The structure of human glucosidase PGGHG reveals a very specific active site accessible through a flat surface for collagen approximation

The enzyme glucosylgalactosylhydroxylysine glucosidase (PGGHG) plays a critical role in collagen metabolism by hydrolyzing the 2-O-α-d-glucopyranosyl-O-β-d-galactopyranose, a natural disaccharide found in the glycosylation of hydroxylysine residues in collagen. We report the X-ray crystallographic structure of human PGGHG, revealing the canonical four-domain fold of enzymes from the GH65 family and representing the first structure reported for a mammalian enzyme in this family. A distinctive flat surface adjacent to the catalytic site, shaped by the N-terminal β-sheet and specific conformations of catalytic loops, is unique to PGGHG among GH65 enzymes. Structural complexes with glucose and the substrate analogue kojibiose (KJB), along with site-directed mutagenesis and enzyme assays, identify residues critical for catalysis and hydroxylysine-collagen binding. Docking studies and AlphaFold3-based predictions suggest that the flat surface facilitates the contact between PGGHG and collagen peptides as well as substrate recognition, and support the enzyme's high specificity toward its glucosaccharide substrate. These findings provide structural insight into the selective recognition of glycosylated hydroxylysines and may inform future therapeutic or biotechnological applications targeting collagen metabolism.