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.

Glyco_18 domain.

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.

Glycosyl hydrolases family 18.

Chitinase, GH18 family [Carbohydrate transport and metabolism].

ChainA, Putative chitinase II [Klebsiella pneumoniae subsp. pneumoniae MGH 78578]

Structureof Chitinase D from Serratia proteamaculans revealed an unusually constrained substrate binding site [Serratia proteamaculans 568]

Keyresidues affecting transglycosylation activity in family 18 chitinases - Insights into donor and acceptor subsites [Serratia proteamaculans 568],6F8N_B Key residues affecting transglycosylation activity in family 18 chitinases - Insights into donor and acceptor subsites [Serratia proteamaculans 568]

Crystalstructure of Chitinase D from Serratia proteamaculans at 1.45 Angstrom resolution [Serratia proteamaculans 568],4Q22_A Crystal structure of Chitinase D from Serratia proteamaculans in complex with N-acetyl glucosamine at 1.93 Angstrom resolution [Serratia proteamaculans 568]

CrystalStructure of Chitinase D from Serratia proteamaculans in complex with N-acetyl glucosamine, a hydrolyzed product of hexasaccharide at 1.7 Angstrom resolution [Serratia proteamaculans 568]

Chitinase A1 OS=Niallia circulans OX=1397 GN=chiA1 PE=1 SV=1

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

Acidic mammalian chitinase OS=Bos taurus OX=9913 GN=CHIA PE=1 SV=1

Acidic mammalian chitinase OS=Mus musculus OX=10090 GN=Chia PE=1 SV=2

Acidic mammalian chitinase OS=Rattus norvegicus OX=10116 GN=Chia PE=2 SV=1