Abstract
Plants have hydroxyproline-rich glycoproteins (HRGPs), containing hydroxyproline residues that are modified by β-arabinooligosaccharides. Several enzymes that catalyze the degradation of β-arabinooligosaccharides in HRGPs were recently discovered in Bifidobacterium longum JCM1217. This review describes three-dimensional structures of one of those enzymes, GH127 β-L-arabinofuranosidase (HypBA1). The X-ray crystal structures of HypBA1 in its apo form and in complex with β-Araf were determined at resolution of 2.2 Å and 2.0 Å respectively. HypBA1 was found to have a novel active center and was suggested to catalyze reactions by an unprecedented mechanism in glycosidases. The proposed reaction mechanism, which was supported by biochemical analysis, uses a Cys residue that coordinates a Zn2+ ion as a nucleophile. There are many homologs of HypBA1 in bacteria, fungi, and plants, and the catalytic residues are highly conserved among them. Therefore, it is predicted that a substantial number of enzymes share a similar reaction mechanism with HypBA1.
