Hexosaminidase
Hexosaminidase (, "beta-acetylaminodeoxyhexosidase", "N-acetyl-beta-D-hexosaminidase", "N-acetyl-beta-hexosaminidase", "N-acetyl hexosaminidase", "beta-hexosaminidase", "beta-acetylhexosaminidinase", "beta-D-N-acetylhexosaminidase", "beta-N-acetyl-D-hexosaminidase", "beta-N-acetylglucosaminidase", "hexosaminidase A", "N-acetylhexosaminidase", "beta-D-hexosaminidase") is an enzyme involved in the hydrolysis of terminal N-acetyl-D-hexosamine residues in N-acetyl-β-D-hexosaminides. Elevated levels of hexosaminidase in blood and/or urine have been proposed as a biomarker of relapse in the treatment of alcoholism. Isozymes and genes. Lysosomal A, B, and S isozymes. Functional lysosomal β-hexosaminidase enzymes are dimeric in structure. Three isozymes  are produced through the combination of α and β subunits to form any one of three active dimers: The α and β subunits are encoded by separate genes, "HEXA" and "HEXB" respectively. Beta-hexosaminidase and the cofactor G activator protein catalyze the degradation of the G gangliosides and other molecules containing terminal N-acetyl hexosamines. Gene mutations in "HEXB" often result in Sandhoff disease; whereas, mutations in "HEXA" decrease the hydrolysis of G gangliosides, which is the main cause of Tay–Sachs disease. Function. Even though the alpha and beta subunits of lysosomal hexosaminidase can both cleave GalNAc residues, only  the alpha subunit is able to hydrolyze G gangliosides because of a key residue, Arg-424, and a loop structure that forms from the amino acid sequence in the alpha subunit. The loop in the alpha subunit, consisting of Gly-280, Ser-281, Glu-282, and Pro-283 which is absent in the beta subunit, serves as an ideal structure for the binding of the G activator protein (GAP), and arginine is essential for binding the N-acetyl-neuraminic acid residue of G gangliosides. The G activator protein transports G gangliosides and presents the lipids to hexosaminidase, so a functional hexosaminidase enzyme is able to  hydrolyze G gangliosides into G gangliosides by removing the N-acetylgalactosamine (GalNAc) residue from G gangliosides. Mechanism of action. A Michaelis complex consisting of a glutamate residue, a GalNAc residue on the G ganglioside, and an aspartate residue leads to the formation of an oxazolinium ion intermediate. A glutamate residue (alpha Glu-323/beta Glu-355) works as an acid by donating its hydrogen to the glycosidic oxygen atom on the GalNAc residue. An aspartate residue (alpha Asp-322/beta Asp-354) positions the C2-acetamindo group so that it can be attacked by the nucleophile (N-acetamido oxygen atom on carbon 1 of  the substrate). The aspartate residue stabilizes the positive charge on the nitrogen atom in the oxazolinium ion intermediate. Following the formation of the oxazolinium ion intermediate, water attacks the electrophillic acetal carbon. Glutamate acts as a base by deprotonating the water leading to the formation of the product complex and the Gganglioside. Gene mutations resulting in Tay–Sachs disease. There are numerous mutations that lead to hexosaminidase deficiency including gene deletions, nonsense mutations, and missense mutations. Tay–Sachs disease occurs when hexosaminidase A loses its ability to function. People with Tay–Sachs disease are unable to remove the GalNAc residue  from the G ganglioside, and as a result, they end up storing 100 to 1000 times more G gangliosides in the brain than the normal person. Over 100 different mutations have been discovered just in infantile cases of Tay–Sachs disease alone. The most common mutation, which occurs in over 80 percent of Tay–Sachs patients, results from a four base pair addition (TATC) in exon 11 of the Hex A gene. This insertion leads to an early stop codon, which causes the Hex A deficiency. Children born with Tay–Sachs usually die between two and four years of age  from aspiration and pneumonia. Tay–Sachs causes cerebral degeneration and blindness. Patients also experience flaccid extremities and seizures. At this point in time, there has been no cure or effective treatment of Tay–Sachs disease. NAG-thiazoline, NGT, acts as mechanism based inhibitor of hexosaminidase A. In patients with Tay–Sachs disease (misfolded hexosaminidase A), NGT acts as a molecular chaperone by binding in the active site of hexosaminidase A which helps create a properly folded hexosaminidase A. The stable dimer conformation of hexosaminidase A has the ability to leave the endoplasmic reticulum and is directed to the lysosome where  it can perform the degradation of G gangliosides. The two subunits of hexosaminidase A are shown below: Cytosolic C and D isozymes. The bifunctional protein NCOAT (nuclear cytoplasmic O-GlcNAcase and acetyltransferase) that is encoded by the "MGEA5" gene possesses both hexosaminidase and histone acetyltransferase activities. NCOAT is also known as hexosaminidase C and has distinct substrate specificities compared to lysosomal hexosaminidase A. A single-nucleotide polymorphism in the human O-GlcNAcase gene is linked to diabetes mellitus type 2. A fourth mammalian hexosaminidase polypeptide which has been designated hexosaminidase D ("HEXDC") has recently been identified.