HEXb

Hexosaminidase B is the beta subunit of the lysosomal enzyme beta-hexosaminidase that, together with the cofactor GM2 activator protein, catalyzes the degradation of the ganglioside GM2, and other molecules containing terminal N-acetyl hexosamines. Beta-hexosaminidase is composed of two subunits, alpha and beta, which are encoded by separate genes. Both beta-hexosaminidase alpha and beta subunits are members of family 20 of glycosyl hydrolases. Mutations in the alpha or beta subunit genes lead to an accumulation of GM2 ganglioside in neurons and neurodegenerative disorders termed the GM2 gangliosidoses. Beta subunit gene mutations lead to Sandhoff disease (GM2-gangliosidosis type II). Alternatively spliced transcript variants encoding different isoforms have been found for this gene. [provided by RefSeq, May 2014]

Full Name

Hexosaminidase Subunit Beta

Function

Hydrolyzes the non-reducing end N-acetyl-D-hexosamine and/or sulfated N-acetyl-D-hexosamine of glycoconjugates, such as the oligosaccharide moieties from proteins and neutral glycolipids, or from certain mucopolysaccharides (PubMed:11707436, PubMed:9694901, PubMed:8672428, PubMed:8123671).

The isozyme B does not hydrolyze each of these substrates, however hydrolyzes efficiently neutral oligosaccharide (PubMed:11707436).

Only the isozyme A is responsible for the degradation of GM2 gangliosides in the presence of GM2A (PubMed:9694901, PubMed:8672428, PubMed:8123671).

During fertilization is responsible, at least in part, for the zona block to polyspermy. Present in the cortical granules of non-activated oocytes, is exocytosed during the cortical reaction in response to oocyte activation and inactivates the sperm galactosyltransferase-binding site, accounting for the block in sperm binding to the zona pellucida (By similarity).

Biological Process

Astrocyte cell migration Source: Ensembl
Cellular calcium ion homeostasis Source: Ensembl
Cellular protein metabolic process Source: Ensembl
Ganglioside catabolic process Source: UniProtKB
Glycosaminoglycan metabolic process Source: ComplexPortal
Lipid storage Source: Ensembl
Locomotory behavior Source: Ensembl
Lysosome organization Source: Ensembl
Male courtship behavior Source: Ensembl
Myelination Source: Ensembl
Neuromuscular process controlling balance Source: Ensembl
Oligosaccharide catabolic process Source: Ensembl
Oogenesis Source: Ensembl
Penetration of zona pellucida Source: Ensembl
Phospholipid biosynthetic process Source: Ensembl
Positive regulation of transcription by RNA polymerase II Source: Ensembl
Regulation of cell shape Source: Ensembl
Sensory perception of sound Source: Ensembl
Single fertilization Source: UniProtKB
Skeletal system development Source: Ensembl

Cellular Location

Lysosome; Cortical granule

Involvement in disease

GM2-gangliosidosis 2 (GM2G2):
An autosomal recessive lysosomal storage disease marked by the accumulation of GM2 gangliosides in the neuronal cells. Clinically indistinguishable from GM2-gangliosidosis type 1, presenting startle reactions, early blindness, progressive motor and mental deterioration, macrocephaly and cherry-red spots on the macula.

PTM

N-linked glycans at Asn-142 and Asn-190 consist of Man3-GlcNAc2 and Man(5 to 7)-GlcNAc2, respectively.
The beta-A and beta-B chains are produced by proteolytic processing of the precursor beta chain.