Lipoxygenase

Lipoxygenase (LOX, EC 1.13.11.12) is an oxidoreductase, a type of non-heme iron-containing protein that specifically catalyzes polyunsaturated fatty acids with a cis,cis-1,4-pentadiene, producing a hydrogen peroxide derivative having a conjugated double bond by intramolecular oxygenation. Lipoxygenase can cause undesirable flavors in processed fruits and vegetables, and can also cause deterioration of color, aroma and taste during storage and processing of oils and oily foods. However, lipoxygenase as a green food additive can improve the quality of wheat flour. The research on lipoxygenase is of great significance to the development of modern food industry, chemical industry, fermentation and other industries.

Sources

Lipoxygenase is widely found in animals and plants, and has high enzyme activity in legumes, especially in soybeans. In 1932, Andre et al. found that the bean flavor in soybeans was mainly caused by lipoxygenase. In 1947, Theorell et al. first extracted lipoxygenase crystals from soybeans. Legume seeds, such as soybeans, broad beans, etc., are rich in lipoxygenase. The protein content in soybean is about 40%, and in mature seeds, lipoxygenase accounts for 1-2% of total protein content. The lipoxygenase activity in soybean is higher than that of lipoxygenase extracted from other plants, and the efficiency of extracting lipoxygenase from soybean is higher. Therefore, most studies have been carried out using soybean lipoxygenase as a model. In addition, the presence of LOX was also found in algae, baker's yeast, fungi, and cyanogen bacteria.

Classification

LOX in soybean seeds has three isoenzyme types, namely LOX-1, LOX-2, and LOX-3. LOX-7 and LOX-8 isoenzymes have also been isolated from soybean leaves shortly after injury. It is also found that these two isozymes are present in young leaves, flowers and immature pods, and have high enzymatic activity. The crystal structure of soybean LOX-1 and LOX-3 has been identified. The researchers also isolated three LOX isozymes from mature rice embryos, named LOX-1, LOX-2 and LOX-3, depending on the elution order of the chromatograms, among which LOX-1 and LOX. -2 belongs to class II LOX, while LOX-3 belongs to class III LOX.

Structure

Lipoxygenase is an oxidoreductase, a type of non-heme iron-containing protein that specifically catalyzes polyunsaturated fatty acids with a cis,cis-1,4-pentadiene, producing a hydrogen peroxide derivative having a conjugated double bond by intramolecular oxygenation. The structure of the active site of the lipoxygenase is not fully understood, and the active site group may contain iron, aromatic amino acid residues and methionine residues. In the soybean LOX-1 model, the iron ion center contains five endogenous ligands and one exogenous ligand. The endogenous ligand includes three histidine residues, one isoleucine residue, and one asparagine residue. The exogenous ligand is a water molecule.

Properties

LOX has structural specificity for substrate. Polyunsaturated fatty acids and fatty acid esters containing cis,cis-1,4-pentadiene structure can be used as substrates for LOX. In plants, the natural substrates are mainly linoleic acid and linlenic acid. In animals, the natural substrate is mainly arachidonic acid. Different sources of LOX have different substrates, resulting in different oxygenation positions and different products.

The substrates for LOX-1 are unsaturated fatty acids, the products are 13-hydroperoxide. The substrates for LOX-2 are esterified substrates, and the products are 9- or 13-hydroperoxide. The optimal pH value and temperature of LOX from different sources are different. The optimum temperature of rice LOX is 30°C, and the optimum pH is 7.6. The optimum temperature of broad bean seeds is 30°C, the optimum pH is 6. The optimum temperature of cucumber LOX is 40°C, the optimum pH is 7. The optimum temperature of tomato LOX is 20°C, and the optimum pH is 6.

Catalytic Mechanism

The process by which LOX catalyzes the oxidation of linoleic acid is different from the automatic oxidation of linoleic acid. First, linoleic acid oxygenates LOX to form a complex. Subsequently, the complex forms a double radical activator on the surface of the enzyme, i.e. a hydrogen ion and an electron are transferred from the linoleic acid to the oxygen molecule. The double radical binds to the surface of the enzyme molecule to form linoleic acid hydrogen peroxide, which is separated from the enzyme and detached. However, the free radical theory holds that, first, the hydrogen atoms leave the substrate while the iron ions are reduced. Molecular oxygen reacts with substrate radicals to form peroxidic free radicals, which may be accompanied by O₂ conversion to O₂ ▪ free radicals. Finally, the peroxy radical is reduced by iron in the LOX to form a hydroperoxide. The iron in LOX is converted to Fe³⁺ and re-transformed into an active state. In recent years, the transfer reaction of proton and electrons in soybean LOX confirmed the hydrogen transfer theory during the LOX reaction. It is theorized that LOX catalyzes the transfer of hydrogen atoms from the substrate linoleic acid to iron ions. Protons and electrons are simultaneously transferred between the donor and the acceptor, resulting in an efficient hydrogen tunneling effect. Therefore, it can be confirmed that the LOX catalytic center has a close relationship with iron ions.

Figure 2. The LOX reaction and the regio-specificity of the reaction mechanism. (Andreou A. et al. 2009)

Application

Lipoxygenase is used in wheat flour to oxidize pigments in flour and whiten flour products. Lipoxygenase oxidizes unsaturated fatty acids to form peroxides, which oxidize sulfhydryl groups in protein molecules to form disulfide bonds, thereby increasing the gluten strength. LOX catalyzes the oxidation of polyunsaturated fatty acids to produce hydroperoxides, which are decomposed by homogenization or β-fission to form secondary oxygen products such as aldehydes and ketones, and further cleavage into unsaturated aldehydes, ketones and alcohols. It is a kind of compound that forms a fruit flavor similar to apple, melon, mango, etc., as well as volatile flavors such as fresh fish, oyster, seaweed, grassy grass, etc.

In addition to the many flavor substances, lipoxygenase can also be used in the industrial production of dyes, coatings, detergents, polyvinyl chloride plasticizers, etc., and as an intermediate for drug synthesis. Most of the plasticizers are prepared from petroleum. If the fatty acid or fatty acid ester is selectively catalyzed by lipoxygenase to form a specific derivative of a hydrogen peroxide fatty acid and its ester, and then epoxidized to form an epoxidized oil, the amount of petroleum can be reduced. The hydrogen peroxide oil is reduced to obtain hydroxy greases, which is oxidized to form unsaturated acids and aldehydes, and further reacted to obtain dibasic acids, which have important applications in food, chemical, and biotechnology industries.

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