Q.104. The graph below shows the activity of enzyme pepsin in the presence of inhibitors aliphatic
alcohols (P) or N-acetyl-1-phenylalanine (Q). Which ONE of the following represents the nature of
inhibition by P and Q , respectively?
(A) Non-competitive and competitive
(B) Competitive and non-competitive
(C) Non-competitive and uncompetitive
(D) Competitive and uncompetitive

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In enzyme kinetics, understanding inhibition types is crucial for biochemistry students and researchers. This article breaks down a classic Lineweaver-Burk plot question on pepsin enzyme activity in the presence of inhibitors P (aliphatic alcohols) and Q (N-acetyl-phenylalanine). We’ll reveal the correct answer, explain why, and analyze all options to clarify nature of inhibition by P and Q.

What the Lineweaver-Burk Plot Reveals About Pepsin Inhibition

The double-reciprocal plot (1/V vs 1/[S]) shows three lines:

• No inhibitor: Standard line with X-intercept -1/Km and Y-intercept 1/Vmax.

• P (aliphatic alcohols): Line intersects Y-axis at the same point as no inhibitor but has a different slope and X-intercept.

• Q (N-acetyl-phenylalanine): Line has the same X-intercept as no inhibitor but a higher Y-intercept.

This pattern directly indicates inhibition types:

• P affects Km (changes X-intercept) but not Vmax (same Y-intercept) → Competitive inhibition.

• Q reduces Vmax (higher Y-intercept) but Km unchanged (same X-intercept) → Non-competitive inhibition.

Correct Answer: A) Non-competitive and competitive

Aliphatic alcohols (P) compete with substrate for the active site, increasing apparent Km. N-acetyl-phenylalanine (Q), a pepsin substrate analog, binds elsewhere, lowering Vmax without altering Km.

Detailed Explanation of All Options

Let’s evaluate each MCQ option based on standard enzyme kinetics principles.

Option A: Non-competitive and Competitive

• Matches the plot: Q (non-competitive, same Km, reduced Vmax); P (competitive, increased Km, same Vmax).

• Why correct? Lineweaver-Burk signatures align perfectly—Q shifts Y-intercept up; P shifts X-intercept rightward.

Option B: Competitive and Non-Competitive

• Reverses P and Q: Would mean P (non-competitive) has same Km (wrong, plot shows changed Km for P); Q (competitive) has changed Km (wrong).

• Incorrect: Plot data contradicts this swap.

Option C: Competitive and Uncompetitive

• Uncompetitive inhibition shows parallel lines (same slope, different intercepts).

• Incorrect: Q’s line isn’t parallel—slope changes, ruling out uncompetitive.

Option D: Non-Competitive and Uncompetitive

• No competitive pattern (P clearly changes Km).

• Incorrect: Fails to account for P’s X-intercept shift; uncompetitive doesn’t fit Q.

Why This Matters for Pepsin and Enzyme Studies

Pepsin, a stomach protease, digests proteins optimally at low pH. Inhibitors like Q mimic substrates to study specificity, while P tests active site binding. Mastering nature of inhibition by P and Q helps in drug design, like HIV protease inhibitors (competitive mimics).