Q.42 Given below are two statements. One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Subtrates are bound to enzymes by multiple weak attraction. The noncovalent interaction in ES
complex are much weaker than covalent bond, which have energies between – 210 and – 460 kj mol-1.
Reason R: The mechanism of catalysis is dynamic, involving structural changes with multiple intermediates
of both reactants and enzymes.
In the light of above, choose the correct answer from the options given below:
1. Both A and R are correct and R is the correct explanation of A
2. Both A and R correct but R is not the correct explanation of A
3. A is correct but R is incorrect
4. A is correct but R is incorrect
Enzymes bind substrates through weak noncovalent interactions, and both Assertion A and Reason R are true but unrelated.
Correct Answer
Option 2: Both A and R are correct but R is not the correct explanation of A
Assertion A Breakdown
Assertion A states that substrates bind enzymes via multiple weak attractions, and noncovalent interactions in the ES complex are much weaker than covalent bonds (-210 to -460 kJ/mol). This is correct, as noncovalent forces like hydrogen bonds, van der Waals, electrostatic, and hydrophobic interactions each contribute 2-30 kJ/mol, allowing reversible binding essential for catalysis turnover. Covalent bonds indeed have far higher energies (210-460 kJ/mol), making noncovalent ones “much weaker” for dynamic ES complex formation.
Reason R Breakdown
Reason R claims catalysis is dynamic, involving structural changes and multiple intermediates of reactants and enzymes. This is true, reflecting induced fit models where enzymes undergo conformational shifts (e.g., loop movements) and form intermediates like enzyme-substrate, transition state, and enzyme-product complexes during multistep reactions. Examples include glycolysis enzymes like hexokinase.
Why R Doesn’t Explain A
R describes the overall catalytic process (dynamic structural changes), not why substrates bind via weak noncovalent forces. Binding specificity arises from cumulative weak interactions for precise active site fit, independent of catalysis dynamics or intermediates. Thus, both statements hold, but R doesn’t justify A.
Options Explained
| Option | Description | Correctness |
|---|---|---|
| 1. Both A and R correct; R explains A | R must directly justify A’s binding mechanism | Incorrect – R unrelated to binding strength |
| 2. Both A and R correct; R doesn’t explain A | Independent true statements | Correct |
| 3. A correct; R incorrect | R falsely denies dynamic catalysis | Incorrect – R true |
| 4. A incorrect; R correct | A wrongly equates binding forces | Incorrect – A true |
This format aids NEET/JEE biology revision on enzyme substrate binding assertion reason topics like Michaelis-Menten kinetics.
