the n-acetyl derivative of glucosamine. Monomer of Chitin. Also in the exopolysaccharide from blue-green alga Cyanospira capsulata (CCD) N-Acetylglucosamine (N-acetyl-D-glucosamine, or GlcNAc, or NAG) is a monosaccharide derivative of glucose. It is part of a biopolymer in the bacterial cell wall, built from alternating units of GlcNAc and N-acetylmuramic acid (MurNAc), cross-linked with oligopeptides at the lactic acid residue of MurNAc. GlcNAc is the monomeric unit of the polymer chitin, which forms the outer coverings of insects and crustaceans. (Wikipedia) All animals and plants dynamically attach and remove beta-N-acetylglucosamine at serine and threonine residues on myriad nuclear and cytoplasmic proteins. b-N-Acetylglucosamine cycling, which is tightly regulated by the concerted actions of two highly conserved enzymes, serves as a nutrient and stress sensor. On some proteins, b-N-Acetylglucosamine competes directly with phosphate for serine/threonine residues. b-N-Acetylglucosamine's subcellular localization in hepatocytes established that it is highly concentrated at the nuclear envelope, particularly at the nuclear pore complex, but is also abundant and widespread within chromatin. However, several cytosolic and cytoskeletal proteins are also found to be glycosylated with b-N-Acetylglucosamine. b-N-Acetylglucosamination is in many ways distinct from 'classical' protein glycosylation. First, it is found mostly within the cytoplasm or nucleoplasm. Second, unlike the extraordinarily complex array of glycans found on extracellular glycoproteins, b-N-Acetylglucosamine is not elongated or further modified. Third, b-N-Acetylglucosamine cycles by means of mechanisms and on a timescale similar to those of phosphorylation and quite different from the cycling of complex extracellular glycans. b-N-Acetylglucosamination is one of the most common post-translational modifications. In terms of high-energy compounds, the intracellular concentration of the direct donor for b-N-Acetylglucosamination, UDP-GlcNAc, is second only to that of ATP, with 2-5% of all glucose being used to generate this sugar nucleotide. b-N-Acetylglucosaminated proteins can be found in almost every intracellular compartment, and there are proteins in almost every functional class that are subject to b-N-Acetylglucosamination. (PMID: 17460662) [HMDB]