Which one of the following statements is CORRECT for enzyme catalyzed reactions? (4G
is Gibbs free energy change, Keq is equilibrium constant)
Options:
(A) Enzymes affect 4G, but not Req
(B) Enzymes affect Kea, but not 4G
(C) Enzymes affect both 4G and Rea
(D) Enzymes do not affect 4G or Req
6 ---
GATE 2019
Biotechnology
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.24)**
**1. Governing Concept**:
- **Topic**: Enzymology & Chemical Thermodynamics
- Catalytic role of enzymes in chemical reactions:
- Enzymes act purely as catalysts by **lowering the activation energy barrier ($\Delta G^\ddagger$)** of both forward and reverse reactions, thereby accelerating reaction velocity ($k_{\text{cat}}$).
- Enzymes **CANNOT alter the standard free energy of reaction ($\Delta G^\circ$)** between initial substrates and final products, because $\Delta G^\circ$ is a state function dependent only on the thermodynamic nature of substrates and products.
- Since equilibrium constant is dictated strictly by free energy change:
$\Delta G^\circ = -RT \ln K_{\text{eq}} \implies K_{\text{eq}} = e^{-\Delta G^\circ / RT}$
enzymes **CANNOT change the equilibrium constant ($K_{\text{eq}}$)** or alter the final equilibrium position; they merely accelerate the rate of reaching equilibrium.
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Enzymes do not affect $\Delta G^\circ$.
- **Option (B) is INCORRECT**: Enzymes do not affect $K_{\text{eq}}$.
- **Option (C) is INCORRECT**: Enzymes affect neither $\Delta G^\circ$ nor $K_{\text{eq}}$.
- **Option (D) [Enzymes affect neither ΔG nor Keq] is CORRECT**: Fundamental thermodynamic law of biological catalysis.
**3. Final Answer**:
- **Correct Choice**: **D**
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