Crystal Structure of Human β-Hexosaminidase B: Understanding the Molecular Basis of Sandhoff and Tay–Sachs Disease

In humans, two major β-hexosaminidase isoenzymes exist: Hex A and Hex B. Hex A is a heterodimer of subunits α and β (60% identity), whereas Hex B is a homodimer of β-subunits. Interest in human β-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 (β-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 (2.4 Å) and in complex with the mechanistic inhibitors GalNAc-isofagomine (2.2 Å) or NAG-thiazoline (2.5 Å). 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 α and β-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 (β-subunit mutations) and Tay–Sachs disease (α-subunit mutations).