Crystallographic Evidence for Substrate-assisted Catalysis in a Bacterial β-Hexosaminidase*

Abstract

β-Hexosaminidase, a family 20 glycosyl hydrolase, catalyzes the removal of β-1,4-linkedN-acetylhexosamine residues from oligosaccharides and their conjugates. Heritable deficiency of this enzyme results in various forms of GalNAc-β(1,4)-[N-acetylneuraminic acid (2,3)]-Gal-β(1,4)-Glc-ceramide gangliosidosis, including Tay-Sachs disease. We have determined the x-ray crystal structure of a β-hexosaminidase from Streptomyces plicatus to 2.2 Å resolution (Protein Data Bank code 1HP4). β-Hexosaminidases are believed to use a substrate-assisted catalytic mechanism that generates a cyclic oxazolinium ion intermediate. We have solved and refined a complex between the cyclic intermediate analogueN-acetylglucosamine-thiazoline and β-hexosaminidase fromS. plicatus to 2.1 Å resolution (Protein Data Bank code1HP5). Difference Fourier analysis revealed the pyranose ring ofN-acetylglucosamine-thiazoline bound in the enzyme active site with a conformation close to that of a ⁴C₁chair. A tryptophan-lined hydrophobic pocket envelops the thiazoline ring, protecting it from solvolysis at the iminium ion carbon. Within this pocket, Tyr³⁹³ and Asp³¹³ appear important for positioning the 2-acetamido group of the substrate for nucleophilic attack at the anomeric center and for dispersing the positive charge distributed into the oxazolinium ring upon cyclization. This complex provides decisive structural evidence for substrate-assisted catalysis and the formation of a covalent, cyclic intermediate in family 20 β-hexosaminidases.