Myrosinase

Mustardase and glucosinolate are stored in different cells. When attacked by diseases and insects, the substrate and the enzyme meet, and the glucosinolate is reduced to a toxic compound for defense. The research progress of plant mustard enzyme defense system is reviewed, including the structure of gene family, gene expression regulation, cell positioning of mustard enzyme, mustard enzyme of other organisms outside the plant, origin and evolution of glucosinolate/ mustard enzyme system and its possible Functions etc. When the mustard plant tissue is damaged, the glucosinolate directly contacts with the myrosinase, and hydrolyzes the glucosinolate to isothiocyanate, nitrile and other toxic substances to protect itself from further damage by herbivores.

Figure 1. Protein structure of Myrosinase.

Structure

Myrosinase exists in the form of a dimer, each subunit being 60-70 kDa. X-ray analysis X-ray crystallography of the myrosinase isolated from mustard leaf mustard found that these two subunits are connected by a zinc atom, and the prominence of salt bridges, disulfide bridges, hydrogen bonding, and glycosylation are considered helpful Due to the stability of the enzyme, especially in the case of insufficient plant growth. Attack and suffer severe tissue damage. A feature of many ß-glucosidases is that they have catalytic glutamate residues in their active site, but two of them have been replaced by single glutamine residues in myrosinase. Ascorbic acid has been shown to replace the activity of glutamate residues.

Mechanism

Myrosinase catalyzes the chemical reaction

a thioglucoside + H2O a sugar + a thiol

Therefore, the two substrates of the enzyme are glucosinolates and H2O, and the two products are sugars and thiols. In the presence of water, myrosinase cleaves the glucose group from glucosinolates. Then, the remaining molecules are quickly converted to thiocyanate, isothiocyanate or nitrile. These are active substances for plant defense. Myrosinase hydrolyzes glucosinolates to produce a variety of products, depending on various physiological conditions, such as pH and the presence of certain cofactors. It has been observed that all known reactions share the same initial steps. First, the myrosinase cleaves the β-glucosinolate bond, releasing D-glucose. The aglycone produced undergoes spontaneous Rosen-like rearrangement, releasing sulfate. The final step of this mechanism is most affected by the physiological conditions under which the reaction occurs. At neutral pH, the main product is isothiocyanate. Under acidic conditions (pH <3), and in the presence of ferrous ions or Epithiospecifer protein, the formation of nitrile is more advantageous.

Functions

It is well known that myrosinase and its natural substrate glucosinolate are part of the plant defense response. When a plant is attacked by pathogens, insects or other herbivores, the plant uses myrosinase to convert the originally benign glucosinolates into toxic products such as isothiocyanate, thiocyanate and nitrile.

Plant compartmentalization

The glucosinolate-Myrosinase defense system is packaged in plants in a unique way. Plants store glucosinolate glucosinolates through compartmentalization, so that the latter are released and activated only when the plant is attacked. Myrosinase is mainly stored in the form of myrosin granules in the vacuoles of specific fibroblasts called myrosin cells, but there are also reports of protein bodies or vacuoles and cytoplasmic enzymes that tend to bind to the membrane. Mustard oil Glycosides are stored in adjacent but separate "S cells". When the plant is damaged by tissues, myrosinase contacts glucosinolate, quickly activating it into an effective antibacterial form. The most effective of these products is isothiocyanate, followed by thiocyanate and nitrile.

Evolution

Plants known to have evolved the myrosinase-glucosinolate defense system include: white mustard (Sinapis alba), cress (Lepidium sativum), mustard (Wasabia japonica), daikon (Raphanus sativus), and several of the cruciferous family Members include yellow mustard (Brassica juncea), rapeseed (Brassica napus), and common dietary crucifers, such as broccoli, cauliflower, cabbage, cabbage, and kale. The bitter aftertaste of many of these vegetables can usually be attributed to the hydrolysis of glucosinolates during tissue preparation or when these vegetable raw materials are destroyed. Papaya seeds use this defense method, but do not use the pulp itself.

Reference

1. Burmeister, W. P.; et al. High Resolution X-ray Crystallography Shows That Ascorbate is a Cofactor for Myrosinase and Substitutes for the Function of the Catalytic Base. Journal of Biological Chemistry. 2000, 275 (50): 39385–39393.