Select the option which is NOT correct with respect to enzyme action.

Addition of lot of succinate does not reverse the inhibition of succinic dehydrogenase by malonate.

Substrate blinds with enzymes at its active site.

A non-competitive inhibitor binds the enzymes at a site distinct from that which binds the substrate.

Malonate is a competitive inhibitor of succinic dehydrogenase.

Competitive Inhibition

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NCERT Class 11 · Biomolecules · 2 NEET questions

Enzymes are not just catalysts; they are precise locks. Competitive inhibition is the molecular equivalent of a key that fits the lock but refuses to turn. If you confuse this with non-competitive inhibition, you lose four marks instantly.

The idea

Competitive inhibition occurs when an inhibitor structurally resembles the substrate. Both compete for the same active site on the enzyme. The inhibitor binds to the active site, preventing substrate attachment. This reduces the reaction velocity, but increasing substrate concentration can overcome this inhibition. A classic NCERT example is malonate inhibiting succinate dehydrogenase.

Why it matters

It serves as a natural regulatory mechanism to control metabolic pathways and prevent the overproduction of certain metabolites.

NEET lens

NEET focuses on the relationship between substrate concentration and inhibition. Remember: Vmax remains unchanged, but Km increases. Do not memorize the mechanism; understand that the inhibitor and substrate are structural mimics. Expect questions matching specific inhibitors to their respective enzymes.

Clinical significance

Competitive inhibition is the basis of many life-saving drugs. Methotrexate acts as a competitive inhibitor of dihydrofolate reductase, effectively stopping cancer cell division. Without this mechanism, we would have no targeted chemotherapy for rapidly dividing malignant cells.

Introduction

Competitive inhibition occurs when an inhibitor molecule resembles the substrate and competes with it for the enzyme's active site, reducing enzyme activity.

Core concept

Mechanism: inhibitor (same shape as substrate) binds active site — blocks substrate binding. Reversible: can be overcome by increasing substrate concentration (more substrate outcompetes inhibitor). Effect on kinetics: Km increases (apparent decrease in enzyme-substrate affinity), Vmax unchanged.

Go deeper

Succinate dehydrogenase inhibition by malonate is classic competitive inhibition. Malonate resembles succinate → competes for active site. Adding more succinate reverses inhibition. This was used to discover competitive inhibition in Krebs cycle study.

From Class 10

Enzymes can be inhibited (slowed down) by molecules that interfere with their function. Competitive inhibition is one type.