Crystal structure of reducing‐end xylose‐releasing exoxylanase in subfamily 7 of glycoside hydrolase family 30

Abstract

AbstractTcXyn30A from Talaromyces cellulolyticus, which belongs to subfamily 7 of the glycoside hydrolase family 30 (GH30‐7), releases xylose from the reducing end of xylan and xylooligosaccharides (XOSs), the so‐called reducing‐end xylose‐releasing exoxylanase (ReX). In this study, the crystal structures of TcXyn30A with and without xylose at subsite +1 (the binding site of the xylose residue at the reducing end) were determined. This is the first report on the structure of ReX in the family GH30‐7. TcXyn30A forms a dimer. The complex structure of TcXyn30A with xylose revealed that subsite +1 is located at the dimer interface. TcXyn30A recognizes xylose at subsite +1 composed of amino acid residues from each monomer and blocks substrate binding to subsite +2 by dimer formation. Thus, the dimeric conformation is responsible for ReX activity. The structural comparison between TcXyn30A and the homologous enzyme indicated that subsite −2 is composed of assembled three stacked Trp residues, Trp49, Trp333, and Trp334, allowing TcXyn30A to accommodate xylan and any branched XOSs decorated with a substitution such as α‐1,2‐linked 4‐O‐methyl‐d‐glucuronic acid or α‐1,2‐ and/or −1,3‐linked L‐arabinofuranose. These findings provide an insight into the structural determinants for ReX activity of TcXyn30A.