(DEC 2017)
14. From the following statements
A. Enzymes enhance reaction rates by a factor of 2 to 10.
B. The activation energy for a reaction is lowered by enzymes.
C. The interactions between enzymes and substrates are hydrogen bonding, hydrophobic and ionic.
D. Substrate concentration does not affect the rate of enzyme catalyzed reactions.
Pick the combination with all INCORRECT statements.
(1) A, B                                                          (2) B, C
(3) A, C                                                          (4) A, D

The correct answer is (4) A, D.

Introduction

Enzymes are biological catalysts that regulate biochemical reactions by increasing reaction rates through specific mechanisms. However, certain misconceptions persist about how enzymes function. This article discusses the correctness of common statements about enzyme catalysis, focusing on rate enhancement magnitudes, activation energy changes, enzyme-substrate interactions, and substrate concentration effects. The goal is to identify which statements are incorrect and explain why, providing clear insights for academic and exam purposes.

Analysis of Each Statement

Statement A

“Enzymes enhance reaction rates by a factor of 2 to 10.”

• Incorrect: Enzymes can enhance reaction rates by factors ranging from thousands to even millions (up to 107107). Limiting enhancement to 2-10 underestimates enzymatic power.

Statement B

“The activation energy for a reaction is lowered by enzymes.”

• Correct: A core function of enzymes is lowering the activation energy (EaEa) to increase the fraction of reactive molecules.

Statement C

“The interactions between enzymes and substrates are hydrogen bonding, hydrophobic and ionic.”

• Correct: Enzyme-substrate binding involves multiple non-covalent interactions including hydrogen bonds, hydrophobic interactions, and ionic bonds that stabilize the complex.

Statement D

“Substrate concentration does not affect the rate of enzyme catalyzed reactions.”

• Incorrect: Substrate concentration strongly influences reaction rates up to enzyme saturation, beyond which rate plateaus (zero-order kinetics).

Why A and D Are Incorrect

• Enzymes theoretically and experimentally show rate enhancements much greater than 2 to 10 times, often accelerating reactions by 103103 to 107107 fold.

• Substrate concentration changes influence enzyme reaction rates, especially at low to moderate concentrations, where the reaction is first-order with respect to substrate.

• Only at very high substrate concentrations (enzyme saturation) does the rate become independent of substrate, showing zero-order kinetics.

Enzyme Catalysis Refined Understanding

• Enzyme catalysis achieves rate enhancements via lowering activation energy, stabilizing transition states, and providing favorable microenvironments.

• These effects enable biological reactions to proceed at life-sustaining speeds.

• The substrate-binding mechanism involves complex molecular interactions crucial for specificity and efficiency.

Summary Table

Conclusion

Among the provided statements, only A and D are incorrect. Enzymes enhance reaction rates far more than 2-10 fold, and substrate concentration significantly affects enzymatic reaction rates. Understanding these points is essential for an accurate grasp of enzyme kinetics and function, vital for life science students and exam preparation.