CRYSTAL-STRUCTURE OF A BACTERIAL CHITINASE AT 2.3-ANGSTROM RESOLUTION

Background: Chitinases cleave the beta-1-4-glycosidic bond 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 imbalan ce, the structure of native chitinase A from Serratia marcescens has b een solved by multiple isomorphous replacement and refined at 2.3 Angs trom resolution, resulting in a crystallographic R-factor of 16.2%. Th e enzyme comprises three domains: an all-beta-strand aminoterminal dom ain, a catalytic alpha/beta-barrel domain, and a small alpha+beta-fold domain. There are several residues with unusual geometries in the str ucture. Structure determination of chitinase A in complex with N,N',N' ',N'''-tetraacetylo-chitotetraose, together with biochemical and seque nce analysis data, enabled the positions of the active-site and cataly tic residues to be proposed. Conclusions: The reaction mechanism seems to be similar to that of lysozyme and most other glycosylhydrolases, i.e. general acid-base catalysis. The role of the amino-ter minal doma in could not be identified, but it has similarities to the fibronectin III domain. This domain may possibly facilitate the interaction of ch itinase A with chitin.