Crystal structure of a bacterial chitinase at 2.3 Å resolution

Background: Chitinases cleave the β-1-4-glycosidicbond between the N-acetyl-D-glucosamine units of which chitin is comprised. Chitinases are present in plants, bacteria and fungi, but whereas structures are available for two prototypic plant enzymes, no structure is available for a bacterial or fungal chitinase. Results To redress this imbalance, the structure of native chitinase A from Serratia marcescens has been solved by multiple isomorphous replacement and refined at 2.3 Å resolution, resulting in a crystallographic R-factor of 16.2%. The enzyme comprises three domains: an all-β-strand amino-terminal domain, a catalytic α/β-barrel domain, and a small α +β-fold domain. There are several residues with unusual geometries in the structure. Structure determination of chitinase A in complex with N, N′, N″, N‴-tetra-acetylo-chitotetraose, together with biochemical and sequence analysis data, enabled the positions of the active-site and catalytic residues to be proposed. Conclusion The reaction mechanism seems to be similar to that of lysozyme and most other glycosylhydrolases, i.e. general acid-base catalysis. The role of theamino-terminal domain could not be identified, but it has similarities to the fibronectin III domain. This domain may possibly facilitate the interaction of chitinase A with chitin.