Glyco_18 domain.

Glycosyl hydrolases family 18.

The GH18 (glycosyl hydrolase, family 18) type II chitinases hydrolyze chitin, an abundant polymer of beta-1,4-linked N-acetylglucosamine (GlcNAc) which is a major component of the cell wall of fungi and the exoskeleton of arthropods. Chitinases have been identified in viruses, bacteria, fungi, protozoan parasites, insects, and plants. The structure of the GH18 domain is an eight-stranded beta/alpha barrel with a pronounced active-site cleft at the C-terminal end of the beta-barrel. The GH18 family includes chitotriosidase, chitobiase, hevamine, zymocin-alpha, narbonin, SI-CLP (stabilin-1 interacting chitinase-like protein), IDGF (imaginal disc growth factor), CFLE (cortical fragment-lytic enzyme) spore hydrolase, the type III and type V plant chitinases, the endo-beta-N-acetylglucosaminidases, and the chitolectins. The GH85 (glycosyl hydrolase, family 85) ENGases (endo-beta-N-acetylglucosaminidases) are closely related to the GH18 chitinases and are included in this alignment model.

GH18 domain of Chitinase D (ChiD). ChiD, a chitinase found in Bacillus circulans, hydrolyzes the 1,4-beta-linkages of N-acetylglucosamine in chitin and chitodextrins. The domain architecture of ChiD includes a catalytic glycosyl hydrolase family 18 (GH18) domain, a chitin-binding domain, and a fibronectin type III domain. The chitin-binding and fibronectin type III domains are located either N-terminal or C-terminal to the catalytic domain. This family includes exochitinase Chi36 from Bacillus cereus.

Narbonin is a plant 2S protein from the globulin fraction of narbon bean (Vicia narbonensis L.) cotyledons with unknown function. Narbonin has a glycosyl hydrolase family 18 (GH18) domain without the conserved catalytic residues and with no known enzymatic activity. Narbonin amounts to up to 3% of the total seed globulins of mature seeds and was thought to be a storage protein but was found to degrade too slowly during germination. This family also includes the VfNOD32 nodulin from Vicia faba.