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.

Glycosyl hydrolases family 18.

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.

GDP-fucose protein O-fucosyltransferase. This is a family of conserved proteins representing the enzyme responsible for adding O-fucose to EGF (epidermal growth factor-like) repeats. Six highly conserved cysteines are present in O-FucT-1 as well as a DXD-like motif (ERD), conserved in mammals, Drosophila, and C. elegans. Both features are characteristic of several glycosyltransferase families. The enzyme is a membrane-bound protein released by proteolysis and, as for most glycosyltransferases, is strongly activated by manganese.

GDP-fucose protein O-fucosyltransferase, plant specific subfamily. Some members of this plant-specific family of O-fucosyltransferases have been annotated as auxin-independent growth promotors. The function of the protein seems unclear. O-fucosyltransferase-like proteins are GDP-fucose dependent enzymes with similarities to the family 1 glycosyltransferases (GT1). They are soluble ER proteins that may be proteolytically cleaved from a membrane-associated preprotein, and are involved in the O-fucosylation of protein substrates, the core fucosylation of growth factor receptors, and other processes.