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).