Q.33 Which of the following statements is not correct?
1. The presence of an enzyme has no effect on ΔGº
2. At equilibrium, the rate constants of forward and backward reaction are equal.
3. Active site histidyl residues frequently aid catalysis by acting as proton donors and acceptors.
4. Enzymes lower the activation energy for a reaction.
Enzymes affect reaction kinetics but not thermodynamics, so option 2 is incorrect as equilibrium equates forward and reverse rate constants only in specific cases.
Enzymes catalyze reactions by stabilizing transition states without altering free energy change (ΔG°) or equilibrium position. They lower activation energy (E_a) via active site interactions like proton shuttling by histidine residues. At equilibrium, forward (k_f) and reverse (k_r) rates equal, but K_eq = k_f / k_r, so constants differ unless K_eq=1.
Correct Answer
Option 2: At equilibrium, the rate constants of forward and backward reaction are equal.
This is false; equilibrium means equal rates (k_f [reactants] = k_r
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Option Explanations
| Option | Statement | Explanation |
|---|---|---|
| 1 | The presence of an enzyme has no effect on ΔGº | Correct. ΔG° is thermodynamic (reactant-product energy difference); enzymes affect kinetics only, not equilibrium constant. |
| 2 | At equilibrium, the rate constants of forward and backward reaction are equal | Incorrect. Rates equalize at equilibrium via concentration ratios, but k_f ≠ k_r generally (defines K_eq). |
| 3 | Active site histidyl residues frequently aid catalysis by acting as proton donors and acceptors | Correct. Histidine’s imidazole pKa (~6-7) enables proton shuttling in enzymes like serine proteases (e.g., chymotrypsin). |
| 4 | Enzymes lower the activation energy for a reaction | Correct. Enzymes stabilize transition state, reducing E_a and accelerating both directions equally. |
Exam Tip
Distinguish thermodynamics (ΔG, K_eq unchanged) from kinetics (E_a lowered, rates sped up); option 2 confuses rate equality with constant equality.
