Q.40 Lineweaver-Burk plot of a viral enzyme in presence of two drugs is shown here.
Which of the following statements regarding the actions of drugs is true?
(Graph showing 1/V on Y-axis and 1/[S] on X-axis with lines for
+ Drug B, + Drug A, and No Drug)
A. Drug A acts as competitive inhibitor.
B. Drug B acts as competitive inhibitor.
C. Drug A acts as non-competitive inhibitor.
D. Drug B acts as non-competitive inhibitor.
- A and B
- A and D
- B and C
- C and D
Understanding the Lineweaver–Burk Plot
The Lineweaver–Burk plot is a double reciprocal plot of enzyme kinetics:
1 / Vi vs 1 / [S]
- Y-intercept = 1 / Vmax
- X-intercept = −1 / Km
- Slope = Km / Vmax
Graph Interpretation
From the graph:
- All lines intersect at the same X-intercept (−1 / Km)
- Y-intercept (1 / Vmax) increases in presence of both drugs
- Slope increases
This means:
- Km remains constant
- Vmax decreases
This pattern represents non-competitive inhibition.
Correct Answer
Option 4: C and D
Both Drug A and Drug B act as non-competitive inhibitors.
Explanation of Each Option
Option A: Drug A acts as competitive inhibitor ❌
Competitive inhibition shows:
- Km increases
- Vmax remains constant
- Lines intersect at Y-axis
Since X-intercept remains same in the graph, Drug A is not competitive.
Option B: Drug B acts as competitive inhibitor ❌
Competitive inhibition would show same Y-intercept and different X-intercepts.
This is not observed. Therefore, Drug B is not competitive.
Option C: Drug A acts as non-competitive inhibitor ✅
- Km unchanged
- Vmax decreases
- Lines intersect at X-axis
Matches the graph perfectly.
Option D: Drug B acts as non-competitive inhibitor ✅
Drug B shows the same characteristics:
unchanged Km, decreased Vmax.
Comparison Table
| Feature | Competitive Inhibition | Non-Competitive Inhibition |
|---|---|---|
| Km | Increases | No change |
| Vmax | No change | Decreases |
| Intersection | Y-axis | X-axis |
| Overcome by ↑[S]? | Yes | No |
Conclusion
Since both drugs decrease Vmax without affecting Km,
both Drug A and Drug B act as non-competitive inhibitors.
Final Answer: 4. C and D
