Myrosinase (, thioglucoside glucohydrolase, sinigrinase, and sinigrase) is a family of involved in plant defense against herbivores, specifically the mustard oil bomb. The three-dimensional structure has been elucidated and is available in the PDB (see links in the infobox).
A member of the glycoside hydrolase family, myrosinase possesses several similarities with the more ubiquitous O-glycosidases.
Myrosinase activity
Myrosinase is regarded as a defense-related enzyme and is capable of hydrolyzing
into various compounds, some of which are toxic.
[ A wound- and methyl jasmonate-inducible transcript coding for a myrosinase-associated protein with similarities to an early nodulin]
Mechanism
Myrosinase
catalysis the chemical reaction
- a thioglucoside + H2O a sugar + a thiol
Thus, the two substrates of this enzyme are thioglucoside and water, whereas its two products are sugar and thiol.
In the presence of water, myrosinase cleaves off the glucose group from a glucosinolate. The remaining molecule then quickly converts to a thiocyanate, an isothiocyanate, or a nitrile; these are the active substances that serve as defense for the plant. The hydrolysis of glucosinolates by myrosinase can yield a variety of products, depending on various physiological conditions such as pH and the presence of certain cofactors. All known reactions have been observed to share the same initial steps. (See Figure 2.) First, the β-thioglucoside linkage is cleaved by myrosinase, releasing D-glucose. The resulting aglycone undergoes a spontaneous Lossen-like rearrangement, releasing a sulfate. The last step in the mechanism is subject to the greatest variety depending on the physiological conditions under which the reaction takes place. At neutral pH, the primary product is the isothiocyanate. Under acidic conditions (pH < 3), and in the presence of ferrous ions or epithiospecifer proteins, the formation of nitriles is favored instead.
Cofactors and inhibitors
Ascorbate is a known cofactor of myrosinase, serving as a base
catalyst in
glucosinolate hydrolysis.
For example, myrosinase isolated from
daikon (
Raphanus sativus) demonstrated an increase in V
max from 2.06 μmol/min per mg of protein to 280 μmol/min per mg of protein on the substrate,
Sinigrin (sinigrin), when in the presence of 500 μM ascorbate.
, a byproduct of
glucosinolate hydrolysis, has been identified as a competitive inhibitor of myrosinase.
In addition, 2-F-2-deoxybenzylglucosinolate, which was synthesized specifically to study the mechanism of myrosinase, inhibits the enzyme by trapping one of the
glutamic acid residues in the
active site, Glu409.
Structure
Myrosinase exists as a
Protein dimer with subunits of 60-70
kDa each.
X-ray crystallography of myrosinase isolated from
White mustard revealed that 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
, 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.
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
, and as cytosolic enzymes that tend to bind to membranes.
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
, followed by
and
.
Evolution
Plants known to have evolved a myrosinase-glucosinolate defense system include:
white mustard (
Sinapis alba),
(
Lepidium sativum),
(
Wasabia japonica),
and
daikon (
Raphanus sativus),
as well as several members of the family
Brassicaceae, including
White 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.
Historical relevance and modern applications
Agriculture
Historically, crops like
rapeseed that contained the glucosinolate-myrosinase system were deliberately
bred to minimize glucosinolate content, since rapeseed in animal feed was proving toxic to
livestock.
The glucosinolate-myrosinase system has been investigated as a possible biofumigant to protect crops against pests. The potent glucosinolate hydrolysis products (GHPs) could be sprayed onto crops to deter herbivory. Another option would be to use techniques in genetic engineering to introduce the glucosinolate-myrosinase system in crops as a means of fortifying their resistance against pests.
Health effects
Isothiocyanates, the primary product of glucosinolate hydrolysis, have been known to prevent
iodine uptake in the
thyroid, causing
goiters.
Isothiocyanates in high concentrations may cause
hepatotoxicity.
There is insufficient scientific evidence that consuming cruciferous vegetables with increased intake of isothiocyanates affects the risk of human diseases.
