|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
Structure of PFK2. Shown: kinase domain (cyan) and the phosphatase domain (green).
Phosphofructokinase 2 (PFK2) or fructose bisphosphatase 2 (FBPase2), is an enzyme responsible for regulating the rates of glycolysis and gluconeogenesis in the human body. It is a homodimer of 55 kDa subunits arranged in a head-to-head fashion, with each polypeptide chain consisting of independent kinase and phosphatase domain. When Ser-32 of the bifunctional protein is phosphorylated, the negative charge causes the conformation change of the enzyme to favor the FBPase2 activity; otherwise, PFK2 activity is favored. The PFK2 domain is closely related to the superfamily of mononucleotide binding proteins including adenylate cyclase, whereas that of FBPase2 is related to a family of proteins that include phosphoglycerate mutases.
The monomers of the bifunctional protein are clearly divided into two functional domains. The kinase domain is located on the N-terminal. It consists of a central six-stranded β sheet, with five parallel strands and an antiparallel edge strand, surrounded by seven α helices. The domain contains nucleotide-binding fold (nbf) at the C-terminal end of the first β-strand, and thus resembles the structure of adenylate kinase.
On the other hand, the phosphatase domain is located on the C-terminal. It resembles the family of proteins that include phosphoglycerate mutases (PGMs) and acid phosphatases. The domain has a mixed α/ β structure, with a six-stranded central β sheet, plus an additional α-helical subdomain that covers the presumed active site of the molecule. Finally, N-terminal region modulates PFK2 and FBPase2 activities, and stabilizes the dimer form of the enzyme.
When glucose level is low, glucagon is released into the bloodstream, triggering a cAMP signal cascade. In the liver Protein kinase A inactivates the PFK-2 domain of the bifunctional enzyme via phosphorylation, however this does not occur in skeletal muscle. The F-2,6-BPase domain is then activated which lowers fructose 2,6-bisphosphate (F-2,6-BP) levels. Because F-2,6-BP normally stimulates phosphofructokinase-1(PFK1), the decrease in its concentration leads to the inhibition of glycolysis and the stimulation of gluconeogenesis.
On the other hand, when the glucose level increases, the level of fructose 6-phosphate (F6P) subsequently rises and the molecule stimulates phosphoprotein phosphatase-1, which removes phosphoryl group from the bifunctional protein. So PFK2 domain is activated and the kinase catalyzes the formation of F-2,6-BP. Thus, glycolysis is stimulated and gluconeogenesis is inhibited.
The allosteric regulation of PFK2 is very similar to the regulation of PFK1. High levels of AMP or phosphate group signifies high levels of glucose and thus stimulates PFK2. On the other hand, a high concentration of phosphoenolpyruvate(PEP) and citrate signifies that there is a high level of biosynthetic precursor and hence inhibits PFK2. However, unlike PFK1, PFK2 is not affected by the ATP concentration.
Glucagon inhibits PFK2 by activating Protein Kinase A (PKA), which phosphorylates the PFK2 complex and causes its FBPase activity to be favored; via PKA and PFK2/FBP, glucagon decreases [F-2,6-BP], which inhibits glycolysis by allosteric inhibition of PFK1. Insulin activates PFK2 by activating Protein Phosphatase, which dephosphorylates the PFK-2 complex and causes its PFK2 activity to be favored; via Protein Phosphatase and PFK2, insulin increases [F-2,6-BP], which activates glycolysis by allosteric activation of PFK1.
PFK2 is likely to catalyze the "simple" transfer of γ-phosphoryl group of ATP onto the hydroxyl present on C-2 of fructose-6-phosphate. Yet, the formation of fructose 2,6-bisphosphate could theoretically occur by a variety of mechanisms, including the intermediary formation of Fructose-6-phosphate 2-pyrophosphate.
The hydrolysis of fructose 2,6-biphosphate is likely to follow the below steps:
- Histidine acts as a nucleophile and attacks the 2-phosphate of F-2,6-BP
- The stabilization of pentacoordinated transition state by several salt bridges and hydrogen bonding.
- The breakdown of the transition state and the release of F6P.
- Histidine increases the nucleophilicity of water, which attacks phosphohistidine, generating phosphate and newly protonated histidine.
The Pfkfb2 gene encoding PFK2/FBPase2 protein is linked to the predisposition to schizophrenia. Furthermore, the control of PFK2/FBPase2 activity was found to be linked to heart functioning and the control against hypoxia.
Five mammalian isozymes of the protein have been reported to date, difference rising by either the transcription of different enzymes or alternative splicing.The isozymes differ radically in their regulation and the discussions above are based on liver isozyme.
Humans genes encoding proteins possessing phosphofructokinase 2 activity include:
- Kurland IJ, el-Maghrabi MR, Correia JJ, Pilkis SJ (March 1992). "Rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Properties of phospho- and dephospho- forms and of two mutants in which Ser32 has been changed by site-directed mutagenesis". J. Biol. Chem. 267 (7): 4416–23. PMID 1339450.
- Kurland I, Chapman B, Lee YH, Pilkis S (August 1995). "Evolutionary reengineering of the phosphofructokinase active site: ARG-104 does not stabilize the transition state in 6-phosphofructo-2-kinase". Biochem. Biophys. Res. Commun. 213 (2): 663–72. doi:10.1006/bbrc.1995.2183. PMID 7646523.
- Hasemann CA, Istvan ES, Uyeda K, Deisenhofer J (September 1996). "The crystal structure of the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase reveals distinct domain homologies". Structure 4 (9): 1017–29. doi:10.1016/S0969-2126(96)00109-8. PMID 8805587.
- Walker JE, Saraste M, Runswick MJ, Gay NJ (1982). "Distantly related sequences in the alpha- and beta-subunits of ATP synthase, myosin, kinases and other ATP-requiring enzymes and a common nucleotide binding fold". EMBO J. 1 (8): 945–51. PMC 553140. PMID 6329717.
- Li L, Lin K, Pilkis J, Correia JJ, Pilkis SJ (October 1992). "Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. The role of surface loop basic residues in substrate binding to the fructose-2,6-bisphosphatase domain". J. Biol. Chem. 267 (30): 21588–94. PMID 1328239.
- Stryer, Lubert; Berg, Jeremy Mark; Tymoczko, John L. (2008). "The Balance Between Glycolysis and Gluconeogenesis in the Liver Is Sensitive to Blood-Glucose Concentration". Biochemistry (Looseleaf). San Francisco: W. H. Freeman. pp. 466–467. ISBN 1-4292-3502-0.
- Tominaga N, Minami Y, Sakakibara R, Uyeda K (July 1993). "Significance of the amino terminus of rat testis fructose-6-phosphate, 2-kinase:fructose-2,6-bisphosphatase". J. Biol. Chem. 268 (21): 15951–7. PMID 8393455.
- Pilkis SJ, Claus TH, Kurland IJ, Lange AJ (1995). "6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase: a metabolic signaling enzyme". Annu. Rev. Biochem. 64: 799–835. doi:10.1146/annurev.bi.64.070195.004055. PMID 7574501.
- Van Schaftingen E, Hers HG (August 1981). "Phosphofructokinase 2: the enzyme that forms fructose 2,6-bisphosphate from fructose 6-phosphate and ATP". Biochem. Biophys. Res. Commun. 101 (3): 1078–84. doi:10.1016/0006-291X(81)91859-3. PMID 6458291.
- Lin K, Li L, Correia JJ, Pilkis SJ (April 1992). "Glu327 is part of a catalytic triad in rat liver fructose-2,6-bisphosphatase". J. Biol. Chem. 267 (10): 6556–62. PMID 1313012.
- Stone WS, Faraone SV, Su J, Tarbox SI, Van Eerdewegh P, Tsuang MT (May 2004). "Evidence for linkage between regulatory enzymes in glycolysis and schizophrenia in a multiplex sample". Am. J. Med. Genet. B Neuropsychiatr. Genet. 127B (1): 5–10. doi:10.1002/ajmg.b.20132. PMID 15108172.
- Wang Q, Donthi RV, Wang J, Lange AJ, Watson LJ, Jones SP, Epstein PN (June 2008). "Cardiac phosphatase-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase increases glycolysis, hypertrophy, and myocyte resistance to hypoxia". Am. J. Physiol. Heart Circ. Physiol. 294 (6): H2889–97. doi:10.1152/ajpheart.91501.2007. PMID 18456722.
- Darville MI, Crepin KM, Hue L, Rousseau GG (September 1989). "5' flanking sequence and structure of a gene encoding rat 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase". Proc. Natl. Acad. Sci. U.S.A. 86 (17): 6543–7. doi:10.1073/pnas.86.17.6543. PMC 297880. PMID 2549541.
- Tsuchiya Y, Uyeda K (May 1994). "Bovine heart fructose 6-P,2-kinase:fructose 2,6-bisphosphatase mRNA and gene structure". Arch. Biochem. Biophys. 310 (2): 467–74. doi:10.1006/abbi.1994.1194. PMID 8179334.
- Sakata J, Abe Y, Uyeda K (August 1991). "Molecular cloning of the DNA and expression and characterization of rat testes fructose-6-phosphate,2-kinase:fructose-2,6-bisphosphatase". J. Biol. Chem. 266 (24): 15764–70. PMID 1651918.
- Fructose 2,6-bisphosphatase at the US National Library of Medicine Medical Subject Headings (MeSH)
- 6-phosphofructokinase of Arabidopsis thaliana at genome.jp