The hexosamine biosynthetic pathway controls O-GlcNAc-modification of proteins.

Cytosolic and nuclear O-GlcNAc glycosylation constitutes a dynamic process that regulates the activity, the localisation or the stability of the modified proteins. O-GlcNAc glycosylation of proteins on serine and threonine residues depends on the flux of glucose through the hexosamine biosynthetic pathway (HBP). A fraction (2 to 5%) of the glucose entering the cell is directed into this pathway for the biosynthesis of UDP-GlcNAc. OGT uses UDP-GlcNAc as a substrate to add GlcNAc on serine or threonine residues, and its activity is tightly dependent on the concentration of UDP-GlcNAc in the cell. These modifications can be reversed by O-GlcNAcase (OGA), which removes the O-GlcNAc moiety from O-GlcNAc-modified proteins. Experimentally, the level of O-GlcNAc in proteins can be increased by incubating cells with glucosamine (which bypasses allosteric inhibition of the rate limiting enzyme GFAT (Glutamine Fructose 6-P amidotransferase)), or with PUGNAc (O-[2-acetamido-2-deoxy-D-glucopyranosylidene] amino-N-phenylcarbamate), which inhibits deglycosylation by OGA.