Protein O-GlcNAcase

Protein O-GlcNAcase (, OGA, glycoside hydrolase O-GlcNAcase, O-GlcNAcase, BtGH84, O-GlcNAc hydrolase) is an enzyme with systematic name (protein)-3-O-(N-acetyl- D -glucosaminyl)- L -serine/threonine N-acetylglucosaminyl hydrolase. OGA is encoded by the OGA gene. This enzyme catalyses the removal of the O-GlcNAc post-translational modification in the following chemical reaction:


 * 1) [protein]-3-O-(N-acetyl-β- D -glucosaminyl)- L -serine + H2O  [protein]- L -serine + N-acetyl- D -glucosamine
 * 2) [protein]-3-O-(N-acetyl-β- D -glucosaminyl)- L -threonine + H2O  [protein]- L -threonine + N-acetyl- D -glucosamine

Nomenclature
Other names include:


 * Nuclear cytoplasmic O-GlcNAcase and acetyltransferase

Isoforms
The human OGA gene is capable of producing two different transcripts, each capable of encoding a different OGA isoform. The long isoform L-OGA, a bifunctional enzyme that possess a glycoside hydrolase activity and a pseudo histone-acetyl transferase domain, primarily resides in the cytoplasm and the nucleus. The short isoform S-OGA, which only exhibit the glycoside hydrolase domain, was initially described as residing within the nucleus. However, more recent work showed that S-OGA is located in mitochondria and regulates reactive oxygen production in this organelle. Another isoform, resulting from proteolytic cleavage of L-OGA, has also been described. All three isoforms exhibit glycoside hydrolase activity.

Homologs
Protein O-GlcNAcases belong to glycoside hydrolase family 84 of the carbohydrate active enzyme classification. Homologs exist in other species as O-GlcNAcase is conserved in higher eukaryotic species. In a pairwise alignment, humans share 55% homology with Drosophila and 43% with C. elegans. Drosophila and C. elegans share 43% homology. Among mammals, the OGA sequence is even more highly conserved. The mouse and the human have 97.8% homology. However, OGA does not share significant homology with other proteins. However, short stretches of about 200 amino acids in OGA have homology with some proteins such as hyaluronidase, a putative acetyltransferase, eukaryotic translation elongation factor-1γ, and the 11-1 polypeptide.

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
X-ray structures are available for a range of O-GlcNAcase proteins. The X-ray structure of human O-GlcNAcase in complex with Thiamet-G identified the structural basis of enzyme inhibition.