Questions: Enzyme Structure and Function

Question 1

An enzyme is added to a reaction that is thermodynamically unfavorable (ΔG > 0). What happens?

• A. The reaction now proceeds spontaneously because the enzyme lowers activation energy
• B. The reaction still does not proceed spontaneously — the enzyme has no effect on ΔG
• C. The enzyme increases ΔG, making the reaction even less favorable
• D. The enzyme converts the reaction from endergonic to exergonic

Answer: (B)

Enzymes lower the activation energy (the kinetic barrier) but cannot change the thermodynamic favorability (ΔG) of a reaction. If ΔG > 0, the reaction is endergonic and will not proceed spontaneously regardless of whether an enzyme is present. Enzymes only speed up reactions that are already thermodynamically possible.

Question 2

A protein enzyme that has been denatured by high temperature will typically regain its original activity when the temperature is returned to normal.

• T. True
• F. False

Answer: False

Denaturation involves the disruption of the non-covalent interactions (hydrogen bonds, hydrophobic interactions, ionic bonds) that maintain the protein's 3D structure. While some small proteins can refold (renature), most denatured enzymes do not spontaneously return to their functional shape. The loss of structure is often irreversible under biological conditions.

Question 3

Why is the induced fit model considered a better description of enzyme-substrate binding than the lock-and-key model?

Model answer: The induced fit model accounts for conformational changes in the active site upon substrate binding. The enzyme flexes to better complement the substrate's shape, which helps stabilize the transition state and explains catalytic activity. The lock-and-key model treats the active site as rigid, which cannot explain why the enzyme actively lowers activation energy or how it can accommodate structurally related substrates.

The induced fit model also better explains competitive inhibition (an inhibitor fits the active site but doesn't induce the productive conformation) and the specificity of enzyme-substrate interactions. Rigid complementarity would predict binding without necessarily explaining catalysis.