Limited proteolysis of native and in vitro phosphorylated muscle phosphofructokinase

Phosphorylation of heart phosphofructokinase by Ca2+/calmodulin protein kinase

Phosphofructokinase (PFK) from sheep heart was shown to be phosphorylated by Ca2+/calmodulin protein kinase (CaM-kinase) as well as by cyclic AMP-dependent protein kinase (PKA). HPLC analysis of phosphorylated PFK indicated that phosphorylation by CaM-kinase occurs at least at two sites that are distinct from those recognized by PKA. Phosphorylation by either CaM-kinase of PKA resulted in an increase in sensitivity to ATP inhibition and a small but consistent decrease in Ki for ATP. Phosphorylation by either protein kinase caused a slight increase in the Km of PFK for fructose-6-P. Protein kinase C failed to phosphorylate PFK. Combinations of PKA, CaM-kinase and protein kinase C did not alter the stoichiometry of phosphorylation and did not change the effect on enzyme activity.

Phosphorylation of 6-phosphofructokinase in rat lung

1. 6-Phosphofructokinase of both fetal and adult rat lung consists of L, M and C subunits in a ratio of 65:25:10. 2. 6-Phosphofructokinase was purified to homogeneity from adult rat lung and subjected to phosphorylation in vitro by the catalytic subunit of cyclic AMP-dependent protein kinase. 3. This resulted in phosphorylation of the L and M subunit of 6-phosphofructokinase. 4. The C subunit was not phosphorylated. 5. However, if the phosphorylation of 6-phosphofructokinase was studied in the cytosol fraction of either fetal or adult lung using endogenous protein kinase(s), only the L subunit was phosphorylated. 6. This phosphorylation was dependent on cyclic AMP. 7. No influence of calcium, calmodulin or phosphatidylserine/diolein on the phosphorylation was observed. 8. It is concluded that although both L and M subunits of rat lung 6-phosphofructokinase are potential substrates for cyclic AMP-dependent protein kinase, their phosphorylation in situ is differentially regulated.

Phosphorylation of glycolytic and gluconeogenic enzymes by the insulin receptor kinase

Various glycolytic and gluconeogenic enzymes were tested as substrates for the insulin receptor kinase. Phosphofructokinase and phosphoglycerate mutase were found to be the best substrates. Phosphorylation of these enzymes was rapid, stimulated 2- to 6-fold by 10(-7) M insulin and occurred exclusively on tyrosine residues. Enolase, fructose 1,6-bisphosphatase, lactate dehydrogenases in decreasing order, were also subject to insulin-stimulated phosphorylation but to a smaller extent than that for phosphofructokinase or phosphoglycerate mutase. The phosphorylation of phosphofructokinase was studied most extensively since phosphofructokinase is known to catalyze a rate-limiting step in glycolysis. The apparent Km of the insulin receptor for phosphofructokinase was 0.1 microM, which is within the physiologic range of concentration of this enzyme in most cells. Tyrosine phosphorylation of phosphofructokinase paralleled autophosphorylation of the beta-subunit of the insulin receptor with respect to time course, insulin dose response (half maximal effect between 10(-9) and 10(-8) M insulin), and cation requirement (Mn2+ greater than Mg2+ much greater than Ca2+). Further study will be required to determine whether the tyrosine phosphorylation of phosphofructokinase plays a role in insulin-stimulated increases in glycolytic flux.

Phosphorylation of phosphofructokinase by protein kinase C changes the allosteric properties of the enzyme

The Ca2+- and phospholipid-dependent protein kinase C from rat brain phosphorylates rabbit muscle phosphofructokinase at the same trypsin-labile site as cyclic AMP-dependent protein kinase. However, protein kinase C also effectively phosphorylates one or more separate sites. Incubation of phosphofructokinase in the presence of protein kinase C, phospholipids, Ca2+, and ATP appears to affect the allosteric properties of phosphofructokinase by shifting the fructose 6-phosphate saturation curve to lower substrate concentrations in a time-dependent manner and decreasing cooperativity of the enzyme.

Purification and characterization of a protein phosphatase from rat liver acting on key enzymes of glucose metabolism

A phosphoprotein phosphatase has been purified from rat liver cytosol. The purification involved chromatography on DEAE-cellulose. Sephacryl S-200, fast protein liquid chromatography (FPLC) and sucrose density gradient centrifugation. It resulted in an almost homogeneous enzyme with a relative molecular mass, Mr, of 90 000 by gel filtration and sucrose gradient centrifugation and Mr = 44 500 by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE). Therefore it seems to be a dimeric enzyme. This protein phosphatase (termed PFK-phosphatase) is completely dependent on Mg2+, which can be replaced partly by Mn2+. It can be eluted from DEAE-cellulose with 120 mM NaCl, is not affected by Ca2+, 100 microM trifluoperazine or the heat-stable inhibitor-2. Inhibition occurs with phosphate, ammonium sulfate and fluoride. PFK-phosphatase dephosphorylates preferentially the alpha subunit of phosphorylase kinase (alpha/beta dephosphorylation ratio 5-10). Phosphorylase a, mixed histone and casein do not serve as substrates. The enzyme dephosphorylates effectively the key enzymes of glucose metabolism 6-phosphofructo-1-kinase, fructose 1,6-bisphosphatase, pyruvate kinase and 6-phosphofructo-2-kinase. Using this protein phosphatase and the catalytic subunit of cAMP-dependent protein kinase, a complete phosphorylation, dephosphorylation and rephosphorylation cycle was possible with 6-phosphofructo-1-kinase as substrate.

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.

Relative phosphate contents of phosphofructokinase and other soluble phosphoproteins in skeletal muscle

In vivo phosphorylation of muscle proteins has been studied by incorporation of [32P]phosphate with emphasis placed upon the phosphorylation of glycolytic enzymes. Of the approximately 25 soluble proteins resolved by two-dimensional electrophoresis that contain significant 32P, phosphofructokinase was the sole glycolytic enzyme identified as a phosphoprotein. The extent of phosphorylation found for this enzyme was the same as determined previously for purified phosphofructokinase and was about the same as the extent of phosphorylation of phosphorylase in resting muscle. Subsequent partial purification of several glycolytic enzymes confirmed the absence of significant amount of phosphate. However, phosphoglycerate mutase contained small amounts of covalently bound 32P that was exchangeable with 3-phosphoglycerate and therefore, most likely was incorporated during the catalytic reaction cycle. Analogous results were obtained for phosphoglucomutase. Both mutases were also phosphorylated at the same sites by the catalytic subunit of cyclic AMP-dependent protein kinase.

Liberation, purification, and properties of thioesterase component of pigeon liver fatty acid synthetase complex

Proteolysis of pigeon liver fatty acid synthetase with elastase results in the quantitative cleavage of the thioesterase component from the enzyme complex. This thioesterase component is two or three times more active catalytically in the isolated state than in the native fatty acid synthetase, and its activity is not affected by the presence or absence of reducing thiols. The proteolytically cleaved thioesterase is separated from the core enzyme in one step by size-exclusion chromatography on a Sephadex G-75 column. The peptide obtained by gel permeation is homogeneous with respect to size and charge, as shown by polyacrylamide gel electrophoresis in the presence and absence of SDS. Size-exclusion chromatography on Bio-Gel A 0.5 m and Sephadex G-75 columns, sucrose density gradient ultracentrifugation, and N-terminal amino acid analysis also indicate that the proteolytically cleaved thioesterase is homogeneous. The sedimentation coefficient of the thioesterase is approximately 2.9 S. Proteolytic cleavage with elastase also quantitatively releases the [1,3-14C]- or [1,3-3H]diisopropylphosphofluoridate-labeled thioesterase component from the correspondingly labeled fatty acid synthetase. Binding studies with 14C- or 3H-labelled diisopropylphosphofluoridate and fatty acid synthetase show that 2 mol of the label are bound per mol of the enzyme when complete loss of fatty acid-synthesizing activity occurs. The molecular weight of the thioesterase component is estimated to be 36000 by size-exclusion chromatography, SDS-polyacrylamide gel electrophoresis and amino acid analysis.