Bifunctional enzyme

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. 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.