(JUNE 2002)
66. Which of following is true for non-competitive inhibition?
(1) Increases Vmax                                  (2) Decreases Vmax
(3) Increases Km                                     (4) Decreases Km

The correct answer is (2) Decreases Vmax.

Introduction

Non-competitive inhibition is a classical mechanism by which enzymes are regulated or inhibited without competing with the substrate for the active site. Unlike competitive inhibition, which primarily affects substrate affinity (KmKm), non-competitive inhibition reduces the overall catalytic capacity (VmaxVmax) of the enzyme. Recognizing how this inhibition impacts kinetic parameters is essential for both biochemistry students and professionals exploring enzyme kinetics and drug design.

What Happens in Non-Competitive Inhibition?

• The inhibitor binds to a site other than the active site, termed the allosteric site, on either the free enzyme or enzyme-substrate complex.

• This binding causes a conformational change that reduces the enzyme’s effectiveness, thereby lowering VmaxVmax.

• Since substrate binding is not directly blocked, the apparent substrate affinity (KmKm) remains unchanged.

Effects on Kinetic Parameters

• The enzyme’s maximal catalytic rate is halved or otherwise reduced, meaning fewer active enzyme molecules or reduced turnover number (kcatkcat).

• Substrate concentration changes cannot overcome inhibition; increasing substrate does not restore VmaxVmax.

Contrasts with Other Inhibition Types

• Competitive inhibition: Increases KmKm, VmaxVmax unchanged.

• Uncompetitive inhibition: Decreases both KmKm and VmaxVmax.

Biological and Practical Significance

• Non-competitive inhibitors are valuable in regulating enzyme activity in complex metabolic networks.

• Many drugs are designed as non-competitive inhibitors to modulate enzyme function effectively.

Summary Table

Conclusion

Non-competitive inhibition leads to a decrease in VmaxVmax while leaving KmKm unaffected. This type of inhibition reduces overall enzyme activity regardless of substrate concentration, reflecting allosteric or alternative site binding.

This detailed guide clarifies the impact of non-competitive inhibitors on enzyme kinetics, equipping students and researchers with critical knowledge for biochemical analysis and drug development.