Phosphorylation of rat hepatic fructose-1,6-bisphosphatase and pyruvate kinase

High Density Protein Kinase Assay With Subattomole Sensitivity

A rapid assay system based on incorporation of [γ-32P]ATP into biotinylated peptide substrates and their subsequent capture onto a high capacity streptavidin-coated membrane, SAM2™, has been developed for the detection of protein kinases. The system uses prenumbered and partially cut membrane squares for analyzing a limited number of samples or can be formatted to analyze up to 1,536 samples per microtiter plate footprint of 7.0 cm X 10.6 cm. The high biotin-binding capacity and low background of the membrane allows the use of nearly saturating amounts of most substrates, giving this system very high signal-to-noise ratios at low enzyme concentrations. Using cAMP-dependent Protein Kinase A (PKA) as a model system, as little as 0.3 amol of purified enzyme in 0.2 μl can be detected with a linear response range of over 3 orders of magnitude. cAMP-dependent kinase activity can be measured directly in tissue extracts by using a specific substrate and harsh washing procedures to reduce nonspecific backgrounds from proteins phosphorylated by other kinases. For increased assay flexibility, results can be analyzed either by PhosphorImager™ quantitation or by scintillation counting.

Are all regions of folded proteins that undergo ligand-dependent order-disorder transitions targets for allosteric peptide mimetics?

Although the classical view of how proteins function relied on well folded structures, it is now recognized that the functions of many proteins are dependent on being intrinsically disordered. The primary consideration in this work is the intermediate group of proteins that are overall well folded, but which contain small regions that undergo order-disorder transitions. In particular, the current focus is on those order-disorder transitions that are energetically coupled to ligand binding. As exemplified by the case of human liver pyruvate kinase (hL-PYK), peptides that mimic the sequence of the order-disorder region can be used as allosteric regulators of the enzyme. On the basis of this example and others reported in the literature, we propose that a similar use of peptides that mimic protein regions that experience ligand-dependent order-disorder transitions can be a generalized initiation point for the development of allosteric drugs.

Transducin-like enhancer of split-6 (TLE6) is a substrate of protein kinase A activity during mouse oocyte maturation

Fully grown oocytes in the ovary are arrested at prophase of meiosis I because of high levels of intraoocyte cAMP that maintain increased levels of cAMP-dependent protein kinase (PKA) activity. Following the luteinizing hormone surge at the time of ovulation, cAMP levels drop, resulting in a reduction in PKA activity that triggers meiotic resumption. Although much is known about the molecular mechanisms of how PKA activity fluctuations initiate the oocyte's reentry into meiosis, significantly less is known about the requirement for PKA activity in the oocyte after exit from the prophase I arrest. Here we show that although PKA activity decreases in the oocyte upon meiotic resumption, it increases throughout meiotic progression from the time of germinal vesicle breakdown (GVBD) until the metaphase II (MII) arrest. Blocking this meiotic maturation-associated increase in PKA activity using the pharmacological inhibitor H89 resulted in altered kinetics of GVBD, defects in chromatin and spindle dynamics, and decreased ability of oocytes to reach MII. These effects appear to be largely PKA specific because inhibitors targeting other kinases did not have the same outcomes. To determine potential proteins that may require PKA phosphorylation during meiosis, we separated oocyte protein extracts on an SDS-PAGE gel, extracted regions of the gel that had corresponding immune reactivity towards an anti-PKA substrate antibody, and performed mass spectrometry and microsequencing. Using this approach, we identified transducin-like enhancer of split-6 (TLE6)-a maternal effect gene that is part of the subcortical maternal complex-as a putative PKA substrate. TLE6 localized to the oocyte cortex throughout meiosis in a manner that is spatially and temporally consistent with the localization of critical PKA subunits. Moreover, we demonstrated that TLE6 becomes phosphorylated in a narrow window following meiotic resumption, and H89 treatment can completely block this phosphorylation when added prior to GVBD but not after. Taken together, these results highlight the importance of oocyte-intrinsic PKA in regulating meiotic progression after the prophase I arrest and offer new insights into downstream targets of its activity.

Computational simulation of ligand docking to L-type pyruvate kinase subunit

Computational blind docking approach was used for mapping of possible binding sites in L-type pyruvate kinase subunit for peptides, RRASVA and the phosphorylated derivative RRAS(Pi)VA, which model the phosphorylatable N-terminal regulatory domain of the enzyme. In parallel, the same docking analysis was done for both substrates of this enzyme, phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP), and for docking of fructose 1,6-bisphosphate (FBP), which is the allosteric activator of the enzyme. The binding properties of the entire surface of the protein were scanned and several possible binding sites were identified in domains A and C of the protein, while domain B revealed no docking sites for peptides or for substrates or the allosteric regulator. It was found that the docking sites of different ligands were partially overlapping, pointing to the possibility that some regulatory effects, observed in the case of L-type pyruvate kinase, may be caused by the competition of different ligands for the same binding sites.

Probing L-pyruvate kinase regulatory phosphorylation site by mutagenesis

The activity of L-type pyruvate kinase (L-PK, ATP:pyruvate 2-O-phosphotransferase, EC 2.7.1.40) is regulated by phosphorylation of serine residue 12 of the N-terminal regulatory domain MEGPAGYLRR(10)AS ( 12 )VAQLTQEL(20)GTAFF of the protein. In this report we studied the effect of the point mutations around this phosphorylation site on the catalytic properties of this enzyme, by introducing amino acids A, L, K, Q and E into positions 9, 10 and 13 of this peptide sequence. It was found that some of these mutations in positions 9 and 10, although occurring at great distances from the enzyme's active site, affected the enzyme's activity by decreasing the effectiveness of phosphoenolpyruvate binding (PEP) with the enzyme, but had practically no influence on the binding effectiveness of the second substrate ADP. A similar asymmetric effect on the binding of these substrates was previously observed after phosphorylation of the enzyme regulatory N-domain peptide, and also after proteolytic truncation of the same N-terminal part of L-PK. All these results could be explained by the internal complex formation between the N-domain peptide and the enzyme's main body. The present study delineated the specificity of the internal binding site and revealed the possibility that the regulatory effect could be modulated by selecting mutation sites and amino acids introduced into the N-terminal domain structure.

An activating interaction between the unphosphorylated n-terminus of human liver pyruvate kinase and the main body of the protein is interrupted by phosphorylation

The initial 26 amino acids of human liver pyruvate kinase (L-PYK) are not present/observed in the crystal structure. This region includes Ser12, the site of hormone-dependent phosphorylation. Truncating the N-terminus of L-PYK mimics the effects of phosphorylation by causing a decrease in apparent phosphoenolpyruvate (PEP) affinity. An N-terminus truncation series was used to map the minimum number of residues that could be removed to result in the decrease in apparent PEP affinity. Results are consistent with a mechanism by which phosphorylation at Ser12 interrupts an activating interaction of N-terminal residues (including those at positions 7-10) with the main body of the protein, as a means of inhibiting substrate affinity.

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 degrees 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.

Expression of inducible 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase/PFKFB3 isoforms in adipocytes and their potential role in glycolytic regulation

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase) catalyzes the synthesis and degradation of fructose 2,6-bisphosphate (F2,6BP), which is a powerful activator of 6-phosphofructo-1-kinase, the rate-limiting enzyme of glycolysis. Four genes encode PFK-2/FBPase (PFKFB1-4), and an inducible isoform (iPFK-2/PFKFB3) has been found to mediate F2,6BP production in proliferating cells. We have investigated the role of iPFK-2/PFKFB3 and related isoforms in the regulation of glycolysis in adipocytes. Human visceral fat cells express PFKFB3 mRNA, and three alternatively spliced isoforms of iPFK-2/PFKFB3 are expressed in the epididymal fat pad of the mouse. Forced expression of the iPFK-2/PFKFB3 in COS-7 cells resulted in increased glucose uptake and cellular F2,6BP content. Prolonged insulin treatment of 3T3-L1 adipocytes led to reduced PFKFB3 mRNA expression, and epididymal fat pads from db/db mice also showed decreased expression of PFKFB3 mRNA. Finally, anti-phospho-iPFK-2(Ser461) Western blotting revealed strong reactivity in insulin-treated 3T3-L1 adipocyte, suggesting that insulin induces the phosphorylation of PFKFB3 protein. These data expand the role of these structurally unique iPFK-2/PFKFB3 isoforms in the metabolic regulation of adipocytes.

Novelty of the Pyruvate Metabolic Enzyme Dihydrolipoamide Dehydrogenasein Spermatozoa

Spermatozoa are cells distinctly different from other somatic cells of the body, capacitation being one of the unique phenomena manifested by this gamete. We have shown earlier that dihydrolipoamide dehydrogenase, a post-pyruvate metabolic enzyme, undergoes capacitation-dependent tyrosine phosphorylation, and the functioning of the enzyme is required for hyperactivation (enhanced motility) and acrosome reaction of hamster spermatozoa (Mitra, K., and Shivaji, S. (2004) Biol. Reprod. 70, 887-899). In this report we have investigated the localization of this mitochondrial enzyme in spermatozoa revealing non-canonical extra-mitochondrial localization of the enzyme in mammalian spermatozoa. In hamster spermatozoa, dihydrolipoamide dehydrogenase along with its host complex, the pyruvate dehydrogenase complex, are localized in the acrosome and in the principal piece of the sperm flagella. The localization of dihydrolipoamide dehydrogenase, however, appears to be in the mitochondria in the spermatocytes, but in spermatids it appears to show a juxtanuclear localization (like Golgi). The capacitation-dependent time course of tyrosine phosphorylation of dihydrolipoamide dehydrogenase appears to be different in the principal piece of the flagella and the acrosome in hamster spermatozoa. Activity assays of this bi-directional enzyme suggest a strong correlation between the tyrosine phosphorylation and the bi-directional enzyme activity. This is the first report of a direct correlation of the localization, tyrosine phosphorylation, and activity of the important metabolic enzyme, dihydrolipoamide dehydrogenase, implicating dual involvement and regulation of the enzyme during sperm capacitation.

A strain-specific alteration of proteomic expression in mouse liver fructose 1,6-bisphosphatase isoforms by alcohol

To study alcohol-related metabolism across inbred mouse strains, liver tissues from C57BL/6J (B6, an alcohol-preferring mouse) and DBA/2J (D2, an alcohol-avoiding strain) mice were analyzed for proteomic expression patterns over time after a single-dose of alcohol (1.5 g/kg ingestion). Despite no significant difference in the elimination rate of blood ethanol, two-dimensional electrophoresis gel images of liver proteins showed that proteins in B6 mice exhibited faster response and more quantitative (spot numbers) and qualitative (spot densities) changes than in D2 mice. Among the differentially expressed metabolic enzymes, four variants (alpha, beta, gamma and delta) of fructose 1,6-bisphosphatase (FBPase), a key regulatory gluconeogenic enzyme, showed remarkable changes in expression with time across the strains. The degree of spot alteration in alpha- and gamma-variants of FBPase in B6 mice was much higher than in D2 mice, while the beta- and delta-forms were not changed as much. Mass spectrometry (MS) analysis showed that the 1714.9 +/- 1 mass peak from the alpha- and gamma-variants of FBPase was much stronger than that of the beta- and delta-variants in both strains regardless of spot density. This MS peak contains 2-ANHAPFETDISTLTR-16, located at the N-terminal of FBPase, where the N-terminal alanine was found to be trimethylated. Thus, we propose this N-terminal fragment as a potential site for enzyme modification in response to ethanol, allowing for differences in two-dimensional gel spot intensity of variants of FBPase in the two mouse strains.

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: suiting structure to need, in a family of tissue-specific enzymes

The present review addresses recent advances in research into a family of bifunctional enzymes that are responsible for the twofold task of synthesizing and hydrolyzing fructose-2,6-bisphosphate (Fru-2,6-P2), which in turn regulates the rate of glycolysis in most cells. The structure of the synthetic kinase, conjoined at its carboxyl-terminus to the phosphatase, is very highly conserved throughout evolution and differentiation, with isotypic expression arising from highly variable amino-terminal and carboxyl-terminal regulatory domains. These domains, which frequently contain protein-kinase-catalyzed phosphorylation motifs, are responsible for the widely divergent kinetics observed in various tissues and species, and for the hormonal modulation that alters intracellular levels of Fru-2,6-P2. The present review discusses recent advances in relating structure to function, and the identification of new pathways of transcriptional regulation of this important family of regulatory enzymes.

Neurotrophin regulation of beta-actin mRNA and protein localization within growth cones

Neurotrophins play an essential role in the regulation of actin-dependent changes in growth cone shape and motility. We have studied whether neurotrophin signaling can promote the localization of beta-actin mRNA and protein within growth cones. The regulated localization of specific mRNAs within neuronal processes and growth cones could provide a mechanism to modulate cytoskeletal composition and growth cone dynamics during neuronal development. We have previously shown that beta-actin mRNA is localized in granules that were distributed throughout processes and growth cones of cultured neurons. In this study, we demonstrate that the localization of beta-actin mRNA and protein to growth cones of forebrain neurons is stimulated by neurotrophin-3 (NT-3). A similar response was observed when neurons were exposed to forskolin or db-cAMP, suggesting an involvement of a cAMP signaling pathway. NT-3 treatment resulted in a rapid and transient stimulation of PKA activity that preceded the localization of beta-actin mRNA. Localization of beta-actin mRNA was blocked by prior treatment of cells with Rp-cAMP, an inhibitor of cAMP-dependent protein kinase A. Depolymerization of microtubules, but not microfilaments, inhibited the NT-3-induced localization of beta-actin mRNA. These results suggest that NT-3 activates a cAMP-dependent signaling mechanism to promote the microtubule-dependent localization of beta-actin mRNA within growth cones.

Heat-stable inhibitor protein derived peptide substrate analogs: phosphorylation by cAMP-dependent and cGMP-dependent protein kinases

The phosphorylation of substrate peptides derived from PKI, the heat-stable inhibitor protein of the cAMP-dependent protein kinase (PKA), has been studied with both PKA and the cGMP-dependent protein kinase (PKG) using a variety of substitution and deletion analogs. On the basis of Km, kcat and kcat/Km values, (Ser21)PKI alpha(14-22) amide (numbering based upon native PKI alpha) is the most effective peptide substrate yet discovered for either kinase, although other peptides, while phosphorylated considerably less efficiently by PKG, are more specific. Although the inhibitory peptide corresponding to this sequence (i.e., with an Ala at position 21) is a much more potent inhibitor of PKA than of PKG (approximately 250-fold), PKG actually exhibits a 60% higher kcat than does PKA with the (Ser21)PKI alpha(14-22) amide substrate peptide, with only a 20-fold higher Km value. The two key PKI residues within this peptide which were found to be essential for substrate activity with both kinases were Arg18 (P-3) and Ile22 (P+1). The Arg19 (P-2) residue, which contributes significantly to both PKI-based peptide inhibitors and substrates of PKA, was only a more minor contributor to PKG substrate efficacy. Of particular note, the Phe10 (P-11) residue, which contributes very substantially to high affinity binding of both PKI and longer PKI peptide inhibitors, neither positively nor negatively affects the kinetics of either PKA or PKG with PKI-based substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro phosphorylation of fructose-1,6-bisphosphatase from rabbit and pig liver with cyclic AMP-dependent protein kinase

Homogeneous preparations of fructose-1,6-bisphosphatase from mouse, man, rabbit, pig, and rat were tested as substrates for cyclic AMP-dependent protein kinase. Up to 1 mol of [32P]phosphate per mole enzyme subunit was incorporated into fructose-1,6-bisphosphatase from pig and rabbit liver, which should be compared with 2.6 mol of phosphate per mole enzyme subunit in the case of the rat liver enzyme. The phosphorylation of fructose-1,6-bisphosphatase from the livers of man and mouse was negligible. Phosphorylation of pig and rabbit fructose-1,6-bisphosphatase decreased the apparent Km for fructose-1,6-bisphosphate, but in contrast to the case of the rat liver enzyme it did not change the inhibition constants for AMP and fructose-2,6-bisphosphate. The phosphorylation sites in rabbit and pig liver fructose-1,6-bisphosphatase were located close to the carboxyterminal of the polypeptide chains, since trypsin treatment of the phosphorylated enzyme quantitatively removed all of the protein-bound radioactivity without significantly altering the subunit molecular weight and with a maintained neutral pH optimum.

Are calcium-dependent protein kinases involved in the regulation of glycolytic/gluconeogenetic enzymes? Studies with Ca2+/calmodulin-dependent protein kinase and protein kinase C

Changes in glycolytic flux have been observed in liver under conditions where effects of cAMP seem unlikely. We have, therefore, studied the phosphorylation of four enzymes involved in the regulation of glycolysis and gluconeogenesis (6-phosphofructo-1-kinase from rat liver and rabbit muscle; pyruvate kinase, 6-phosphofructo-2-kinase and fructose-1,6-bisphosphatase from rat liver) by defined concentrations of two cAMP-independent protein kinases: Ca2+/calmodulin-dependent protein kinase and Ca2+/phospholipid-dependent protein kinase (protein kinase C). The results were compared with those obtained with the catalytic subunit of cAMP-dependent protein kinase. The following results were obtained. 1. Ca2+/calmodulin-dependent protein kinase phosphorylates 6-phosphofructo-1-kinase and L-type pyruvate kinase at a slightly lower rate as compared to cAMP-dependent protein kinase. 2. 6-Phosphofructo-1-kinase is phosphorylated by the two kinases at a single identical position. There is no additive phosphorylation. The final stoichiometry is 2 mol phosphate/mol tetramer. The same holds for L-type pyruvate kinase except that the stoichiometry with either kinase or both kinases together is 4 mol phosphate/mol tetramer. 3. Rabbit muscle 6-phosphofructo-1-kinase is phosphorylated by cAMP-dependent protein kinase but not by Ca2+/calmodulin-dependent protein kinase. 4. Fructose-1,6-bisphosphatase from rat but not from rabbit liver is phosphorylated at the same position but at a markedly lower rate by Ca2+/calmodulin-dependent protein kinase when compared to the phosphorylation by cAMP-dependent protein kinase. 5. 6-Phosphofructo-2-kinase is phosphorylated by Ca2+/calmodulin-dependent protein kinase only at a negligible rate. 6. Protein kinase C does not seem to be involved in the regulation of the enzymes examined: only 6-phosphofructo-2-kinase became phosphorylated to a significant degree. In contrast to the phosphorylation by cAMP-dependent protein kinase, this phosphorylation is not associated with a change of enzyme activity. This agrees with our observation that the sites of phosphorylation by the two kinases are different. The results indicate that Ca2+/calmodulin-dependent protein kinase but not protein kinase C could be involved in the regulation of hepatic glycolytic flux under conditions where changes in the activity of cAMP-dependent protein kinase seem unlikely.

Phosphorylation- and ligand-induced conformational changes of rat liver fructose-1,6-bisphosphatase

The effects of cyclic AMP-dependent phosphorylation on the structural properties of rat liver fructose-1,6-bisphosphatase were investigated by uv difference spectroscopy and circular dichroism. The incorporation of 4 mol of phosphate per mole of fructose-1,6-bisphosphatase induces a significant increase in the alpha-helix content of the enzyme without affecting its spectrophotometric properties. The addition of fructose 1,6-bisphosphate or fructose 2,6-bisphosphate also affects the conformation of the enzyme. However, both the phosphorylated and the nonphosphorylated forms exhibit similar ligand-induced conformational changes. These results show that cyclic AMP-dependent phosphorylation of fructose-1,6-bisphosphatase induces a specific conformational change. They also suggest that this modification does not alter the interaction of the enzyme protein with fructose 1,6-bisphosphate and fructose 2,6-bisphosphate.

The protein phosphatases involved in cellular regulation. Glycolysis, gluconeogenesis and aromatic amino acid breakdown in rat liver

The identities of the protein phosphatases involved in the regulation of hepatic glycolysis, gluconeogenesis and aromatic amino acid breakdown were investigated using 6-phosphofructo-1-kinase, fructose-1,6-bisphosphatase, L-pyruvate kinase, phenylalanine hydroxylase and the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as substrates. Purified preparations of protein phosphatases-1, 2A, 2B and 2C exhibited activity towards all five substrates in vitro, although phosphatases-1 and 2B were only weakly active. Studies in liver extracts using inhibitor-2 and trifluoperazine, which inhibit protein phosphatase-1 and 2B, respectively, confirmed that these phosphatases are unlikely to be important in dephosphorylating these substrates in vivo. Sequential fractionation of rat liver extracts by anion-exchange chromatography and gel-filtration failed to resolve any protein phosphatases acting on each substrate, apart from protein phosphatases-2A and 2C. The present results, together with those described in the following paper (in this journal) indicate that under the assay conditions used, protein phosphatase-2A is the most powerful phosphatase acting on each substrate, although protein phosphatase-2C contributes a significant percentage of the activity towards 6-phosphofructo-1-kinase. No clear evidence was obtained for a role of metabolites in the regulation of dephosphorylation of the five substrates. This study reinforces our contention that only a few serine-specific and threonine-specific protein phosphatase catalytic subunits participate in cellular regulation.

In vitro phosphorylation of a synthetic collagen peptide by cyclic AMP-dependent protein kinase

A synthetic tridecapeptide that corresponds closely to amino acid residues 98 to 110 in chick collagen alpha 1(I) contains several determinants of specificity required for recognition and phosphorylation by the cyclic AMP-dependent protein kinase. The peptide Gly-Leu-Hyp-Gly-Nle-Lys-Gly-His-Arg-Gly-Phe-Ser-Gly was predicted to be a substrate for the cyclic AMP-dependent protein kinase because it contained multiple basic amino acids NH2-terminal to a potentially phosphorylatable seryl residue. When tested as a substrate for the enzyme, the peptide was stoichiometrically phosphorylated. Phosphoserine was identified as the only phosphoamino acid in a partial hydrolysate of the phosphorylated peptide. The peptide and several of its analogs were phosphorylated by the cyclic AMP-dependent protein kinase with Km values from 1 to 6 mM and Vmax values from 1 to 3 mumol of phosphate/min/mg of enzyme. Although the Km of the kinase for the collagen peptide was high, these results confirmed the prediction made from knowledge of the substrate specificity of cyclic AMP-dependent protein kinase. The potential for such a phosphorylation reaction to occur in vivo is discussed.

Rat hepatic 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase: a unique bifunctional enzyme

Fructose 2,6-bisphosphate is a potent allosteric activator of 6-phosphofructo 1-kinase and an inhibitor of fructose 1,6-bisphosphatase. It potentiates the effect of AMP on both enzymes. A great deal of compelling evidence supports the hypothesis that fructose 2,6-bisphosphate plays a key role in the hormonal and substrate regulation of substrate cycling at the fructose 6-phosphate/fructose 1,6-bisphosphate level in liver. This regulation is exerted at the level of the enzyme activities responsible for the synthesis and degradation of fructose 2,6-bisphosphate. Synthesis of the compound is catalyzed by a unique enzyme which transfers the gamma-phosphate of ATP to the C2 position of fructose 6-phosphate (ATP:D fructose 6-phosphate 2-phosphotransferase) while degradation is catalyzed by a phosphohydrolase activity which is specific for the C-2 position of fructose 2,6-bisphosphate (D-fructose 2,6-bisphosphate 2-phosphohydrolase). These activities are distinct from the classical 6-phosphofructo 1-kinase and fructose 1,6-bisphosphatase with regard to molecular weight, interaction with ligands, and the efficiency with which phosphoryl transfer occurs. Both activities have been purified to homogeneity and have been shown to be present in a single enzyme protein, i.e. the enzyme is bifunctional. Incubation of the 6-phosphofructo 2-kinase/fructose 2,6-bisphosphatase with cAMP-dependent protein kinase and ATP leads to phosphorylation of the enzyme resulting in inactivation of the phosphotransferase activity and stimulation of the phosphohydrolase activity. Since fructose 2,6-bisphosphate is not further metabolized and can only be recycled to fructose 6-phosphate, simultaneous modulation of the synthesis and degradation of the compound by covalent modification of a single protein provides a very efficient and sensitive regulatory mechanism. The bifunctional enzyme was also shown to possess an ATPase activity which was nearly equal to the activity of the kinase reaction. However, in the presence of fructose 6-phosphate the enzyme did not transfer phosphate to water but rather to the C-2 position of the phosphorylated sugar. The ability of the enzyme to catalyze a partial reaction at a rate nearly equal to that of the forward reaction suggested that the reaction mechanism of the kinase proceeds by a two step transfer, i.e. via a phosphoryl enzyme intermediate.(ABSTRACT TRUNCATED AT 400 WORDS)

The demonstration in rat liver cell sap of protein kinase and phosphoprotein phosphatase active on fructose-bisphosphatase

A protein kinase active on fructose-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) was demonstrated in rat liver cell sap. The protein kinase activity was stimulated by cyclic AMP and coincided with the activity of cyclic AMP-dependent protein kinase type I. In addition, three different peaks of phosphoprotein phosphatase active on [32P] phosphofructose-bisphosphatase were found on chromatography of rat liver cell sap on a DEAE-cellulose column. These phosphatases needed divalent cations for full activity. 5'-AMP, a negative modulator of fructose-bisphosphatase, had no effect on the phosphorylation-de-phosphorylation reactions of the enzyme. ATP and Ca2+ did not influence the dephosphorylation reaction of fructose-bisphosphatase.

The kinetics of unphosphorylated, phosphorylated and proteolytically modified fructose bisphosphatase from fat liver

Phosphorylation of fructose-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) by the catalytic subunit of cyclic AMP-dependent protein kinase from pig muscle decreased the K0.5 for fructose-bisphosphate from 21 to 11 microM. When the phosphorylated fructose-bisphosphatase was treated with trypsin the K0.5 increased to 22 microM. The K0.5 also increased when the phosphoenzyme was treated with a partially purified phosphatase from rat liver. There was no difference between the unphosphorylated and phosphorylated enzyme with respect to pH dependence, the pH optimum being about 7.0 for both. Limited treatment of fructose-bis-phosphatase with subtilisin, which cleaves the enzyme at its unphosphorylatable N-terminal part, increased the pH optimum more than limited treatment with trypsin, which releases the phosphorylated peptide at the C-terminal part of fructose-bisphosphatase. The phosphorylated site on the phosphorylated fructose-bisphosphatase was more easily split off by trypsin treatment than the corresponding unphosphorylated site. The results suggest in addition to the glucagon-induced phosphorylation of fructose-bisphosphatase described by Claus et al. [1] that the phosphorylation-dephosphorylation of fructose-bisphosphatase could be of importance for the hormonal regulation of the enzyme in vivo.