Meta-Biol

• Pathways

Chitin is a linear polymer made up of repeating units of the sugar N-acetylglucosamine (GlcNAc) and is the second most abundant polysaccharide in nature after cellulose. It is found in the cell walls of bacteria and fungi, the exoskeleton of crustaceans and insects, and the microfilarial sheath of parasitic nematodes. Chitinases are evolutionarily ancient enzymes that hydrolyse the chitin polymer into di- and trisaccharides. This process produces differentially sized chitin fragments that can trigger the release of type 2 cytokines, including interleukin IL-4, IL-5, IL-13 by CD4 T helper (Th2) and other immune cells which play critical roles in the pathogenesis of asthma and allergic responses. Humans express two active chitinases; acidic mammalian chitinase (CHIA, AMCase) and chitotriosidase (CHIT1) . CHIA is a secreted enzyme that can randomly hydrolyse chitin (and chitotriose, not shown here) (Boot et al. 2001, Chou et al. 2006, Harti et al. 2008, Olland et al. 2009, Seibold et al. 2009).