Structure of a two-domain chitotriosidase from Serratia marcescens at 1.9-angstrom resolution

in this paper, we describe the structure of chitinase B from Serratia marce scens. which consists of a catalytic domain with a TIM-barrel fold and a 49 -residue C-terminal chitin-binding domain. This chitinase is the first stru cture of a bacterial exochitinase, and it represents one of only a few exam ples of a glycosyl hydrolase structure having interacting catalytic and sub strate-binding domains. The chitin-binding domain has exposed aromatic resi dues that contribute to a 55-Angstrom long continuous aromatic stretch exte nding into the active site. Binding of chitin oligomers is blocked beyond t he -3 subsite, which explains why the enzyme has chitotriosidase activity a nd degrades the chitin chain from the nonreducing end. Comparison of the ch itinase B structure with that of chitinase A explains why these enzymes act synergistically in the degradation of chitin.