Q.34 The activity of an enzyme was measured by varying the concentration of the substrate (S) in the
presence of three different concentrations of inhibitor (I) 0, 2 and 4 mM.The double reciprocal plot
given belowsuggests that the inhibitor (I) exhibits
(A) substrate inhibition (B) uncompetitive inhibition
(C) mixed inhibition (D) competitive inhibition
Introduction
Enzyme inhibition is a crucial topic in biochemistry and is frequently tested in competitive examinations such as GATE, IIT-JAM, CSIR-NET, and NEET PG. One of the most reliable ways to identify the type of enzyme inhibition is by analyzing the Lineweaver–Burk plot, also known as the double reciprocal plot.
In this problem, enzyme activity was measured at different substrate concentrations in the presence of three inhibitor concentrations (0, 2, and 4 mM). Based on the nature of the double reciprocal plot, we must determine the type of inhibition exhibited by the inhibitor.
Understanding the Given Graph
The graph is a Lineweaver–Burk plot, where:
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Y-axis: 1/V0
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X-axis: 1/[S]1
Key Observations from the Graph
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All straight lines intersect at the same point on the Y-axis
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As inhibitor concentration increases, the slope increases
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The Y-intercept (1/Vmax) remains constant
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The X-intercept shifts toward zero
Interpretation of the Graph
From Michaelis–Menten kinetics:
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Same Y-intercept ⇒ Vmax remains unchanged
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Increasing slope ⇒ Km increases
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Increasing Km with constant Vmax is the hallmark of competitive inhibition
Correct Conclusion
The inhibitor competes with the substrate for the active site of the enzyme, increasing the apparent Km but not affecting Vmax.
Evaluation of Options
Option (A): Substrate Inhibition
❌ Incorrect
Substrate inhibition occurs at very high substrate concentrations and is not diagnosed using intersecting straight lines in a Lineweaver–Burk plot.
Option (B): Uncompetitive Inhibition
❌ Incorrect
In uncompetitive inhibition:
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Both Km and Vmax decrease
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Lines are parallel in a Lineweaver–Burk plot
This does not match the given graph.
Option (C): Mixed Inhibition
❌ Incorrect
Mixed inhibition results in:
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Change in both Km and Vmax
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Lines intersect left of the Y-axis, not on it
Option (D): Competitive Inhibition
✅ Correct
Competitive inhibition shows:
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Same Y-intercept
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Increasing slope with inhibitor concentration
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Increase in Km, constant Vmax
Final Answer
✅ Correct Option: (D) Competitive Inhibition
Key Takeaways
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Lineweaver–Burk plots are powerful tools to identify enzyme inhibition types
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Same Y-intercept = Vmax unchanged
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Increasing slope with inhibitor concentration = competitive inhibition
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Competitive inhibitors can be overcome by increasing substrate concentration
