Chitinase

Chitinase from barley seeds

Chitinases (EC 3.2.1.14, chitodextrinase, 1,4-beta-poly-N-acetylglucosaminidase, poly-beta-glucosaminidase, beta-1,4-poly-N-acetyl glucosamidinase, poly[1,4-(N-acetyl-beta-D-glucosaminide)] glycanohydrolase, (1->4)-2-acetamido-2-deoxy-beta-D-glucan glycanohydrolase) are hydrolytic enzymes that break down glycosidic bonds in chitin.^[1]

As chitin is a component of the cell walls of fungi and exoskeletal elements of some animals (including worms and arthropods), chitinases are generally found in organisms that either need to reshape their own chitin^[2] or dissolve and digest the chitin of fungi or animals.

Species distribution

Chitinivorous organisms include many bacteria^[3] (Aeromonads, Bacillus, Vibrio,^[4] among others), which may be pathogenic or detritivorous. They attack living arthropods, zooplankton or fungi or they may degrade the remains of these organisms.

Fungi, such as Coccidioides immitis, also possess degradative chitinases related to their role as detritivores and also to their potential as arthropod pathogens.

Chitinases are also present in plants (barley seed chitinase: PDB 1CNS, EC 3.2.1.14); some of these are pathogenesis related (PR) proteins that are induced as part of systemic acquired resistance. Expression is mediated by the NPR1 gene and the salicylic acid pathway, both involved in resistance to fungal and insect attack. Other plant chitinases may be required for creating fungal symbioses.^[5]

Although mammals do not produce chitin, they have two functional chitinases (Chitotriosidase - CHIT1 and acidic mammalian chitinase - AMCase) as well as chitinase-like proteins (such as YKL-40) that have high sequence similarity but lack chitinase activity.^[6]

Function

Like cellulose, chitin is an abundant biopolymer that is relatively resistant to degradation.^[7] It is typically not digested by animal, though certain fish are able to digest chitin.^[8] It is currently assumed that chitin digestion by animals requires bacterial symbionts and lengthy fermentations, similar to cellulase digestion by ruminants. Nevertheless, chitinases have been isolated from the stomachs of certain mammals, including humans.^[9] Chitinase activity can also be detected in human blood^[10][11][11] and possibly cartilage.^[12] As in plant chitinases this may be related to pathogen resistance.^[13][14]

Clinical significance

Human chitinases may be related to allergies, and asthma has been linked to enhanced chitinase expression levels.^{[15][16][17][18][19]}

Human chitinases may explain the link between some of the most common allergies (dust mites, mold spores - both of which contain chitin) and worm (helminth) infections, as part of one version of the hygiene hypothesis^[20][21][22] (worms have chitinous mouthparts to hold the intestinal wall). Finally, the link between chitinases and salicylic acid in plants is well established - but there is a hypothetical link between salicylic acid and allergies in humans.^[23]

Presence in food

Chitinase occurs naturally in many common foods. This is at least one cause of the cross-reaction phenomenon in latex-fruit syndrome. Bananas, chestnuts, kiwis, avocados, papaya, and tomatoes, for example, all contain significant levels of chitinase.

See also

• Chitin
• Ligninase

References

External links

• Chitinase at the US National Library of Medicine Medical Subject Headings (MeSH)
• X-ray structure of a chitinase from the pathogenic fungus Coccidioides immitis