Solution structure of the chitin-binding domain of Bacillus circulans WL-12 chitinase A1

The three-dimensional structure of the chitin-binding domain (ChBD) of chit inase A1 (ChiA1) from a Grampositive bacterium, Bacillus circulans WL-12, w as determined by means of multidimensional heteronuclear NMR methods. ChiA1 is a glycosidase that hydrolyzes chitin and is composed of an N-terminal c atalytic domain, two fibronectin type Iii-like domains, and C-terminal ChBD (ChiA1), (45 residues, Ala(655)-Gln(699)), which binds specifically to inso luble chitin, ChBD(ChiA1) has a compact and globular structure with the top ology of a twisted beta-sandwich, This domain contains two antiparallel bet a-sheets, one composed of three strands and the other of two strands. The c ore region formed by the hydrophobic and aromatic residues makes the overal l structure rigid and compact. The overall topology of ChBD(ChiA1) is simil ar to that of the cellulose-binding domain (CBD) of Erwinia chrysanthemi en doglucanase Z (CBDEGZ). However, ChBD(ChiA1) lacks the three aromatic resid ues aligned linearly and exposed to the solvent, which probably interact wi th cellulose in CBDs. Therefore, the binding mechanism of a group of ChBDs including ChBD(ChiA1) may be different from that proposed for CBDs.