Crystal structures of the catalytic domain of a novel glycohydrolase family 23 chitinase from sp. A-471 reveals a unique arrangement of the catalytic residues for inverting chitin hydrolysis

Arimori, Takao*; Kawamoto, Noriko*; Shinya, Shoko*; Okazaki, Nobuo*; Nakazawa, Masami*; Miyatake, Kazutaka*; Fukamizo, Tamo*; Ueda, Mitsuhiro*; Tamada, Taro

Chitinase C from sp. A-471 (Ra-ChiC) has a catalytic domain sequence similar to goose type (G-type) lysozymes and, unlike other chitinases, belongs to glycohydrolase (GH) family 23. Using NMR spectroscopy, however, Ra-ChiC was found to interact only with the chitin dimer but not with the peptideglycan fragment. Here we report the crystal structures of wild-type, E141Q, and E162Q of the catalytic domain of Ra-ChiC with or without chitin oligosaccharides. Ra-ChiC has a substrate-binding site including a tunnel-shaped cavity, which determines the substrate specificity. Mutation analyses based on this structural information indicated that a highly conserved Glu141 acts as a catalytic acid, and that Asp226 located at the roof of the tunnel activates a water molecule as a catalytic base. The unique arrangement of the catalytic residues makes a clear contrast to the other GH23 members and also to inverting GH19 chitinases.