Characterization and structural analysis of a GH51 arabinofuranosidase Cabf51 catalyzing the transformation of...

In this study, a GH51 α-l-arabinofuranosidase (Cabf51) from Cellulosimicrobium sp. TH-20 was recombinantly expressed, characterized, and functionally analyzed. The Cabf51 exhibited optimal activity at pH 7.0 and 60 °C with arabinofuranose-induced activation. The recombinant enzyme selectively hydrolyzed the arabinofuranosyl residue at the C-20 position of ginsenoside Rc (G-Rc) and compound Mc1 (C-Mc1) to produce ginsenoside Rd (G-Rd) and ginsenoside F2 (G-F2), respectively. The k_cat/K_m values of Cabf51 for pNP-Araf, C-Mc1, and G-Rc were 102.32, 27.49, and 14.75 s^-1 mM^-1, respectively. Structural analysis of the Cabf51-ligand complex revealed a novel substrate selectivity mechanism for GH51 arabinofuranosidases toward ginsenosides. The three-dimensional structure of Cabf51 indicated the presence of a semi-open groove that gradually narrowed from the protein surface toward the interior. Gatekeeper residues (Trp178 and Lys309) were key determinants of substrate binding. The hierarchical architecture of the catalytic channel of Cabf51 functioned as a geometrically driven selective filter, with the narrow binding pocket preventing the accommodation of larger ginsenosides. This unique structural feature enabled Cabf51 to recruit Asn72 as an auxiliary catalytic residue. The Asn72 anchored G-Rc or C-Mc1 and stabilized the hydrogen bond network, thereby effectively driving the hydrolytic process to completion. These findings enhanced our understanding of the catalytic mechanism of GH51 family arabinofuranosidases and provided a theoretical foundation for the development of more efficient and versatile enzymes to accelerate ginsenoside biotransformation.