title: Crystal structure of human beta-hexosaminidase B: understanding the molecular basis of Sandhoff and Tay-Sachs disease.
abstract: In humans, two major beta-hexosaminidase isoenzymes exist: Hex A and Hex B. Hex A is a heterodimer of subunits alpha and beta (60% identity), whereas Hex B is a homodimer of beta-subunits. Interest in human beta-hexosaminidase stems from its association with Tay-Sachs and Sandhoff disease; these are prototypical lysosomal storage disorders resulting from the abnormal accumulation of G(M2)-ganglioside (G(M2)). Hex A degrades G(M2) by removing a terminal N-acetyl-D-galactosamine (beta-GalNAc) residue, and this activity requires the G(M2)-activator, a protein which solubilizes the ganglioside for presentation to Hex A. We present here the crystal structure of human Hex B, alone (2.4A) and in complex with the mechanistic inhibitors GalNAc-isofagomine (2.2A) or NAG-thiazoline (2.5A). From these, and the known X-ray structure of the G(M2)-activator, we have modeled Hex A in complex with the activator and ganglioside. Together, our crystallographic and modeling data demonstrate how alpha and beta-subunits dimerize to form either Hex A or Hex B, how these isoenzymes hydrolyze diverse substrates, and how many documented point mutations cause Sandhoff disease (beta-subunit mutations) and Tay-Sachs disease (alpha-subunit mutations).
authors.affiliation: Canadian Institutes of Heath Research Group in Protein Structure and Function, Department of Biochemistry, University of Alberta, T6G 2H7, Edmonton, Alt., Canada.
authors.forename: Brian L
authors.initials: BL
authors.lastname: Mark
authors.forename: Don J
authors.initials: DJ
authors.lastname: Mahuran
authors.forename: Maia M
authors.initials: MM
authors.lastname: Cherney
authors.forename: Dalian
authors.initials: D
authors.lastname: Zhao
authors.forename: Spencer
authors.initials: S
authors.lastname: Knapp
authors.forename: Michael N G
authors.initials: MN
authors.lastname: James
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D018360:Crystallography, X-Ray; D019281:Dimerization; D054818:Hexosaminidase A; D054819:Hexosaminidase B; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D012497:Sandhoff Disease / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid; D013661:Tay-Sachs Disease / Q000201:enzymology*; D001619:beta-N-Acetylhexosaminidases / Q000737:chemistry*
chemical_list: D054818:Hexosaminidase A; D054819:Hexosaminidase B; D001619:beta-N-Acetylhexosaminidases