Glyco_18 domain.

Glycosyl hydrolases family 18.

The GH18 (glycosyl hydrolases, family 18) type II chitinases hydrolyze chitin, an abundant polymer of N-acetylglucosamine and have been identified in bacteria, fungi, insects, plants, viruses, and protozoan parasites. The structure of this domain is an eight-stranded alpha/beta barrel with a pronounced active-site cleft at the C-terminal end of the beta-barrel.

This conserved domain family includes a large number of catalytically inactive chitinase-like lectins (chitolectins) including YKL-39, YKL-40 (HCGP39), YM1, oviductin, and AMCase (acidic mammalian chitinase), as well as catalytically active chitotriosidases. The conserved domain is an eight-stranded alpha/beta barrel fold belonging to the family 18 glycosyl hydrolases. The fold has a pronounced active-site cleft at the C-terminal end of the beta-barrel. The chitolectins lack a key active site glutamate (the proton donor required for hydrolytic activity) but retain highly conserved residues involved in oligosaccharide binding. Chitotriosidase is a chitinolytic enzyme expressed in maturing macrophages, which suggests that it plays a part in antimicrobial defense. Chitotriosidase hydrolyzes chitotriose, as well as colloidal chitin to yield chitobiose and is therefore considered an exochitinase. Chitotriosidase occurs in two major forms, the large form being converted to the small form by either RNA or post-translational processing. Although the small form, containing the chitinase domain alone, is sufficient for the chitinolytic activity, the additional C-terminal chitin-binding domain of the large form plays a role in processing colloidal chitin. The chitotriosidase gene is nonessential in humans, as about 35% of the population are heterozygous and 6% homozygous for an inactivated form of the gene. HCGP39 is a 39-kDa human cartilage glycoprotein thought to play a role in connective tissue remodeling and defense against pathogens.

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.

Crystalstructure of the GH18 chitinase ChiB from the chitin utilization locus of Flavobacterium johnsoniae [Flavobacterium johnsoniae UW101]

Crystalstructure of a chitinase from Bacteroides thetaiotaomicron [Bacteroides thetaiotaomicron VPI-5482],3FND_A Crystal structure of a chitinase from Bacteroides thetaiotaomicron [Bacteroides thetaiotaomicron]

Crystalstructure of Ostrinia furnacalis Group IV chitinase [Ostrinia furnacalis],6JMB_A Chain A, ofchtiv-allosamidin [Ostrinia furnacalis]

Crystalstructure of Ostrinia furnacalis Group IV chitinase [Ostrinia furnacalis]

ChitinaseChiA74 from Bacillus thuringiensis [Bacillus thuringiensis],6BT9_B Chitinase ChiA74 from Bacillus thuringiensis [Bacillus thuringiensis]

Probable chitinase 2 OS=Drosophila melanogaster OX=7227 GN=Cht2 PE=1 SV=1

Chitinase 4 OS=Candida albicans (strain SC5314 / ATCC MYA-2876) OX=237561 GN=CHT4 PE=3 SV=1

Chitinase-like protein 3 OS=Mus musculus OX=10090 GN=Chil3 PE=1 SV=2

Chitinase-like protein 4 OS=Mus musculus OX=10090 GN=Chil4 PE=1 SV=2

Endochitinase OS=Manduca sexta OX=7130 PE=2 SV=1