Glutamine-Fructose-6-Phosphate Transaminase 1, 2 (GFPT1, 2).

l-Glutamine-d-fructose-6-phosphate transaminase (GFPT) is the first and rate-limiting enzyme of the hexosamine biosynthesis pathway (HBP). GFPT converts d-fructose-6-phosphate (Fru-6-P) and l-glutamine to d-glucosamine-6-phosphate (GlcN-6-P) and l-glutamate. GlcN-6-P is an essential precursor of hexosamines, such as uridine 5′-diphosphate-N-acetyl-d-glucosamine (UDP-GlcNAc). UDP-GlcNAc is a substrate for synthesis of glycoproteins, proteoglycans, and glycolipids and moreover for the posttranslational modification of proteins with O-linked β-N-acetylglucosamine (O-GlcNAc) on serine and threonine residues, which is an important step for the regulation of intracellular signaling. Figure 129.1 is a schematic representation of the HBP, which includes the synthesis of GlcNAcylated proteins.