Crystal structure of endo-β- <i>N</i> -acetylglucosaminidase HSα

The crystal structure of endo-β-N-acetylglucosaminidase HSα (Endo HSα) was determined at 1.8 Å resolution, revealing that the enzyme is composed of five distinct domains. Domains I to III adopt a fold that is conserved among GH85 enzymes, with catalytic residues Asn216, Glu218 and Tyr252 corresponding to conserved positions, while Tyr282 is newly implicated in catalysis based on the Endo HSα structure. A long loop unique to Endo HSα constricts the active site in domain I. Domain IV represents a novel structural element that is not observed in other GH85 enzymes. Its glycan-binding model and structural similarity to known sugar-binding domains play a role in substrate recognition. The minimal contacts with other domains allow it to remain flexible, accommodating bulky substrates at the active site. These features provide insights into the structural basis for substrate specificity and expand the structural diversity of the GH85 family.