Q.88 The action of which class of enzyme inhibitors can be reversed by adding an excess
of substrate?
(A) Uncompetitive inhibitors
(B) Competitive inhibitors
(C) Non-specific inhibitors
(D) Allosteric inhibitors
Competitive inhibitors are the class of enzyme inhibitors whose action can be reversed by adding excess substrate, as they compete directly with the substrate for the enzyme’s active site.
Question Solution
The correct answer is (B) Competitive inhibitors. This holds true because competitive inhibitors bind reversibly to the enzyme’s active site, mimicking the substrate’s structure and blocking substrate access. Increasing substrate concentration displaces the inhibitor, restoring enzyme activity.
Option Analysis
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(A) Uncompetitive inhibitors: These bind only to the enzyme-substrate (ES) complex at an allosteric site, not the active site, lowering both Km and Vmax. Excess substrate worsens inhibition by forming more ES complexes.
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(B) Competitive inhibitors: Bind reversibly to the free enzyme’s active site, increasing apparent Km but not affecting Vmax. Excess substrate outcompetes the inhibitor, reversing the effect.
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(C) Non-specific inhibitors: These broadly disrupt enzyme function without targeting specific sites, often irreversibly or through denaturation. Excess substrate does not reverse them.
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(D) Allosteric inhibitors: Bind to sites other than the active site, inducing conformational changes that alter activity. This is typically non-competitive or uncompetitive, not reversible by substrate excess.
Competitive inhibitors represent a key reversible enzyme inhibition type where excess substrate reverses the effect by outcompeting the inhibitor at the active site. This concept is crucial for CSIR NET Life Sciences aspirants studying enzyme kinetics and biochemistry.
Mechanism of Competitive Inhibition
Competitive inhibitors structurally resemble the substrate and bind reversibly to the enzyme’s active site, forming an enzyme-inhibitor (EI) complex that prevents substrate binding. This raises the apparent Michaelis constant (Km), indicating reduced substrate affinity, but maximum velocity (Vmax) remains unchanged since the enzyme functions normally once substrate binds. Examples include methotrexate inhibiting dihydrofolate reductase and penicillin blocking bacterial cell wall enzymes.
Comparison of Inhibitor Types
| Inhibitor Type | Binding Site | Effect on Km | Effect on Vmax | Reversed by Excess Substrate? |
|---|---|---|---|---|
| Competitive | Active site | Increases | Unchanged | Yes |
| Uncompetitive | ES complex (allosteric) | Decreases | Decreases | No |
| Non-competitive | Allosteric | Unchanged | Decreases | No |
| Allosteric | Allosteric | Varies | Decreases | No |
Exam Relevance for CSIR NET
In competitive inhibition, Lineweaver-Burk plots show intersecting lines on the y-axis, confirming unchanged Vmax. Practice distinguishing from non-competitive (parallel lines) to master enzyme kinetics questions. Real-world applications include drug design, like statins competing with cholesterol precursors.
