Protein O-GlcNAcylation O-GlcNAcylation is a form of
glycosylation, the site-specific enzymatic addition of saccharides to proteins and lipids. This form of glycosylation is with
O-linked β-
N-acetylglucosamine or β-
O-linked 2-acetamido-2-deoxy-D-glycopyranose (
O-GlcNAc). In this form, a single sugar (β-
N-acetylglucosamine) is added to serine and threonine residues of nuclear or cytoplasmic proteins. Two conserved enzymes control this glycosylation of serine and threonine:
O-GlcNAc transferase (OGT) and
O-GlcNAcase (OGA). While OGT catalyzes the addition of
O-GlcNAc to serine and threonine, OGA catalyzes the hydrolytic cleavage of
O-GlcNAc from post-transitionally modified proteins. OGA is a member of the family of
hexosaminidases. However, unlike lysosomal hexosaminidases, OGA activity is the highest at neutral pH (approximately 7) and it localizes mainly to the cytosol. OGA and OGT are synthesized from two conserved genes and are expressed throughout the human body with high levels in the brain and pancreas. The products of
O-GlcNAc and the process itself plays a role in embryonic development, brain activity, hormone production, and a myriad of other activities. Over 600 proteins are targets for
O-GlcNAcylation. While the functional effects of
O-GlcNAc modification is not fully known, it is known that
O-GlcNAc modification impacts many cellular activities such as lipid/carbohydrate metabolism and hexosamine biosynthesis. Modified proteins may modulate various downstream signaling pathways by influencing transcription and proteomic activities.
Mechanism and inhibition OGA catalyzes
O-GlcNAc hydrolysis via an
oxazoline reaction intermediate. Stable compounds which mimic the reaction intermediate can act as selective enzyme inhibitors.
Thiazoline derivatives of GlcNAc can be used as a reaction intermediate. An example of this includes Thiamet-G as shown on the right. A second form of inhibition can occur from the mimicry of the
transition state. The GlcNAcstatin family of inhibitors exploit this mechanism in order to inhibit OGA activity. For both types of inhibitors, OGA can be selected apart from the generic lysosomal hexosaminidases by elongating the C2 substituent in their chemical structure. This takes advantage of a deep pocket in OGA's active site that allow it to bind analogs of GlcNAc. There is potential for regulation of
O-GlcNAcase for the treatment of
Alzheimer's disease. When the
tau protein in the brain is hyperphosphorylated,
neurofibrillary tangles form, which are a pathological hallmark for neurodegenerative diseases such as Alzheimer's disease. In order to treat this condition, OGA is targeted by inhibitors such as Thiamet-G in order to prevent
O-GlcNAc from being removed from tau, which assists in preventing tau from becoming phosphorylated. == Structure ==