Polyphenol oxidase

When it comes to polyphenol oxidase, many people may not know what it is. In fact, polyphenol oxidase is a protein, which is generally a catalyst in tea processing. Generally, polyphenol oxidase is also found in fruits and vegetables. The lower the polyphenol oxidase, the better for the plant. Polyphenol oxidase is often used as a catalyst and is suitable for fruits and vegetables. The following is a detailed introduction to polyphenol oxidase.

Polyphenol oxidase and its function in plants

In plant tissues (such as apples, lychees, spinach, potatoes, beans, tea leaves, mulberry leaves, tobacco, etc.), PPO is combined with the endoplasmic reticulum membrane and is inactive in its natural state. However, after tissue homogenization or damage, PPO is activated and becomes active. In fruit and vegetable cell tissues, the location of PPO varies depending on the type, variety and maturity of the raw materials. Most of the PPO activity in green leaves is present in chloroplasts [7]; almost all subcellular parts of potato tubers contain PPO, and the content is approximately the same as that of the protein part [8]; PPO in tea leaves is divided into free state and bound state. The former is mainly present in the cell sap and is a soluble PPO, while the latter is mainly present in organelles such as chloroplasts and mitochondria, and is bound to the membrane system or other specific parts of these organelles and is insoluble [9]. Thanaraj S.N. (1990) studied the effects of PPO activity and polyphenol content in tea shoots on the quality of black tea and found that strong PPO activity and high polyphenol content are beneficial to the quality of black tea, while the opposite is beneficial to the production of green tea [10]; in fresh apples, polyphenol oxidase is almost entirely present in chloroplasts and mitochondria. The substrate specificity of PPO prepared from these two parts is slightly different[11]. Liu Gangang believes that PPO exists not only in chloroplasts and mitochondria within cells, but may also exist in cell walls and affect fermentation. PPO will be affected as long as the cells are slightly damaged. Polyphenol oxidase is a plastid enzyme. Some researchers believe that polyphenol oxidase may only exist in plastids[12]. Tissues that lack plastids, such as sieve tubes and sieve cells, do not have polyphenol oxidase. However, tissues that have plastids may also lack polyphenol oxidase, such as leaves of C4 plants. Polyphenol oxidase is not necessarily present in all plant tissues containing plastids, but polyphenol oxidase is definitely present in plant tissues containing plastids.

With the development of molecular biology, the genes of polyphenol oxidase from tomatoes, apples, etc. have been cloned. Zhao Dong et al. from Zhejiang University [12] cloned and compared the sequences of tea plant polyphenol oxidase. From the cloned polyphenol oxidase genes, we can see that they all belong to a gene family, with as many as 6-7 genes. The expression of these genes has temporal and spatial differences and tissue specificity (PPO is expressed in young tissues but not in mature tissues), indicating that the genes of polyphenol oxidase play different roles in plants. The browning of higher plant tissues is mainly the result of the action of PPO, which catalyzes the oxidation of polyphenols to quinones. Quinones polymerize and react with amino acids in intracellular proteins, resulting in melanin precipitation.

Microbial laccase

Laccase is the enzyme with the broadest substrates among the three major classes of polyphenol oxidases. Laccase was first discovered by Yoshida in 1883 from lacquer tree sap, and later people discovered laccase from a large number of fungi. Laccases come from many sources with different structures, and the catalytic properties of laccases from different sources vary greatly. Even from the same source, such as the same white rot fungus species, laccase components with different properties can be secreted, including oxidation ability, optimal pH, substrate specificity, etc., so the catalytic oxidation effects are also different. The copper ion in the laccase molecule is the active center of the laccase catalytic reaction and plays a decisive role in the catalytic oxidation process.

Among fungi, laccases are mostly distributed in Basidiomycetes, Polyporus, A-somycetes, Neurospora, Po-dospora and Aspergillus. White rot fungi among basidiomycetes are currently the main source of laccase. Givaudan et al. also isolated bacterial laccase from Azospirillum lipoferum on rice roots.

Huang Ganming et al. used Trametes gallica as the starting bacteria, treated its basidiospores with ultraviolet mutagenesis, conducted primary screening by the PDA-RBBR plate color change method, and rescreened by determining the laccase activity of the culture fluid using the ABTS method, and obtained a high-producing laccase mutant strain SAH-12.

Huang Jun et al. (2006) isolated a bacterial strain with laccase activity from the soil at the roots of forest trees and identified the bacterium as belonging to the genus Klebsiella, naming it Klebsiella sp-601. This is the first report of laccase activity in Klebsiella bacteria.