O-GlcNAc transferase (EC 2.4.1.255, O-GlcNAc transferase, OGTase, O-linked N-acetylglucosaminyltransferase, uridine diphospho-N-acetylglucosamine:polypeptide beta-N-acetylglucosaminyltransferase, protein O-linked beta-N-acetylglucosamine transferase) is an enzyme with systematic name UDP-N-acetyl-D-glucosamine:protein-O-beta-N-acetyl-D-glucosaminyl transferase. In humans the enzyme is encoded by the OGT gene.

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) catalyzes the addition of a single N-acetylglucosamine in O-glycosidic linkage to serine or threonine residues of intracellular proteins. Since both phosphorylation and O-GlcNAcylation compete for similar serine or threonine residues, the two processes may compete for sites, or they may alter the substrate specificity of nearby sites by steric or electrostatic effects. Two transcript variants encoding cytoplasmic and mitochondrial isoforms have been found for this gene. OGT glycosylates many proteins including: Histone H2B,AKT1,PFKL, KMT2E/MLL5,MAPT/TAU,Host cell factor C1, and SIN3A.

O-GlcNAc transferase is a part of a host of biological functions within the human body. OGT is involved in the resistance of insulin in muscle cells and adipocytes by inhibiting the Threonine 308 phosphorylation of AKT1, increasing the rate of IRS1 phosphorylation (at Serine 307 and Serine 632/635), reducing insulin signaling, and glycosylating components of insulin signals. Additionally, O-GlcNAc transferase catalyzes intracellular glycosylation of serine and threonine residues with the addition of N-acetylglucosamine. Studies show that OGT alleles are vital for embryogenesis, and that OGT is necessary for intracellular glycosylation and embryonic stem cell vitality. O-GlcNAc transferase also catalyzes the posttranslational modification that modifies transcription factors and RNA polymerase II, however the specific function of this modification is mostly unknown.

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