Elucidation of the Molecular Basis for Arabinoxylan-Debranching Activity of a Thermostable Family GH62 alpha-L-Arabinofuranosidase from Streptomyces thermoviolaceus

Xylan-debranching enzymes facilitate the complete hydrolysis of xylan and can be used to alter xylan chemistry. Here, the family GH62 α-L-arabinofuranosidase from Streptomyces thermoviolaceus (SthAbf62A) was shown to have a half-life of 60 min at 60°C and the ability to cleave α-1,3 L-arabinofuranose (L-Araf) from singly substituted xylopyranosyl (Xylp) backbone residues in wheat arabinoxylan; low levels of activity on arabinan as well as 4-nitrophenyl α-L-arabinofuranoside were also detected. After selective removal of α-1,3 L-Araf substituents from disubstituted Xylp residues present in wheat arabinoxylan, SthAbf62A could also cleave the remaining α-1,2 L-Araf substituents, confirming the ability of SthAbf62A to remove α-L-Araf residues that are (1¡2) and (1¡3) linked to monosubstituted β-D-Xylp sugars. Three-dimensional structures of SthAbf62A and its complex with xylotetraose and L-arabinose confirmed a five-bladed β-propeller fold and revealed a molecular Velcro in blade V between the β1 and β21 strands, a disulfide bond between Cys27 and Cys297, and a calcium ion coordinated in the central channel of the fold. The enzyme-arabinose complex structure further revealed a narrow and seemingly rigid L-arabinose binding pocket situated at the center of one side of the β propeller, which stabilized the arabinofuranosyl substituent through several hydrogen-bonding and hydrophobic interactions. The predicted catalytic amino acids were oriented toward this binding pocket, and the catalytic essentiality of Asp53 and Glu213 was confirmed by site-specific mutagenesis. Complex structures with xylotetraose revealed a shallow cleft for xylan backbone binding that is open at both ends and comprises multiple binding subsites above and flanking the L-arabinose binding pocket.