Bovine heart fructose 6-P,2-kinase:fructose 2,6-bisphosphatase mRNA and gene structure

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: head-to-head with a bifunctional enzyme that controls glycolysis

Fru-2,6-P2 (fructose 2,6-bisphosphate) is a signal molecule that controls glycolysis. Since its discovery more than 20 years ago, inroads have been made towards the understanding of the structure-function relationships in PFK-2 (6-phosphofructo-2-kinase)/FBPase-2 (fructose-2,6-bisphosphatase), the homodimeric bifunctional enzyme that catalyses the synthesis and degradation of Fru-2,6-P2. The FBPase-2 domain of the enzyme subunit bears sequence, mechanistic and structural similarity to the histidine phosphatase family of enzymes. The PFK-2 domain was originally thought to resemble bacterial PFK-1 (6-phosphofructo-1-kinase), but this proved not to be correct. Molecular modelling of the PFK-2 domain revealed that, instead, it has the same fold as adenylate kinase. This was confirmed by X-ray crystallography. A PFK-2/FBPase-2 sequence in the genome of one prokaryote, the proteobacterium Desulfovibrio desulfuricans, could be the result of horizontal gene transfer from a eukaryote distantly related to all other organisms, possibly a protist. This, together with the presence of PFK-2/FBPase-2 genes in trypanosomatids (albeit with possibly only one of the domains active), indicates that fusion of genes initially coding for separate PFK-2 and FBPase-2 domains might have occurred early in evolution. In the enzyme homodimer, the PFK-2 domains come together in a head-to-head like fashion, whereas the FBPase-2 domains can function as monomers. There are four PFK-2/FBPase-2 isoenzymes in mammals, each coded by a different gene that expresses several isoforms of each isoenzyme. In these genes, regulatory sequences have been identified which account for their long-term control by hormones and tissue-specific transcription factors. One of these, HNF-6 (hepatocyte nuclear factor-6), was discovered in this way. As to short-term control, the liver isoenzyme is phosphorylated at the N-terminus, adjacent to the PFK-2 domain, by PKA (cAMP-dependent protein kinase), leading to PFK-2 inactivation and FBPase-2 activation. In contrast, the heart isoenzyme is phosphorylated at the C-terminus by several protein kinases in different signalling pathways, resulting in PFK-2 activation.

Alternative splicing of novel exons of rat heart-type fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase gene

Three C-terminal sequences (VSIPVV, VCKWT, and LTLLS) of rat heart-type fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase have been reported. To elucidate the mechanism of generating the heterogeneity of the enzyme, we carried out reverse transcriptase PCR using three pairs of specific primers. The existence of mRNAs encoding VSIPVV and LTLLS was confirmed but not that of VCKWT. In addition to these cDNAs, we found four novel mRNAs that encode the C-termini of the enzyme. The genomic sequence reveals that the heterogeneity is generated by alternative splicing of exon 15 and four novel exons (exon 16a-d).

Molecular cloning, expression, and chromosomal localization of a ubiquitously expressed human 6-phosphofructo-2-kinase/ fructose-2, 6-bisphosphatase gene (PFKFB3)

We report the identification of a human 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene (PFKFB3) isolated from a human fetal brain cDNA library. The gene was localized to 10p15-->p14 by fluorescence in situ hybridization. The entire cDNA (4,322 bp) codes for a polypeptide of 520 amino acid residues (molecular weight, 59.571 kDa). Structural analysis showed the presence of a kinase domain located at the amino terminus and a bisphosphatase domain at the carboxy terminus, characteristic of previously described 6-phosphofructo-2-kinase/fructose 2, 6-bisphosphatase isozymes. In addition, a phosphorylation site for cAMP-dependent protein kinase was found at the carboxy terminus. Northern blot analysis showed the presence of a unique 4.8-kb mRNA expressed in the different tissues studied. In mammalian COS-1 cells, this cDNA drives the expression of an active isozyme. Taken together, these results identify the presence of a gene coding for a human 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase isozyme which is ubiquitously expressed.

Structure and heterologous expression of a gene encoding fructose-6-phosphate,2-kinase/fructose-2,6-bisphosphatase from Arabidopsis thaliana

A full-length cDNA clone encoding fructose-6-phosphate, 2-kinase/fructose-2,6-bisphosphatase from Arabidopsis thaliana (AtF2KP) was isolated. The encoded protein is composed of two different regions: (i) a 400 amino acid COOH-terminal region, covering the catalytic region of the protein which is homologous to enzymes from other eukaryotes. This region is highly conserved among plant species (88% identity to spinach F2KP). (ii) A 345 amino acid plant-specific NH(2)-terminal region, with 59% identity to spinach F2KP, which is composed of homologous motifs and intermittent variable sequences. Western blots show that F2KP from several plant species migrates in sodium dodecyl sulphate-polyacrylamide gel electrophoresis as a similar sized (93 kDa) protein. AtF2KP was expressed in Escherichia coli as a full length and a truncated (without the NH(2)-terminal region) fusion protein. Both forms had kinase as well as phosphatase activity, but presence of the NH(2)-terminal region influenced the ratio between the two activities. It is suggested that the NH(2)-terminal region represents a regulatory region, which defines specific properties of the plant enzymes. A genomic clone for the corresponding gene, AtF2KP, was isolated. The clone (9519 bp) included 23 exons, 22 introns and the promoter sequence. Southern blot analysis showed only one copy of the gene in the A. thaliana genome.

N- and C-termini modulate the effects of pH and phosphorylation on hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

Liver and skeletal muscle isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-P(2)ase) isoenzymes are products of alternatively spliced first exons of the same gene, with common kinase and bisphosphatase domains. The muscle-specific exon-1 encodes nine unique amino acids, that lack the cAMP-dependent protein kinase (PK-A) phosphorylation site, and differ in sequence from those encoded by the liver-specific exon-1 (32 amino acids), contributing to its much lower affinity for fructose 6-phosphate (Fru-6-P). PK-A phosphorylation of the liver isoform at Ser(32) reduces the affinity of the kinase for Fru-6-P, and stimulates the bisphosphatase V(max). In the present study, we have defined the locus of interaction of the N-terminal residues with the N-terminal kinase and C-terminal domains by successive N- and C-terminal deletions. This study shows that: (1) residues Gly(5)-Glu(6)-Leu(7) of the liver isoform are responsible for increasing the affinity of 6PF2K for Fru-6-P, maintaining the inhibition of Fru-2,6-P(2)ase activity, and mediating the effects of PK-A phosphorylation on the two activities; (2) the loss of Fru-6-P inhibition of the bisphosphatase and the enhancement of its V(max), rather than the inhibition of the kinase, may be responsible for the behaviour of the muscle isoform primarily as a bisphosphatase; (3) the composition of residues 24-32 of the liver form appears to confer the enhanced kinase catalytic rate of this form over that of the muscle isoform. It is concluded that specific regions of the N-terminus of liver and skeletal muscle 6PF2K/Fru-2,6-P(2)ase have a role in adapting the two activities to work in the physiological range of pH and substrate concentrations found in each particular tissue.

Tissue-specific alternative splicing of rat brain fructose 6-phosphate 2-kinase/fructose 2,6-bisphosphatase

We have reported the occurrence of eight splice variants of rat brain fructose 6-phosphate 2-kinase/fructose 2,6-bisphosphatase (RB2K). In the present study, we quantified these splice variants in various tissues using a RNAse protection assay and found a tissue-specific pattern of alternative splicing of the RB2K gene. Splice variants containing exon F were specifically expressed in brain. Moreover, exons D and E were spliced in brain, skeletal muscle and heart. Consequently, eight, six, four and two splice variants were expressed in brain, skeletal muscle, heart and liver plus testis, respectively. These results suggest that distinct RB2K isoforms could be involved in regulation of glycolysis in a tissue-specific manner.

Cloning, expression and chromosomal localization of a human testis 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene

6-Phosphofructo-2-kinase/fructose 2,6-bisphosphatase (PFK-2/FBPase-2) is a bifunctional enzyme responsible for the synthesis and breakdown of Fru-2,6-P2, a key metabolite in the regulation of glycolysis. Several genes encode distinct PFK-2/FBPase-2 isozymes that differ in their tissue distribution and enzyme regulation. In this paper, we present the isolation of a cDNA from a human testis cDNA library that encodes a PFK-2/FBPase-2 isozyme. Sequencing data show an open reading frame of 1407 nucleotides that codifies for a protein of 469 amino acids. This has a calculated molecular weight of 54kDa and 97% similarity with rat testis PFK-2/FBPase-2, with complete conservation of the amino acid residues involved in the catalytic mechanism. Fluorescence in-situ hybridization (FISH) localized testis PFK-2/FBPase-2 gene (PFKFB4) in human chromosome 3 at bands p21-p22. A Northern blot analysis of different rat tissues showed the presence of a 2.4-kb mRNA expressed specifically in testis. In mammalian COS-1 cells, the human testis cDNA drives expression of an isozyme with a molecular weight of 55kDa. This isozyme shows clear PFK-2 activity. Taken together, these results provide evidence for a new PFK-2/FBPase-2 gene coding for a human testis isozyme.

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.

Separate bisphosphatase domain of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: the role of the C-terminal tail in modulating enzyme activity

The separate bisphosphatase domain (amino acid residues 243-468) of the chicken liver bifunctional enzyme 6-phosphofructo-2-kinase-fructose-2,6-bisphosphatase was expressed in Escherichia coli and purified to homogeneity. The fructose-2, 6-bisphosphatase activity of the separate domain was 7-fold higher than that of the native bifunctional enzyme, and exhibited substrate inhibition characteristic of the native enzyme. The inhibition of the enzymes by fructose 2,6-bisphosphate could be overcome by Pi, glycerol 3-phosphate and GTP. Deletion of 30 amino acid residues from the C-terminus of the separate domain resulted in around a 5-fold increase in the Vmax of the bisphosphatase. Also, the truncated form was more accessible to chemical modification by diethyl pyrocarbonate and N-ethylmaleimide, suggesting a more open structure than the wild-type form. In addition, the mutation of cysteine-389 to alanine increased bisphosphatase activity by 20% and the Km value for fructose 2,6-bisphosphate by 3-fold, and both the point mutation at cysteine-389 and the deletional mutation led to the predominantly insoluble expression of the enzyme. The results indicated that the C-terminal tail plays a role in modulating the enzyme activity and suggested that the difference in the C-terminal tail sequence is responsible for the difference in activity of the chicken and rat liver fructose-2,6-bisphosphatases. It is postulated that an interaction between the C-terminal tail and the active site might be present.

The crystal structure of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase reveals distinct domain homologies

BACKGROUND

Glucose homeostasis is maintained by the processes of glycolysis and gluconeogenesis. The importance of these pathways is demonstrated by the severe and life threatening effects observed in various forms of diabetes. The bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase catalyzes both the synthesis and degradation of fructose-2,6-bisphosphate, a potent regulator of glycolysis. Thus this bifunctional enzyme plays an indirect yet key role in the regulation of glucose metabolism.

RESULTS

We have determined the 2.0 A crystal structure of the rat testis isozyme of this bifunctional enzyme. The enzyme is a homodimer of 55 kDa subunits arranged in a head-to-head fashion, with each monomer consisting of independent kinase and phosphatase domains. The location of ATPgammaS and inorganic phosphate in the kinase and phosphatase domains, respectively, allow us to locate and describe the active sites of both domains.

CONCLUSIONS

The kinase domain is clearly related to the superfamily of mononucleotide binding proteins, with a particularly close relationship to the adenylate kinases and the nucleotide-binding portion of the G proteins. This is in disagreement with the broad speculation that this domain would resemble phosphofructokinase. The phosphatase domain is structurally related to a family of proteins which includes the cofactor independent phosphoglycerate mutases and acid phosphatases.

Covalent control of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: insights into autoregulation of a bifunctional enzyme

The hepatic bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF-2-K/Fru-2,6-P2ase), E.C. 2.7-1-105/E.C. 3-1-3-46, is one member of a family of unique bifunctional proteins that catalyze the synthesis and degradation of the regulatory metabolite fructose-2,6-bisphosphate (Fru-2,6-P2). Fru-2,6-P2 is a potent activator of the glycolytic enzyme 6-phosphofructo-1-kinase and an inhibitor of the gluconeogenic enzyme fructose-1,6-bisphosphatase, and provides a switching mechanism between these two opposing pathways of hepatic carbohydrate metabolism. The activities of the hepatic 6PF-2-K/Fru-2,6-P2ase isoform are reciprocally regulated by a cyclic AMP-dependent protein kinase (cAPK)-catalyzed phosphorylation at a single NH2-terminal residue, Ser-32. Phosphorylation at Ser-32 inhibits the kinase and activates the bisphosphatase, in part through an electrostatic mechanism. Substitution of Asp for Ser-32 mimics the effects of cAPK-catalyzed phosphorylation. In the dephosphorylated homodimer, the NH2- and COOH-terminal tail regions also have an interaction with their respective active sites on the same subunit to produce an autoregulatory inhibition of the bisphosphatase and activation of the kinase. In support of this hypothesis, deletion of either the NH2- or COOH-terminal tail region, or both regions, leads to a disruption of these interactions with a maximal activation of the bisphosphatase. Inhibition of the kinase is observed with the NH2-truncated forms, in which there is also a diminution of cAPK phosphorylation to decrease the Km for Fru-6-P. Phosphorylation of the bifunctional enzyme by cAPK disrupts these autoregulatory interactions, resulting in inhibition of the kinase and activation of the bisphosphatase. Therefore, effects of cyclic AMP-dependent phosphorylation are mediated by a combination of electrostatic and autoregulatory control mechanisms.