more strongly to DEAE-cellulose than does native enzyme; the converse is true for proteolytic product 2. 6. Proteolytic product 1 is a tetramer with a monomer Mr about 74 300; proteolytic product 2 is also a tetramer. 7. Native enzyme can only be prepared in the presence of proteinase inhibitors; partial purifications based on simple fractionation of crude mucosal extracts in the absence of proteinases inhibitors contain proteolytic product 2 as the main component and proteolytic product 1 together with little native enzyme. 8. Purified native mucosal phosphofructokinase displayed little co-operativity with respect to fructose 6-phosphate at pH 7.0 and was only weakly inhibited by ATP.

Partial purification and regulatory properties of phosphofructokinase from Aspergillus niger

Phosphofructokinase (EC 2.7.1.11) from a citric acid-producing strain of Aspergillus niger was partially purified by the application of affinity chromatography on Blue Dextran--Sepharose and the use of fructose 6-phosphate and glycerol as stabilizers in the working buffer. The resulting preparation was still impure, but free of enzyme activities interfering with kinetic investigations. Kinetic studies showed that the enzyme exhibits high co-operativity with fructose 6-phosphate, but shows Michaelis--Menten kinetics with ATP, which inhibits at concentrations higher than those for maximal activity. Citrate and phosphoenolpyruvate inhibit the enzyme; citrate increases the substrate (fructose 6-phosphate) concentration for half-maximal velocity, [S]0.5, and the Hill coefficient, h. The inhibition by citrate is counteracted by NH4+, AMP and phosphate. Among univalent cations tested only NH4+ activates by decreasing the [S]0.5 for fructose 6-phosphate and h, but has no effect on Vmax. AMP and ADP activate at low and inhibit at high concentrations of fructose 6-phosphate, thereby decreasing the [S]0.5 for fructose 6-phosphate. Phosphate has no effect in the absence of citrate. The results indicate that phosphofructokinase from A. niger is a distinct species of this enzyme, with some properties similar to those of the yeast enzyme and in some other properties resembling the mammalian enzyme. The results of determinations of activity at substrate and effector concentrations resembling the conditions that occur in vivo support the hypothesis that the apparent insensitivity of the enzyme to citrate during the accumulation of citric acid in the fungus is due to counteraction of citrate inhibition by NH4+.

Regulation of fructose-6-phosphate 2-kinase by phosphorylation and dephosphorylation: possible mechanism for coordinated control of glycolysis and glycogenolysis

The kinetic properties and the control mechanism of fructose-6-phosphate 2-kinase (ATP: D-fructose-6-phosphate 2-phosphotransferase) were investigated. The molecular weight of the enzyme is approximately 100,000 as determined by gel filtration. The plot of initial velocity versus ATP concentration is hyperbolic with a Km of 1.2 mM. However, the plot of enzyme activity as a function of fructose-6-phosphate is sigmoidal. The apparent K0.5 for fructose-6-phosphate is 20 microM. Fructose-6-phosphate 2-kinase is inactivated by the catalytic subunit of cyclic AMP-dependent protein kinase, and the inactivation is closely correlated with phosphorylation. The enzyme is also inactivated by phosphorylase kinase in the presence of Ca2+ and calmodulin. The phosphorylated fructose-6-phosphate 2-kinase, which is inactive, is activated by phosphorylase phosphatase and alkaline phosphatase. The possible physiological significance of these observations in the coordinated control of glycogen metabolism and glycolysis is discussed.

The stereochemical course of phosphoryl transfer catalysed by Bacillus stearothermophilus and rabbit skeletal-muscle phosphofructokinase with a chiral [16O,17O,18O]phosphate ester

Bacillus stearothermophilus and rabbit skeletal-muscle phosphofructokinases catalyse the transfer of the chiral [16O,17O,18O]phosphoryl group from D-fructose 1[(S)-16O,17O,18O],6-bisphosphate to ADP with inversion of configuration at the phosphorus atom. D-Fructose 1[(S)-16O,17O,18O],-bisphosphate was synthesized in situ from sn-glycerol 3[(S)-16O,17O,18O]phosphate. The simplest interpretation of these results is that the phosphoryl group is transferred between substrates in the enzyme substrate ternary complexes by an 'in-line' mechanism.

Control of liver 6-phosphofructokinase by fructose 2,6-bisphosphate and other effectors

Rat liver 6-phosphofructokinase (ATP-D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) was partially purified free of interfering enzymes by a rapid procedure. Fructose 2,6-bisphosphate, at micromolar concentrations, greatly stimulated the enzyme by increasing its affinity for fructose 6-phosphate and relieving the inhibition by ATP. Its action was synergistic with that of AMP. As a stimulator of liver phosphofructokinase, fructose 2,6-bisphosphate was approximately 1000- and 2500-fold more efficient than fructose 1,6-bisphosphate and glucose 1,6-bisphosphate, respectively. The concentration at which a half-maximal effect was obtained with the hexose bisphosphates was dependent upon the experimental conditions. It was relatively high at physiological concentrations of substrates, AMP, and Pi, and under these conditions the positive effect of fructose 1,6-bisphosphate was no longer detectable. This was probably due to the negative effect of fructose 1,6-bisphosphate as a reaction product inhibitor. It is concluded that fructose 2,6-bisphosphate rather than fructose 1,6-bisphosphate controls, in association with other effectors, the activity of phosphofructokinase in the liver.

An activation factor of liver phosphofructokinase

Pure phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) from liver is strongly inhibited by ATP, whereas crude phosphofructokinase is only slightly inhibited by ATP. A factor that is removed from the enzyme during purification and can prevent the inhibition of phosphofructokinase by ATP has been isolated. The factor can be resolved into three components that differ in molecular weights, as shown by gel filtration on Sephadex G-25. These factors overcome the ATP inhibition but have no effect on the catalytic activity under the optimum assay conditions. Furthermore, AMP acts syngeristically with the activation factor in reversing ATP inhibition. It is proposed that the activation of phosphofructokinase by the activation factor and AMP is sufficient to account for the glycolytic flux in the liver.