Chitinase

Chitin and Chitinase are widely found in nature. Chitin, also known as chitin, is a linear polymer with β-1,4-N-acetylglucosamine as the basic unit, and its content ranks second in natural polymers. Chitin is also a major component of most fungal cell walls and is also found in the outer shell and intestines of insects. Chitinase has been found in microorganisms, plants and animals, and its functions are also different.

Distribution

As early as 1905, Benecke isolated a microorganism that could use chitin as its nutrient and named it Bacillus chitinovirous. Since then, people have isolated and purified chitinase from more microorganisms, plants and animals.

Table 1 Distribution and basic characteristics of chitinase

Classification

There are generally three methods for classifying chitinase. First, the classification system determined by the Nomenclature Committee of the International Union of Biochemistry. Since chitinase and cellulose have many similarities and both belong to the same glycosylation hydrolase, chitinase is named EC 3.2.1.14 by an internationally accepted enzyme classification system. The disadvantage of this classification method is that it does not reflect the evolutionary relationship within the chitinase. Second, chitinase and N-acetylhexosaminidase are classified into three families according to the amino acid sequence of the enzyme, namely, the family 18, 19, and 20. The family 18 and 19 are composed of intracellular chitinase from various sources such as viruses, bacteria, fungi, insects and plants. The family 19 is mainly composed of plant chitinase. The family 20 chitinase is relatively small. Third, since plants produce a large amount of chitinase, plant chitinase is classified into six types according to its gene sequence, namely, type I-VI. The description of the enzyme by this classification method mainly includes the following aspects: N-terminal sequence, enzyme localization, isoelectric point, signal peptide and inducer.

Biological Functions

Chitinase has a different role in microorganisms, animals, and plants. In fungi, protozoa and invertebrates, chitinase plays a role in their growth and morphogenesis. In some bacteria, chitinase can decompose insoluble chitin and use it as a nutrient, thereby using chitin as a carbon source and energy source. In yeast (Saocharomyces), although chitin only constitutes 1% of its cell wall, there is a large amount of chitin deposition in the separator between the two generations of cells, indicating that chitinase is required in the process of yeast cell proliferation and isolation. As far as current research is concerned, chitinase is a defense protein in higher plants and vertebrates. Chitinase has a degrading effect on certain nodulation factors. It is speculated that chitinase balances plants and rhizobium by controlling the level of nodulation factors, thus participating in symbiotic nitrogen fixation.

Characteristics

Chitinase is inducible. Many studies have shown that chitinase in vertebrates and plants is inducible. After the higher plants are induced by pathogens and their elicitors, ethylene, salicylic acid, ultraviolet light, heavy metals, mechanical damage, etc., the chitinase activity is rapidly increased, and the plants are protected to some extent. Certain microbial and animal chitinases can form chitinase with a lower molecular weight after degradation. The expression of chitinase is also time-series and tissue-specific. Although the production of plant chitinase is related to plant resistance, the presence of chitinase has also been found in certain developmental stages and special tissues of many plants. In plants, almost no chitinase is detected at the top, but it is abundant in the old leaves and roots of the base. Therefore, chitinase is a protective mechanism developed in order to adapt to the unfavorable external environment during some developmental stages of plants to ensure the smooth completion of the development process. The chitinase produced by parasitic nematodes is specific and time-dependent, and plays an important role in its development and transmission.

Applications

• Application of chitinase in the control of plant pathogenic fungi

The main component of the cell wall of many phytopathogenic fungi is chitin. In vitro bacteriostatic tests have shown that chitinase can inhibit spore germination and mycelial growth of some pathogenic fungi. When plants are infected by pathogens, they produce a series of active defense responses, including increased activity of chitinase. Therefore, chitinase has long been regarded as a potential substance against plant fungal diseases. Since the 1980s, scientists have transferred chitinase genes to plants such as tobacco, tomato, soybean, potato, lettuce, grape and sugar beet, and obtained transgenic plants expressing chitinase activity. Compared to non-transgenic plants, transgenic plants are not only resistant to fungi, but also resistant to nematodes, insects and other pathogenic organisms.

• Application of chitinase in biological control

Because the parasites Brugia malayi and Wuchereria bancrofti have chitinase involved in the process of reproduction and transmission, chitinase can be used as a candidate vaccine to control these two parasitic nematodes. In 1992, Furhman et al. reported that the monoclonal antibody MF1 can degrade the peripheral components of parasitic microfilariae in gerbils, and this antibody has high homology with several bacterial and yeast chitinases. Some biocontrol bacteria, such as Trichoderma harzianum, Rhizoctonia solani, Leishococcus, Bacillus, etc., one of the mechanisms of biocontrol is the production of cell wall degrading enzymes such as chitinase and glucanase. In addition, studies have found that chitinase can kill or prevent pests and mosquitoes in certain stages of development, so it is envisaged that chitinase can be used to control the number of mosquitoes and these pests without using a large amount of pesticides.

• Application of chitinase degradation products

Amino oligosaccharide, the product produced by hydrolysis of chitin, plays a very important role in regulating the life metabolism of plants and animals. N-acetylglucosamine itself can be used as an anti-inflammatory drug, and has therapeutic effects on large intestine ulcers and gastrointestinal ulcer disorders. In addition, amino oligosaccharides can be used as an immune inducer to induce plants to produce a defense response. In animals, amino oligosaccharides also have physiological activities such as improving immunity and inhibiting tumor cell growth.