Chitinases (, chitodextrinase, 1,4-β-poly-N-acetylglucosaminidase, poly-β-glucosaminidase, β-1,4-poly-N-acetyl glucosamidinase, poly[1,4-(N-acetyl-β-D-glucosaminide)] glycanohydrolase, (1→4)-2-acetamido-2-deoxy-β-D-glucan glycanohydrolase; systematic name (1→4)-2-acetamido-2-deoxy-β-D-glucan glycanohydrolase) are hydrolytic enzymes that break down glycosidic bonds in chitin. They catalyse the following reaction:
Random endo-hydrolysis of N-acetyl-β-D-glucosaminide (1→4)-β-linkages in chitin and chitodextrins
As chitin is a component of the cell walls of fungi and exoskeletal elements of some animals (including mollusks and arthropods), chitinases are generally found in organisms that either need to reshape their own chitin or dissolve and digest the chitin of fungi or animals.
Species distribution
Chitinivorous organisms include many bacteria (Aeromonads, Bacillus, Vibrio, 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 – for example barley seed chitinase: , . Barley seeds are found to produce clone 10 in Ignatius et al 1994(a). They find clone 10, a Class I chitinase, in the seed aleurone during development. Leaves produce several isozymes (as well as several of β-1,3-glucanase). Ignatius et al 1994(b) find these in the leaves, induced by powdery mildew. Ignatius et al also find these (seed and leaf isozymes) to differ from each other. 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.
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.
Classification
Endochitinases (EC 3.2.1.14) randomly split chitin at internal sites of the chitin microfibril, forming soluble, low molecular mass multimer products. The multimer products includes di-acetylchitobiose, chitotriose, and chitotetraose, with the dimer being the predominant product. Exochitinases have also been divided into two sub categories: Chitobiosidases () act on the non-reducing end of the chitin microfibril, releasing the dimer, di-acetylchitobiose, one by one from the chitin chain. Therefore, there is no release of monosaccharides or oligosaccharides in this reaction. β-1,4- N-acetylglucosaminidases () split the multimer products, such as di-acetylchitobiose, chitotriose, and chitotetraose, into monomers of N-acetylglucoseamine (GlcNAc). Chitinases were also classified based on the amino acid sequences, as that would be more helpful in understanding the evolutionary relationships of these enzymes to each other. Therefore, the chitinases were grouped into three families: 18, 19, and 20. Both families 18 and 19 consists of endochitinases from a variety of different organisms, including viruses, bacteria, fungi, insect, and plants. However, family 19 mainly comprises plant chitinases. Family 20 includes N-acetylglucosaminidase and a similar enzyme, N-acetylhexosaminidase.
And as the gene sequences of the chitinases were known, they were further classified into six classes based on their sequences. Characteristics that determined the classes of chitinases were the N-terminal sequence, localization of the enzyme, isoelectric pH, signal peptide, and inducers.
chitinases had a cysteine-rich N-terminal, leucine- or valine-rich signal peptide, and vacuolar localization. And then, Class I chitinases were further subdivided based on their acidic or basic nature into and , respectively. Class 1 chitinases were found to comprise only plant chitinases and mostly endochitinases.
chitinases did not have the cysteine-rich N-terminal but had a similar sequence to Class I chitinases. Class II chitinases were found in plants, fungi, and bacteria and mostly consisted of exochitinases.
chitinases did not have similar sequences to chitinases in Class I or Class II.
chitinases had similar characteristics, including the immunological properties, as Class I chitinases. However, Class IV chitinases were significantly smaller in size compared to Class I chitinases.