A novel chitin-binding mode of the chitin-binding domain of chitinase A1 from Bacillus circulans WL-12 revealed by solid-state NMR.

Chitin-binding domain of chitinase A1 (ChBD ) is characteristic because it binds only to insoluble crystalline chitin. While binding sites of major carbohydrate-binding modules carry multiple aromatic rings aligned on a surface, lethal mutations for ChBD were reported only at W687, a location completely different from the site mentioned above, in spite of their similar main-chain folds. Here, the structural mechanism underlying its crystalline chitin binding was uncovered by solid-state NMR. Based on C- and N-signal assignment of microcrystalline ChBD , the chemical shift perturbation on chitin binding was carefully examined. The perturbation was greatest at W687 and nonaromatic residues surrounding it, revealing their direct involvement in chitin binding. These residues and Q679 should provide a novel chitin-binding platform parallel to the W687 ring.