Phosphofructokinase 2
| 6-phosphofructo-2-kinase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| EC number | 2.7.1.105 | ||||||||
| CAS number | Template:CAS | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / EGO | ||||||||
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| fructose-2,6-bisphosphate 2-phosphatase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| EC number | 3.1.3.46 | ||||||||
| CAS number | Template:CAS | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| Gene Ontology | AmiGO / EGO | ||||||||
|
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| 6-phosphofructo-2-kinase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
Structure of PFK2. Shown: kinase domain (cyan) and the phosphatase domain (green).
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| Identifiers | |||||||||
| Symbol | 6PF2K | ||||||||
| Pfam | PF01591 | ||||||||
| InterPro | IPR013079 | ||||||||
| PROSITE | PDOC00158 | ||||||||
| SCOP | 1bif | ||||||||
| SUPERFAMILY | 1bif | ||||||||
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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.[1] 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.
Contents
Structure
The monomers of the bifunctional protein are clearly divided into two functional domains. The kinase domain is located on the N-terminal.[2] It consists of a central six-stranded β sheet, with five parallel strands and an antiparallel edge strand, surrounded by seven α helices.[3] The domain contains nucleotide-binding fold (nbf) at the C-terminal end of the first β-strand,[4] and thus resembles the structure of adenylate kinase.
On the other hand, the phosphatase domain is located on the C-terminal.[5] It resembles the family of proteins that include phosphoglycerate mutases (PGMs) and acid phosphatases.[6] 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.[3] Finally, N-terminal region modulates PFK2 and FBPase2 activities, and stabilizes the dimer form of the enzyme.[6][7]
Function
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.[8]
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.
Regulation
The allosteric regulation of PFK2 is very similar to the regulation of PFK1.[9] High levels of AMP or phosphate group signifies a low energy state 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, signalling an abundance of glucose
Reaction mechanism
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.[9]
The hydrolysis of fructose 2,6-biphosphate is likely to follow the below steps:[10]
- 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.
Clinical significance
The Pfkfb2 gene encoding PFK2/FBPase2 protein is linked to the predisposition to schizophrenia.[11] Furthermore, the control of PFK2/FBPase2 activity was found to be linked to heart functioning and the control against hypoxia.[12]
Isozymes
Five mammalian isozymes of the protein have been reported to date, difference rising by either the transcription of different enzymes or alternative splicing.[13][14][15]The isozymes differ radically in their regulation and the discussions above are based on liver isozyme.[3]
Humans genes encoding proteins possessing phosphofructokinase 2 activity include:
References
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External links
- Fructose 2,6-bisphosphatase at the US National Library of Medicine Medical Subject Headings (MeSH)
- 6-phosphofructokinase of Arabidopsis thaliana at genome.jp
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