Lysine Oxidase

Lysine-Oxidase (EC 1.4.3.14) belongs to the class of oxidoreductases, acts on the CH-NH₂ group of donors. The systematic name of this enzyme class is L-lysine:oxygen 2-oxidoreductase. Other names in common use include L-lysine α-oxidase, L-lysyl-α-oxidase. The first lysine-oxidase was isolated from Trichoderma viride Y 244-2. Later, another lysine-oxidase was indentified in Trichoderma harzianum Rifai.

Physicochemical properties

The molecular weight of the enzyme from Trichoderma viride Y 244-2 is about 120 kDa, the protein molecule consisting of two identical subunits of 60 kDa. The isoelectric points of this enzyme is 4.25. The molecular weight of the enzyme from Trichoderma harzianum Rifai is 100 kDa, the isoelectric points is 5.6. The second lysine-oxidase likely to be a product of proteolysis of the native lysine-oxidase. Lysine-oxidases contain one molecule of FAD per subunit as the coenzyme. Their optical absorption spectra exhibit maxima at 360 and 460 nm, as well as a peak at 280 nm. The molecule contains from 3 to 98 carbohydrate residues, only 3 of 43 lysine residues revealed on the surface of the molecule by amino acid analysis. Implying that the spatial structure of lysine-oxidase tends to minimize surface amino groups. The amino groups of lysine-oxidase are inaccessible. The optimal PH of lysine-oxidase from Trichoderma harzianum Rifai range from 4.5 to 10.5.

Structure

The two lysine-oxidases have different structures.The lysine-oxidase from Trichoderma viride Y 244-2 contain 13% of α-helix and 18% of β-form. The lysine-oxidase from Trichoderma harzianum Rifai contains 12% of α-helix and 33% of β-form. The contant of amino acid of the two lysine-oxidases are also different. The lysine-oxidase from Trichoderma harzianum Rifai contains about 2.6-fold glycine residues more than the lysine-oxidase from Trichoderma viride Y 244-2. But the tryptophan and arginine residues are about 4 and 2-fold less than the lysine-oxidase from Trichoderma viride Y 2442.

Stability

The thermal stability assay of the lysine-oxidase from Trichoderma harzianum Rifai showed that this enzyme is low thermal stability compare with the lysine-oxidase from Trichoderma viride Y 244-2. After incubation at 75 and 80 °C, the inactivation proceeds very quickly. Thermal stability of lysine-oxidase significantly increases in diluted solutions. Kinetic curves showed that the thermal inactivation significantly decelerate when dissociation of the enzyme into subunits. The mechanism of the inactivation of lysine-oxidase indicated that the enzyme should be stored as the diluted solution. Lyophilized storage at 4°C or as a frozen solution at -18°C, the enzyme retained its activity constant for a year.

Catalytic Mechanism

Lysine-oxidase only acts on L-lysine and participates in lysine degradation with a low K_m. During the reaction, oxygen is consumed to produce ammonia, hydrogen peroxide and α-keto-ε-aminocaproic acid, α-keto-ε-aminocaproic acid can be spontaneously converted into a cyclic derivative. The reduction of lysine and the production of hydrogen peroxide are the basis of the physiological effects of lysine-oxidase.

Figure 1. Catalytic mechanism of Lysine-Oxidase. (Lukasheva E.V. 2002)

Physiological effects

In vitro, lysine-oxidase inhibits the synthesis of DNA, RNA and proteins in leukemia, human ovarian cancer, Burkitt's lymphoma. In addition, in vitro experiments have also demonstrated that lysine-oxidase has anti-invasive, antibacterial, antiviral effects. It inhibited the replication of type I Herpes simplex virus and also inhibited the expression of viral antigens. The injection of lysine-oxidase into mice observed that lysine-oxidase exhibits high activity in vivo, its concentration has a time-dependent effect, and the half-life time is 2 h. Besides, lysine-oxidase exhibits immunomodulating action, antiviral therapeutic effect, antitumor effect and antimetastatic effect in vivo. Lysine-oxidase is highly cytotoxic to many tumor cell lines and exhibits anti-leukemia and anti-metastatic effects at very small doses. Experiments on mice bearing Lewis lung cancer demonstrated that the anti-metastatic effect of lysine-oxidase increased with increasing dose and had a dose-dependent effect. In addition, lysine-oxidase can affect the cell membrane of macrophages.

Applications

As an essential amino acid, lysine is an important component of nutritional additions. Its wide range of industrial applications requires convenient and specific determination of lysine concentrations in culture and final products. Lysine-oxidase acts only on lysine and has no effect on other amino acids. Therefore, this enzyme is used to determine the content of lysine. The lysine-oxidase and horseradish peroxidase are immobilized on the porous support membrane for analysis of the concentration of lysine. The membrane can be used repeatedly, and the sensitivity varies from 20 to 120 µM, and the reaction time is 2-20 min.

Reference