Human beta-
hexosaminidases A and B (EC 3.2.1.52) are dimeric lysosomal
glycosidases composed of evolutionarily related alpha and/or beta subunits. Both
isozymes hydrolyze terminal beta-linked GalNAc or GlcNAc residues from numerous artificial and natural substrates; however, in vivo
GM2 ganglioside is a substrate for only the heterodimeric A
isozyme. Thus, mutations in either gene encoding its alpha or beta subunits can result in
GM2 ganglioside storage and Tay-Sachs or
Sandhoff disease, respectively. All glycosyl
hydrolases ae believed to have one or more acidic residues in their catalytic site. We demonstrate that incubation of
hexosaminidase with a chemical modifier specific for carboxyl side chains produces a time-dependent loss of activity, and that this effect can be blocked by the inclusion of a strong competitive inhibitor in the reaction mix. We hypothesized that the catalytic
acid residue(s) should be located in a region of overall homology and be invariant within the aligned deduced primary sequences of the human alpha and beta subunits, as well as
hexosaminidases from other species, including bacteria. Such a region is encoded by exons 5-6 of the HEXA and HEXB genes. This region includes beta Arg211 (invariant in 15 sequences), which we have previously shown to be an active residue. This region also contains two invariant and one conserved acidic residues. A fourth acidic residue, Asp alpha 258, beta 290, in exon 7 was also investigated because of its association with the B1 variant of
Tay-Sachs disease. Conservative substitutions were made at each candidate residue by in vitro mutagenesis of a beta
cDNA, followed by cellular expression. Of these, only the beta Asp196Asn substitution decreased the kcat (350-910-fold) without any noticeable effect on the K(m). Mutagenesis of either beta Asp240 or beta Asp290 to Asn decreased kcat by 10- or 1.4-fold but also raised the K(m) of the
enzyme 11- of 3-fold, respectively. The above results strongly suggest that beta Asp196 is a catalytic
acid residue in
beta-hexosaminidase.