Chitinase C chitin binding domain (ChBD ChiC)
Streptomyces griseus HUT6037
Chitinases (EC 22.214.171.124) are enzymes that catalyze the hydrolytic degradation of chitin. Chitinases exist ubiquitously, not only in organisms such as crabs, insects, fungi, and invertebrates, which contain chitin, but also in bacteria, plants, and vertebrates. Chitin is a linear β-1,4-linked polymer of N-acetyl-D-glucosamine (NAG). The quantity of chitin produced by these organisms is almost equal to that of cellulose, the second most abundant biomass on earth.
Chitinases are classified into two families, family 18 (mainly from bacteria, fungi, etc.) and family 19 (mainly from plants) chitinases, on the basis of the amino acid sequences of their catalytic domains. Chitinase C from Streptomyces griseus HUT6037 was discovered as the first bacterial chitinase in family 19, other than chitinases found in higher plants.
ChiC, composed of 294 amino acid residues (31.4 kDa), has two functional domains: an N-terminal chitin-binding domain (ChBD ChiC) for attachment to chitin, and a C-terminal chitin-catalytic domain (CatD ChiC) for breaking chitin into pieces. In spite of the high sequence similarity between CatD ChiC and the chitin-catalytic domains of other plant chitinases in family 19, the sequence of ChBD ChiC has similarity to those found in some bacterial family 18 chitinases and cellulases. ChiC has a chimeric structure of the bacterial type chitin-binding domain and the plant type chitin-catalytic domain.
The backbone structure of ChBD ChiC, which consists of two β-sheets composed of two and three antiparallel β-strands, belongs to the carbohydrate-binding module (CBM) 5 family. This structure is very similar to the backbone conformations of the cellulose-binding domain of endoglucanase Z from Pectobacterium chrysanthemi (CBD EGZ, PDB:1aiw) and of the chitin-binding domain of chitinase A1 from Bacillus circulans WL-12 (ChBD ChiA1, PDB:1ed7).
Protein Data Bank (PDB)
author: Ken-ichi Akagi