Identification of the substrate interaction region of the chitin-binding domain of Streptomyces griseus chitinase C

Chitinase C from Streptomyces griseus HUT6037 was discovered as the first bacterial chitinase in family 19 other than chitinases found in higher plants. Chitinase C comprises two domains: a chitin-binding domain (ChBD Chic ) for attachment to chitin and a chitin-catalytic domain for digesting chitin. The structure of ChBD Chic was determined by means of 13 C-, 15 N-, and 1 H-resonance nuclear magnetic resonance (NMR) spectroscopy. The conformation of its backbone comprised two β-sheets composed of two and three antiparallel p-strands, respectively, this being very similar to the backbone conformations of the cellulose-binding domain of endoglucanase Z from Erwinia chrysanthemi (CBD EGZ ) and the chitin-binding domain of chitinase Al from Bacillus circulans WL-12 (ChBD ChiA1 ). The interaction between ChBD Chic and hexa-N-acetyl-chitohexaose was monitored through chemical shift perturbations, which showed that ChBD Chic interacted with the substrate through two aromatic rings exposed to the solvent as CBD EGZ interacts with cellulose through three characteristic aromatic rings. Comparison of the conformations of ChBD ChiA1 , ChBD ChiC , and other typical chitin- and cellulose-binding domains, which have three solvent-exposed aromatic residues responsible for binding to polysaccharides, has suggested that they have adopted versatile binding site conformations depending on the substrates, with almost the same backbone conformations being retained.