hexosaminidase

Hexosaminidase refers to a group of lysosomal hydrolases that cleave terminal N-acetylhexosamine residues from glycoconjugates, including glycolipids and glycoproteins. In humans, the principal lysosomal forms are hexosaminidase A, a heterodimer composed of an alpha and a beta subunit, and hexosaminidase B, a beta-beta homodimer. The enzymes are encoded by the HEXA and HEXB genes, respectively, and belong to the glycoside hydrolase family 20. They catalyze hydrolysis of N-acetylglucosamine or N-acetylgalactosamine residues from the non-reducing ends of substrates.

Hexosaminidases play a key role in the degradation of gangliosides and other glycoconjugates within lysosomes. The degradation of GM2 ganglioside in particular is a major function, and is facilitated by the activity of hexosaminidase A in concert with the GM2 activator protein. Hexosaminidase B has broader exoglycosidase activity and can compensate to some extent for Hex A deficiency in certain contexts, though Hex A is essential for efficient GM2 processing in vivo.

Clinically important disorders arise from HEXA or HEXB mutations. Deficiency of hexosaminidase A causes Tay-Sachs disease (GM2 gangliosidosis type 1), a progressive neurodegenerative disorder that typically presents in infancy or early childhood. Deficiency of both Hex A and Hex B due to HEXB mutations leads to Sandhoff disease, which tends to be more severe. Other related disorders involve defects in the GM2 activator protein and can yield similar lysosomal storage phenotypes.

Diagnosis is based on decreased hexosaminidase activity in leukocytes or fibroblasts and confirmed by genetic testing of HEXA and HEXB. There is currently no approved cure; management is supportive. Research into gene therapy and other disease-modifying approaches is ongoing, with the aim of restoring enzyme activity and slowing neurodegeneration.