(DEC 2012)
70. An enzyme catalyzed reaction was measured in the presence and absence of an inhibitor for an uncompetitive inhibition,
(1) only Km is increased
(2) only is Vmax decreased
(3) both Km and Vmax are decreased
(4) both Km and Vmax are not affected

The correct answer is (3) both Km and Vmax are decreased.

Introduction

Enzyme inhibition is a vital aspect of biochemical regulation and drug action, with uncompetitive inhibition being a distinct mode where the inhibitor binds exclusively to the enzyme-substrate (ES) complex. This unique binding mode causes simultaneous decreases in both the Michaelis constant (KmKm) and the maximum reaction velocity (VmaxVmax), distinguishing uncompetitive inhibition from other types. This article elaborates on the mechanism and kinetic consequences of uncompetitive inhibition to clarify how and why KmKm and VmaxVmax both decrease.

Mechanism of Uncompetitive Inhibition

• The uncompetitive inhibitor binds only to the enzyme-substrate complex (ES), forming an inactive ESI complex.

• This binding prevents product formation, trapping the enzyme in an inactive state.

• Because the inhibitor binds after substrate binding, it does not affect substrate affinity in a typical way but lowers the apparent KmKm due to shift in equilibrium.

Effects on Kinetic Parameters

• Decrease in VmaxVmax:

  • The effective concentration of active enzyme is reduced because some enzyme is locked in the inactive ESI complex, lowering the maximum achievable reaction rate.

• Decrease in apparent KmKm:

  • The inhibitor binding shifts equilibrium towards ES complex formation, effectively increasing enzyme’s apparent affinity for substrate, reducing KmKm.

Mathematical Relationships

• Modified Michaelis-Menten equation with inhibitor:

V=(Vmax/(1+[I]/Ki))[S](Km/(1+[I]/Ki))+[S]V=(Km/(1+[I]/Ki))+[S](Vmax/(1+[I]/Ki))[S]

• Both VmaxVmax and KmKm are divided by the same factor, lowering their observable values in kinetics.

Distinguishing Uncompetitive Inhibition Graphs

• Lineweaver-Burk plots show parallel lines for inhibited and uninhibited reactions.

• Both slopes and intercepts change proportionally, reflecting simultaneous decreases in KmKm and VmaxVmax.

Comparison Table of Inhibition Types

Biological and Pharmaceutical Implications

• Uncompetitive inhibitors can be highly effective at high substrate concentrations because they bind exclusively to ES.

• Their distinct kinetic signature aids in identifying binding modes and designing targeted inhibitors.

Conclusion

In uncompetitive inhibition, both KmKm and VmaxVmax decrease due to inhibitor binding the ES complex, trapping the enzyme-substrate form and preventing product formation. Option (3) both Km and Vmax are decreased correctly describes this inhibition type.

This detailed review strengthens biochemical understanding of inhibition kinetics, aiding students and researchers in enzyme characterization and drug discovery.