(DEC 2014)
59. Reaction products inhibit catalysis in enzymes by
(1) covalently binding to the enzyme.
(2) altering the enzyme structure
(3) occupying the active site.
(4) form a complex with the substrate.

The correct answer is (3) occupying the active site.

Introduction

In enzyme-catalyzed reactions, the accumulation of reaction products often slows down or halts further catalysis—a natural regulatory mechanism essential for metabolic control. Product inhibition occurs through various mechanisms, but predominantly through the occupation of the enzyme’s active site by the product molecules. This article explains how product binding to active sites inhibits enzyme activity and impacts metabolic regulation.

Mechanisms of Product Inhibition

1. Occupying the Active Site (Competitive Product Inhibition)

   • Reaction product binds the active site, competing with substrate for binding.

   • Prevents substrate access and formation of the enzyme-substrate complex, slowing catalysis.

   • Typical for products structurally similar to substrates or transition states.

2. Altering Enzyme Structure (Allosteric Inhibition)

   • Product binds to an allosteric site distinct from the active site.

   • Induces conformational changes that reduce enzyme affinity or catalytic efficiency.

   • Not as common as active site occupation for direct product inhibition.

3. Covalent Binding

   • Less common for typical product inhibition. Usually involves irreversible or mechanism-based inhibitors, different from the reversible product inhibition.

4. Forming Complexes with Substrate (Rare)

   • Products forming complexes with substrates can indirectly reduce substrate availability but is not a primary product inhibition mode.

Why Active Site Occupation Is Predominant

• Proximity in structure between product and substrate.

• Direct competition offers immediate and reversible feedback.

• Provides efficient control in metabolic pathways.

Biological Importance

• Prevents substrate depletion and energy waste.

• Maintains metabolic flux balance.

• Protects cells from toxic product accumulation.

Summary Table

Conclusion

The primary way reaction products inhibit enzymatic catalysis is by occupying the enzyme’s active site, directly competing with substrate binding and slowing further reaction progress. This competitive form of product inhibition is a fundamental mechanism for self-regulation in metabolic pathways.