The X-ray Crystal Structure of Human β-Hexosaminidase B Provides New Insights into Sandhoff Disease

Human lysosomal β-hexosaminidases are dimeric enzymes composed of α and β-chains, encoded by the genes HEXA and HEXB. They occur in three isoforms, the homodimeric hexosaminidases B (ββ) and S (αα), and the heterodimeric hexosaminidase A (αβ), where dimerization is required for catalytic activity. Allelic variations in the HEXA and HEXB genes cause the fatal inborn errors of metabolism Tay–Sachs disease and Sandhoff disease, respectively. Here, we present the crystal structure of a complex of human β-hexosaminidase B with a transition state analogue inhibitor at 2.3 Å resolution (pdb 1o7a). On the basis of this structure and previous studies on related enzymes, a retaining double-displacement mechanism for glycosyl hydrolysis by β-hexosaminidase B is proposed. In the dimer structure, which is derived from an analysis of crystal packing, most of the mutations causing late-onset Sandhoff disease reside near the dimer interface and are proposed to interfere with correct dimer formation. The structure reported here is a valid template also for the dimeric structures of β-hexosaminidase A and S.