Phosphofructokinase 2

6-phosphofructo-2-kinase
Structure of PFK2. Shown: kinase domain (cyan) and the phosphatase domain (green).
Identifiers
Symbol	6PF2K
Pfam	PF01591
InterPro	IPR013079
PROSITE	PDOC00158
SCOP	1bif
SUPERFAMILY	1bif
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

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.

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]

1. Histidine acts as a nucleophile and attacks the 2-phosphate of F-2,6-BP
2. The stabilization of pentacoordinated transition state by several salt bridges and hydrogen bonding.
3. The breakdown of the transition state and the release of F6P.
4. 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:

• PFKFB1, PFKFB2, PFKFB3, PFKFB4

References

Further reading

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