Phosphofructokinase from Fasciola hepatica: activation by phosphorylation and other regulatory properties distinct from the mammalian enzyme

citrate inhibition-resistant form of 6-phosphofructo-1-kinase from Aspergillus niger

Two forms of Aspergillus niger 6-phosphofructo-1-kinase (PFK1) have been described recently, the 85-kDa native enzyme and 49-kDa shorter fragment that is formed from the former by posttranslational modification. So far, kinetic characteristics have never been determined on the enzyme purified to near homogeneity. For the first time, kinetic parameters were determined for individual enzymes with respect to citrate inhibition. The native 85-kDa enzyme was found to be moderately inhibited by citrate, with the Ki value determined to be 1.5 mM, in the system with 5 mM Mg2+ ions, while increasing magnesium concentrations relieved the negative effect of citrate. An identical inhibition coefficient was determined also in the presence of ammonium ions, although ammonium acted as a strong activator of enzyme activity. On the other hand, the shorter fragment of PFK1 proved to be completely resistant to inhibition by citrate. Allosteric citrate binding sites were most probably lost after the truncation of the C-terminal part of the native protein, in which region some binding sites for inhibitor are known to be located. At near physiological conditions, characterized by low fructose-6-phosphate concentrations, a much higher efficiency of the shorter fragment was observed during an in vitro experiment. Since the enzyme became more susceptible to the positive control by specific ligands, while the negative control was lost after posttranslational modification, the shorter PFK1 fragment seems to be the enzyme most responsible for generating undisturbed metabolic flow through glycolysis in A. niger cells.

Fructose-2,6-bisphosphate and control of carbohydrate metabolism in eukaryotes

Fructose-2,6-bisphosphate is an important intracellular biofactor in the control of carbohydrate metabolic fluxes in eukaryotes. It is generated from ATP and fructose-6-phosphate by 6-phosphofructo-2-kinase and degraded to fructose-6-phosphate and phosphate ion by fructose-2,6-bisphosphatase. In most organisms these enzymatic activities are contained in a single polypeptide. The reciprocal modulation of the kinase and bisphosphatase activities by post-translational modifications places the level of the biofactor under the control of extra-cellular signals. In general, these signals are generated in response to changing nutritional states, therefore, fructose-2,6-bisphosphate plays a role in the adaptation of organisms, and the tissues within them, to changes in environmental and metabolic states. Although the specific mechanism of fructose-2,6-bisphosphate action varies between species and between tissues, most involve the allosteric activation of 6-phosphofructo-1-kinase and inhibition of fructose-1,6-bisphosphatase. These highly conserved enzymes regulate the fructose-6-phosphate/fructose-1,6-bisphosphate cycle, and thereby, determine the carbon flux. It is by reciprocal modulation of these activities that fructose-2,6-bisphosphate plays a fundamental role in eukaryotic carbohydrate metabolism.

In vivo phosphorylation of phosphofructokinase from the bivalve mollusk Mytilus galloprovincialis

The phosphorylation state of phosphofructokinase from the mantle tissue of the facultative anaerobe mollusk Mytilus galloprovincialis was determined by a back-phosphorylation technique. The incubation of intact mantle tissue with 8-bromoadenosine 3':5'-cyclic monophosphate increased significantly the phosphate content of phosphofructokinase, which indicates that the enzyme can be phosphorylated in vivo by endogenous cAMP-dependent protein kinase. The phosphate content of mussel phosphofructokinase changes significantly during the year, in agreement with the kinetic data that show a more active enzyme form in earlier autumn. These results suggest that cAMP-dependent phosphorylation of phosphofructokinase can be partially responsible for the observed glycolytic changes associated with the annual gametogenic cycle that takes place in the mantle tissue of the mollusk. On the contrary, no differences were observed between aerobic and 24-h hypoxic mussels with regard to the phosphorylation state and the kinetic constants of phosphofructokinase. This result is inconsistent with the hypothesis that phosphorylation of phosphofructokinase is involved in the glycolytic depression that takes place during the long-term environmental hypoxia that the mollusk can undergo.

Cloning of a cDNA encoding phosphofructokinase from Haemonchus contortus

Phosphofructokinase (PFK), the key regulatory enzyme in glycolysis, has been cloned from the pathogenic parasitic nematode Haemonchus contortus by functional complementation in Escherichia coli. An E. coli strain deleted for both PFK loci (strain DF1020) was transformed with plasmid DNA from a lambda ZAP II H. contortus cDNA library. Two out of 3 x 10(7) transformants were able to grow on minimal medium with mannitol as the sole carbon source. A plasmid, pPFK, containing a 2.7-kb insert, was isolated from one of these transformants and conferred on DF1020 the ability to grow on mannitol (the PFK phenotype). The complemented cells contain PFK enzyme activity, absent in the E. coli mutant, at levels considerably higher than in wild type E. coli. Sequence analysis of the 2.7-kb insert shows an open reading frame that predicts a 789-amino acid protein that has approximately 70% similarity to mammalian PFKs. The amino acid sequence around asp182, thought to be the catalytic site, is completely conserved from nematodes to mammals.

Phosphofructokinase from Fasciola hepatica: sequence of the cAMP-dependent protein kinase phosphorylation site

We have reported previously that phosphofructokinase from the liver fluke Fasciola hepatica is activated by phosphorylation with cAMP-dependent protein kinase, and that this event appears to be important in vivo for regulation of PFK (E. S. Kamemoto, L. Lan, and T. E. Mansour (1989) Arch. Biochem. Biophys. 271, 553-559). Here, we report the amino acid sequence of the single tryptic phosphopeptide generated after phosphorylation of the purified enzyme with cAMP-dependent protein kinase and [gamma 32P]ATP. Through a combination of Edman microsequence analysis, fast atom bombardment mass spectroscopy, and phosphoamino acid analysis, the sequence of the phosphorylated peptide was determined to be: R-S-T(P)-M-M-I-P-G-M-E-G-K. This sequence is not homologous to any previously determined phosphofructokinase phosphopeptides. Regulatory differences between the mammalian and parasite enzymes are discussed with particular emphasis on regulation by protein phosphorylation.

Phosphofructokinase from a vertebrate facultative anaerobe: effects of temperature and anoxia on the kinetic parameters of the purified enzyme from turtle white muscle

The effects of low temperature and anoxia were determined on phosphofructokinase (PFK) purified from white skeletal muscle of the freshwater turtle, Pseudemys scripta. These effects were assayed by comparing PFK kinetic constants measured at a high (20 degrees C) and low (6 degrees C) temperature using enzyme obtained from animals held under normoxic and anoxic conditions. When assayed at 20 degrees C, PFK from anoxic animals had a lower Ka for phosphate, a lower Ka for AMP and showed no inhibition with increasing concentrations of ATP (up to 10 mM) when compared to enzyme from normoxic animals. At 6 degrees C, anoxic enzyme had a higher Km for fructose 6-phosphate and a higher I50 value for citrate with respect to normoxic enzyme. Decreasing temperature also had a differential effect on PFK kinetic parameters depending on the source of the enzyme. When normoxic enzymes were compared at 20 and 6 degrees C, the enzyme measured at 6 degrees C showed a lower Km for ATP and a lower Ka for AMP. Comparison of anoxic enzymes at these two temperatures showed that anoxic PFK at 6 degrees C had a higher Ka for phosphate, a higher Ka for AMP, and a larger Hill coefficient. A comparison of maximal velocities at varying temperature showed that normoxic enzyme (Q10 = 2.22) was more temperature sensitive than the anoxic enzyme (Q10 = 1.80). It is possible to interconvert the normoxic and anoxic forms of PFK by incubating normoxic enzyme with the active subunit of protein kinase, suggesting that the kinetic changes observed during anoxia resulted from enzyme phosphorylation. These data are discussed with respect to the mechanisms underlying white muscle function during diving and hibernation in red-eared turtles.

Phosphofructokinase from Dirofilaria immitis: effect of fructose 2,6-bisphosphate and AMP on the non-phosphorylated and phosphorylated forms of the enzyme

The enzyme responsible for the synthesis of fructose 2,6-bisphosphate (Fru-2,6-P2), 6-phosphofructo-2-kinase, was shown to be present in the heart worm, Dirofilaria immitis. The level of Fru-2,6-P2 was determined to be 4 +/- 0.3 nmol(g wet weight)-1 in the tissues of the filariid. Fru-2,6-P2 stimulated the activity of both the non-phosphorylated and phosphorylated forms of the D. immitis phosphofructokinase (PFK). The Kact values for Fru-2,6-P2 were 378 +/- 18 nM and 65 +/- 6 nM for the non-phosphorylated and phosphorylated forms, respectively, at 1 mM fructose 6-phosphate (Fru-6-P) and 1 mM ATP at pH 6.8. AMP also stimulated the activity of both forms of the enzyme with Kact values of 230 +/- 10 microM and 37.3 +/- 6.1 microM for the non-phosphorylated and phosphorylated forms, respectively. In the absence of any effectors, the S0.5 values for Fru-6-P were 17.4 mM and 11.0 mM for the non-phosphorylated and phosphorylated forms, respectively, of the D. immitis PFK at 1 mM ATP, pH 6.8. These S0.5 values were lowered to 0.03 mM by the combined effects of saturating levels of Fru-2,6-P2 and AMP. A physiological assay was developed based on the level of metabolites in the parasite that influence the activity of PFK. This assay contained the known effectors of the PFK at concentrations approximating those found in the parasite. Under these conditions the KFru-6-P values were 153 microM and 60 microM for the non-phosphorylated and phosphorylated forms of the PFK, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo regulation of phosphorylation level and activity of phosphofructokinase by serotonin in Fasciola hepatica

The level of phosphorylation and activation of phosphofructokinase by serotonin (5-hydroxytryptamine) was studied in intact liver flukes Fasciola hepatica. The enzyme was immunoprecipitated with antiserum prepared against pure enzyme from the liver flukes. The resuspended immunoprecipitated enzyme retained most of its original activity and its kinetic properties. The level of phosphorylation was determined by a "back phosphorylation" technique. According to this technique, the immunoprecipitated phosphofructokinase was phosphorylated with the catalytic subunit of pure cAMP-dependent protein kinase. Incubation of intact liver flukes with serotonin caused an increase in the level of enzyme phosphorylation which was concomitant with an increase in enzyme activity. The level of phosphorylation was increased by 0.08 mol per protomer as a result of maximal activation by serotonin. It is proposed that phosphorylation plays, at least in part, a functional role in the regulation of phosphofructokinase from the liver fluke F. hepatica under in vivo conditions.