residues in the active site, Glu 409. Structure Myrosinase exists as a dimer with subunits of 60-70 kDa each. X-ray crystallography of myrosinase isolated from Sinapis alba revealed the two subunits are linked by a zinc atom. The prominence of salt bridges, disulfide bridges, hydrogen bonding, and glycosylation are thought to contribute to the enzyme’s stability, especially when the plant is under attack and experiences severe tissue damage. A feature of many β-glucosidases are catalytic glutamate residues at their active sites, but two of these have been replaced by a single glutamine residue in myrosinase. Ascorbate has been shown to substitute for the activity of the glutamate residues. (See Figure 3 for mechanism.) Biological function Myrosinase and its natural substrate, glucosinolate, are known to be part of the plant’s defense response. When the plant is attacked by pathogens, insects, or other herbivores, the plant uses myrosinase to convert glucosinolates, which are otherwise-benign, into toxic products like isothiocyanates, thiocyanates, and nitriles. Compartmentalization in plants The glucosinolate-myrosinase defensive system is packaged in the plant in a unique manner. Plants store myrosinase glucosinolates by compartmentalization, such that the latter is released and activated only when the plant is under attack. Myrosinase is stored largely as myrosin grains in the vacuoles of particular idioblasts called myrosin cells, but have also been reported in protein bodies or vacuoles, and as cytosolic enzymes that tend to bind to membranes. Glucosinolates are stored in adjacent but separate "S-cells." When the plant experiences tissue damage, the myrosinase comes into contact with glucosinolates, quickly activating them into their potent, antibacterial form. The most potent of such products are isothiocyanates, followed by thiocyanates and nitriles. Evolution Plants known to have evolved a myrosinase-glucosinolate defense system include: white mustard (Sinapis alba), garden cress (Lepidium sativum), wasabi (Wasabia japonica), and daikon (Raphanus sativus), as well as several members of the family Brassicaceae, including yellow mustard (Brassica juncea), rape seed (Brassica napus), and common dietary brassicas like broccoli, cauliflower, cabbage, bok choy, and kale. The bitter aftertaste of many of these vegetables can often be attributed to the hydrolysis of glucosinolates upon tissue damage during food preparation or when consuming these vegetables raw. Papaya seeds use this method of defense, but not the fruit pulp itself. Myrosinase has also been isolated from the cabbage aphid. This suggests coevolution of the cabbage aphid with its main food source. The aphid employs a similar defense