Hexosaminidase

Class of enzymes

β-N-Acetylhexosaminidase

Hexosaminidase A (Hex A)

Identifiers EC no.

3.2.1.52

CAS no.

9012-33-3

Databases IntEnz

IntEnz view

BRENDA

BRENDA entry

ExPASy

NiceZyme view

KEGG

KEGG entry

MetaCyc

metabolic pathway

PRIAM

profile

PDB structures

RCSB PDB PDBe PDBsum

Gene Ontology

AmiGO / QuickGO

Search PMC

articles

PubMed

articles

NCBI

proteins

Hexosaminidase (EC 3.2.1.52, β-acetylaminodeoxyhexosidase, N-acetyl-β-D-hexosaminidase, N-acetyl-β-hexosaminidase, N-acetyl hexosaminidase, β-hexosaminidase, β-acetylhexosaminidinase, β-D-N-acetylhexosaminidase, β-N-acetyl-D-hexosaminidase, β-N-acetylglucosaminidase, hexosaminidase A, N-acetylhexosaminidase, β-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.

Hereditary inability to form functional hexosaminidase enzymes are the cause of lipid storage disorders Tay-Sachs disease and Sandhoff disease.

β-hexosaminidase subunit α Identifiers Symbol

HEXA

NCBI gene

3073

HGNC

4878

OMIM

606869

RefSeq

NM_000520

UniProt

P06865

Other data EC number

3.2.1.52

Locus

Chr. 15 q24.1

Search for Structures

Swiss-model

Domains

InterPro

β-hexosaminidase subunit β Identifiers Symbol

HEXB

NCBI gene

3074

HGNC

4879

OMIM

606873

RefSeq

NM_000521

UniProt

P07686

Other data EC number

3.2.1.52

Locus

Chr. 5 q13.3

Search for Structures

Swiss-model

Domains

InterPro

Function

Even though the α and β subunits of lysosomal hexosaminidase can both cleave GalNAc residues, only the α subunit is able to hydrolyze GM2 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 α subunit, consisting of Gly-280, Ser-281, Glu-282, and Pro-283 which is absent in the β subunit, serves as an ideal structure for the binding of the GM2 activator protein (GM2AP), and arginine is essential for binding the N-acetyl-neuraminic acid residue of GM2 gangliosides. The GM2 activator protein transports GM2 gangliosides and presents the lipids to hexosaminidase, so a functional hexosaminidase enzyme is able to hydrolyze GM2 gangliosides into GM3 gangliosides by removing the N-acetylgalactosamine (GalNAc) residue from GM2 gangliosides.

Mechanism of action

A Michaelis complex consisting of a glutamate residue, a GalNAc residue on the GM2 ganglioside, and an aspartate residue leads to the formation of an oxazolinium ion intermediate. A glutamate residue (α Glu-323/β Glu-355) works as an acid by donating its hydrogen to the glycosidic oxygen atom on the GalNAc residue. An aspartate residue (α Asp-322/β 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 GM3ganglioside.

Hydrolysis of GM2 ganglioside to GM3 ganglioside catalyzed by hexosaminidase A.The mechanism of the hydrolysis of a GM2 ganglioside and removal of a GalNAc residue to produce GM3 ganglioside.

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 GM2 ganglioside, and as a result, they end up storing 100 to 1000 times more GM2 gangliosides in the brain than the unaffected 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 present 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 GM2 gangliosides. The two subunits of hexosaminidase A are shown below:

The α subunit active site shown bound to NAG-thiazoline (NGT) in β-hexosaminidase. PDB: 2GK1​ The light green outline surrounding NGT represents the Van der Waals surface of NGT. The critical amino acids in the active site that are able to hydrogen bond with NGT include Arginine 178 and Glutamate 462.The β subunit active site shown bound to NAG-thiazoline (NGT) in β-hexosaminidase. PDB: 2GK1​ The light blue outline surrounding NGT represents the Van der Waals surface of NGT. The critical amino acids in the active site that are able to hydrogen bond with NGT include Glutamate 491 and Aspartate 452.

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.

hexosaminidase C Identifiers Symbol

MGEA5

NCBI gene

10724

HGNC

7056

OMIM

604039

RefSeq

NM_012215

UniProt

O60502

Other data EC number

3.2.1.52

Locus

Chr. 10 q24.1-24.3

Search for Structures

Swiss-model

Domains

InterPro

hexosaminidase D Identifiers Symbol

HEXDC

Alt. symbols

FLJ23825

NCBI gene

284004

HGNC

26307

RefSeq

NM_173620

UniProt

Q8IYN4

Other data EC number

3.2.1.52

Locus

Chr. 17 q25.3

Search for Structures

Swiss-model

Domains

InterPro

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