Endo-β-N-acetylglucosaminidases Acting on the Carbohydrate Moieties of Glycoproteins

Endo-β-N-acetyglucosaminidse H from Streptomyces griseus completely hydrolyzed thyroglobulin Unit A (mannose-N-acetylglucosamine unit) glycopeptides, but did not act on “side-chain*” thyroglobulin Unit B (complex heteropolysaccharide unit) ,glycopeptides. Endo-β-N-acetyglucosaminidase H also did not act on intact or “side-chain-free” IgG glycopeptides. α-L-Fucosidase [EC 3.2.1.51] treatment did not make the above glycopeptides susceptible to endo-β-N-acetyglucosaminidse H. [14C]-Acetyl-AsnGlcNA2Man1 prepared from [14C]-Acetyl-AsnGlcNA2Man5 by .α-mannosidase [EC 3.2. 1. 24] digestion was resistant not only to endo-β-N-acetyglucosaminidase H, but also to endo-β-N-acetylglucosaminidase D from Diploaccus pneumoniae. A disaccharide isolated from a “side-chain-free” bovine IgG glycopeptide by treatment with endo-β-N-acetyglucosaminidase D, followed by αa-mannosidase digestion had a probable structure of Manβ1→4GlcNAc, suggesting that a common sequence of the mannose-N-acetylglucosamine unit (Manβ1→4GlcNAcβ1→4GlcNAc→Asn) is present in glycopeptides of complex heteropolysaccharide unit. We propose that endo-β-N-acetyglucosaminidse H and endo-β-N-acetyglucosaminidse D have opposite specificities; i.e., the former generally hydrolyzes glyco peptides of the mannose-N-acetylglucosamine unit, and the latter generally hydrolyzes “side-chain-free” glycopeptides of the complex heteropolysaccharide unit, and that the two enzymes recognize the structural differences of α-mannosyl residues in the oligomannosyl cores of the two units.