Question:
Which of the following enzymes is involved in the plant defense against herbivores and requires ascorbic acid as its cofactor?
Which of the following enzymes is involved in the plant defense against herbivores and requires ascorbic acid as its cofactor?
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Myrosinase is a $\beta$-thioglucosidase involved in the "mustard oil bomb" defense system of cruciferous plants.
It is uniquely regulated and activated by ascorbic acid (Vitamin C) acting as a cofactor.
It is uniquely regulated and activated by ascorbic acid (Vitamin C) acting as a cofactor.
Updated On: Aug 7, 2026
- Lipoxygenase
- Myrosinase
- Phenylalanine Ammonia Lyase
- Chorismate Synthase
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
Plants produce specialized secondary metabolites and defensive enzymes to protect themselves against herbivory and insect attacks.
Some of these enzymes require specific organic cofactors to undergo the conformational changes needed for catalytic activity.
Step 2: Detailed Explanation:
Myrosinase ($\beta$-thioglucosidase) is a key enzyme in the glucosinolate-myrosinase defense system, commonly referred to as the "mustard oil bomb" in plants of the Brassicaceae family.
In intact plant tissues, glucosinolates are stored in the vacuole, while myrosinase is sequestered in specialized cells called myrosin cells.
When an herbivore chews the tissue, the cells are ruptured, bringing myrosinase into contact with its glucosinolate substrates.
Myrosinase hydrolyzes the glucose moiety of glucosinolates, producing unstable aglycones that spontaneously rearrange into toxic, pungent compounds like isothiocyanates, nitriles, and thiocyanates that deter the herbivore.
Myrosinase is unique among glycosidases because it requires ascorbic acid (Vitamin C) as an essential cofactor.
Ascorbic acid binds to an allosteric site on the enzyme, inducing a conformational change that aligns the catalytic residues and accelerates the hydrolysis reaction.
The other enzymes listed (lipoxygenase, phenylalanine ammonia lyase, and chorismate synthase) are involved in plant metabolism but do not use ascorbic acid as a cofactor.
Step 3: Final Answer:
The enzyme involved in plant defense that requires ascorbic acid as a cofactor is Myrosinase, which corresponds to Option (B).
Plants produce specialized secondary metabolites and defensive enzymes to protect themselves against herbivory and insect attacks.
Some of these enzymes require specific organic cofactors to undergo the conformational changes needed for catalytic activity.
Step 2: Detailed Explanation:
Myrosinase ($\beta$-thioglucosidase) is a key enzyme in the glucosinolate-myrosinase defense system, commonly referred to as the "mustard oil bomb" in plants of the Brassicaceae family.
In intact plant tissues, glucosinolates are stored in the vacuole, while myrosinase is sequestered in specialized cells called myrosin cells.
When an herbivore chews the tissue, the cells are ruptured, bringing myrosinase into contact with its glucosinolate substrates.
Myrosinase hydrolyzes the glucose moiety of glucosinolates, producing unstable aglycones that spontaneously rearrange into toxic, pungent compounds like isothiocyanates, nitriles, and thiocyanates that deter the herbivore.
Myrosinase is unique among glycosidases because it requires ascorbic acid (Vitamin C) as an essential cofactor.
Ascorbic acid binds to an allosteric site on the enzyme, inducing a conformational change that aligns the catalytic residues and accelerates the hydrolysis reaction.
The other enzymes listed (lipoxygenase, phenylalanine ammonia lyase, and chorismate synthase) are involved in plant metabolism but do not use ascorbic acid as a cofactor.
Step 3: Final Answer:
The enzyme involved in plant defense that requires ascorbic acid as a cofactor is Myrosinase, which corresponds to Option (B).
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