Lysozyme is a powerful bactericidal substance discovered in nasal mucus by British bacteriologist Fenin in 1929, and was subsequently named lysozyme.

Lysozyme, also known as muramidase or glycoside hydrolase or N-acetyl muramidyl glycanhydrolase, is a hydrolase that specifically acts on the cell wall of microorganisms. Basic globulin is composed of 129 anylases, which is a white or slightly yellow crystalline or amorphous powder; it is non-toxic, odorless, sweet, easily soluble in water, insoluble in acetone, ether, and ethanol. It is a kind of basic globulin, which is relatively stable to pH changes. It is a protein that is stable to heat under acidic conditions and has the most abundant content in egg white.
Lysozyme is widely present in the egg whites of birds and poultry. In mammals’ tears, saliva, blood, urine, milk, white blood cells and their body fluids (such as lymph) and tissues (such as liver and kidney) cells, egg white is the most abundant, about 0.3-0.5%.

Classification of Lysozyme
With the deepening of research, it was discovered that lysozyme not only has an effect on bacterial cell walls, but also on fungal cell walls. According to different sources, lysozyme can be divided into the following three categories:

1. Animal lysozyme
Chicken egg white contains about 3.5% lysozyme, which can decompose Gram-positive bacteria, but has no effect on Gram-negative bacteria. Its molecular weight is 14,000. In addition, lysozyme has also been isolated from other bird proteins, mammalian milk and body fluids.

2. Plant lysozyme
Researchers conducted a census of 410 species of 116 families and found that 168 plants contained lysozyme. Among them, lysozyme can be isolated from papaya, fig, barley and other plants. Its molecular weight is larger, about 24000-29000, and its lysozyme activity against Micrococcus lysodeikticus is not more than 1/3 of that of egg white lysozyme.

3. Microbial lysozyme
It was discovered in the 1960s that microorganisms also produce lysozyme, which can be divided into two categories according to its targets, namely bacterial lysozyme and fungal lysozyme.

1) Bacterial lysozyme
Bacterial lysozymes can generally be divided into three categories: N-acetylhexosaminidase, which catalyzes the hydrolysis of β(1→4) glycosidic bonds in the sugar backbone of peptidoglycan. N-acetylmuramyl-alanine amidase, which catalyzes the cleavage of sugar and peptide groups in peptidoglycans. Endopeptidase, which catalyzes the cleavage of peptide bonds in peptidoglycan peptide bridges. Studies have found that the antibacterial activity of lysozyme is not only manifested in its decomposition of bacterial cell walls, but when the enzyme is irreversibly inhibited, it still shows antibacterial effects, which may be related to the alkalinity of lysozyme. Lysozymes from different sources have different antibacterial ranges and are specific to different types of peptidoglycans, especially their ability to decompose O-acetylated peptidoglycans.

Because Gram-positive bacteria have no outer membrane, lysozyme has a strong effect on them. The combined use of lysozyme and EDTA can enhance the inhibition of Gram-negative bacteria. Treating lysozyme with perillaldehyde or attaching lysozyme to galactomannan can strengthen the enzyme to penetrate the outer membrane into target cells, thereby enhancing its effect on gram-negative bacteria. In addition, lysozyme combined with other antimicrobial enzymes (glucose oxidase, lactoperoxidase) or with traditional preservative measures (such as sorbate, ethanol, temperature and low pH) can improve food microbial safety.

2) Fungal lysozyme
Fungal lysozyme mainly includes chitinase and -beta glucanase.

Chitinase
Although some exochitinases (EC3.2.1.30) also show antifungal properties, the antifungal chitinases are mainly endochitinases (EC3.2.1. 14). Many chitinases from plants and microorganisms have been studied, and the effects of some chitinases on inhibiting fungal growth/lysing fungal cells have been studied. Scientists have discovered the antifungal effect of chitinase in plants. This type of chitinase can fight fungal pathogens that invade plants.

β-glucanase
β-glucanases (EC 3.2.1.39) has antifungal effect mainly because it can hydrolyze β(1→3) glycosidic bonds. Studies have shown that β(1→3) glucanase has a significant synergistic effect on chitin degradation of fungal cell walls. If the purified chitinase and β-glucanase are combined, the effect of anti-Botrytis cinera is increased by 10 times. Endo-glucanase, exo-glucanase, and different endo-glucanases also have synergistic antifungal effects.

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