GH18

GH18 refers to glycoside hydrolase family 18, a group of enzymes catalogued in the CAZy database. Members of this family are primarily chitinases, catalyzing the hydrolysis of the beta-1,4-glycosidic bonds in chitin, a polymer of N-acetylglucosamine that forms fungal cell walls, insect exoskeletons, and crustacean shells. GH18 enzymes are found across bacteria, fungi, plants, and animals and are frequently secreted or localized to periplasmic spaces to access insoluble chitin.

The catalytic mechanism of GH18 enzymes typically involves a conserved catalytic motif and a key acidic residue, with many studies supporting a substrate-assisted retaining mechanism that can proceed through an oxazolinium ion intermediate. This mechanism generally results in retention of the anomeric configuration after bond cleavage. The active site is adapted to accommodate both soluble oligosaccharides and crystalline chitin, enabling diverse substrate specificities within the family.

Structural features of GH18 enzymes commonly include a catalytic core that adopts a beta/alpha-barrel or related fold, forming a deep cleft for chitin binding. Many enzymes in this family also possess carbohydrate-binding modules appended to the catalytic domain, which enhance interaction with crystalline chitin and improve catalytic efficiency.

Biological roles and applications are broad. In nature, GH18 enzymes participate in nutrient acquisition for microbes, fungal development, and plant defense responses. They have potential applications in biocontrol, bioconversion of chitin waste, and various industrial processes where chitin degradation is desired. CAZy further subdivides GH18 into subfamilies reflecting sequence diversity and substrate preferences.