This page has been approved by the Responsible Curator as essentially complete. CAZypedia is a living document, so further improvement of this page is still possible. If you would like to suggest an addition or correction, please contact the page's Responsible Curator directly by e-mail.
The glycoside hydrolases of this family are endo-β-galactosidases. No other activities have been reported. Family 98 glycoside hydrolases are unique in their specificity towards cleavage of the type II core [gal(β1-4)glcNAc] in the AB blood group antigens (EABase, Sp3GH98) and LewisY antigens (SpGH98) [1, 2, 3]. These enzymes are capable of processing these glycans when presented on cell surfaces thus destroying the antigens [1, 2].
Kinetics and Mechanism
Family GH98 galactosidases are inverting enzymes, as first shown by NMR monitoring of the Sp4GH98-catalyzed hydrolysis of the LewisY tetrasaccharide [2]. EABase was also shown to act through an inverting enzyme by NMR monitoring of the EABase catalysed hydrolysis of an artificial substrate, DNP-A-trisaccharide [3]. These results are contrary to the initial predictions made by Rigden [4]. EABase follows normal Michaelis-Menten kinetics [3].
Catalytic Residues
The general base, an aspartate and glutamate diad, and the general acid, glutamate, were identified through the crystal structures of Sp3GH98 and Sp4GH98 in complex with the A trisaccharide and H disaccharide, respectively, and confirmed by site-directed mutagenesis [2]. Similar results were obtained through kinetic studies of the corresponding acid and base mutants of EABase with the artificial substrate dinitrophenyl A-trisaccharide. Further biochemical evidence for the catalytic acid residue was obtained by comparison between the activity of the acid mutant and the pKa of the leaving group of the substrate [3].
Three-dimensional structures
The first crystal structures from family 98 were the Sp3GH98 and Sp4GH98 enzymes from S. pneumoniae in complex with the A trisaccharide and H disaccharide respectively [2]. Both structures feature a (α/β)8 barrel in the catalytic domain with an adjoining β-sandwich domain that contributes to the architecture of the active site and helps define the respective specificities of the enzymes [2]
Family Firsts
First sterochemistry determination
Streptococcus pneumoniae TIGR4 endo-beta-galactosidase Sp4GH98 by NMR [2].
