Enzyme inhibition

Overview of Enzyme Inhibition

  • Enzymatic inhibition is the process of decreasing enzyme activity when necessary.

  • Enzyme inhibitors are compounds that reduce the activity of enzymes.

Types of Inhibition

Irreversible Inhibitors

  • Definition: Inhibit enzyme activity permanently by forming covalent bonds.

  • Mechanism: Removal of the inhibitor does not restore enzyme activity.

  • Effects: Once an irreversible inhibitor binds, the enzyme cannot be reactivated.

  • Characteristics: Often structurally similar to the substrate and highly reactive.

    • Affinity Labels: Compounds that mimic substrates to bond irreversibly with enzyme active sites.

    • Suicide Substrates: Substrates that become more reactive post-modification and inhibit the enzyme.

  • Examples: Aspirin and clavulanic acid are drugs that act as irreversible inhibitors.

Reversible Inhibitors

  • Definition: Temporary binding to enzymes, allowing for the possibility of restoring enzyme activity.

  • Mechanism: Bind through non-covalent interactions (ionic, hydrogen bonds).

  • Behavior: If the inhibitor is removed, the enzyme can regain its activity.

  • Categorization: There are three main types of reversible inhibitors:

1. Competitive Inhibitors

  • Mechanism: Bind to the active site of the enzyme competing with the substrate.

  • Effects: Inhibition does not affect catalysis. If the inhibitor dissociates, the substrate can then bind and react.

  • Overcoming Inhibition: Increasing substrate concentration can outcompete the inhibitor for the active site.

  • Lineweaver-Burk Plot Characteristics: Lines intersect on the y-axis indicating constant Vmax but increased apparent Km.

    • Apparent Km: Given by equation: extK′<em>m=extK</em>m(1+rac[I]K<em>I)ext{K'}<em>m = ext{K}</em>m(1 + rac{[I]}{K<em>I}) where [I] is the inhibitor concentration and $KI$ is the inhibitor's affinity.

  • Transition State Analogs: Many competitive inhibitors are designed to resemble the transition state of substrates, facilitating higher binding affinity.

2. Uncompetitive Inhibitors

  • Mechanism: Inhibitors bind to the enzyme-substrate complex, not the free enzyme.

  • Effects: Prevents product formation but can dissociate, leading to reversible behavior.

  • Characteristics: Often seen in bisubstrate reactions, and requires one substrate to be in excess to act.

  • Lineweaver-Burk Plot Characteristics: Display parallel lines indicating a reduction in Vmax and apparent Km.

    • Influence on Vmax: V<em>max′=racV</em>maxextalpha′V<em>{max}' = rac{V</em>{max}}{ ext{alpha}'} where alpha' indicates the inhibitor's affinity.

3. Mixed Inhibitors

  • Mechanism: Can bind to either the free enzyme or the enzyme-substrate complex.

  • Characteristics: Affects both substrate binding and catalytic function, and its effects on Km depend on the binding affinity.

  • Lineweaver-Burk Plot Characteristics: Lines intersect left of the y-axis, but changes occur in both Vmax and apparent Km.

    • Effects on Vmax: Similar to uncompetitive inhibition, indicating lowered Vmax.

Noncompetitive Inhibitors

  • Special Case of Mixed Inhibitors: Where the binding affinity for free and bound enzyme is equal.

  • Mechanism: Inhibits both substrate binding and catalysis but does not change apparent Km (stays the same).

  • Lineweaver-Burk Plot Characteristics: Intersection on the x-axis indicating no change in Km but a decrease in Vmax.

Summary of Km and Vmax Changes

  • Competitive Inhibition: Increases Km, Vmax unchanged.

  • Uncompetitive Inhibition: Decreases both Km and Vmax.

  • Mixed Inhibition: Vmax decreases, Km can increase or decrease depending on binding characteristics.

  • Noncompetitive Inhibition: Decreases Vmax, Km unchanged.

Concept Check

  • Question: Which factor could not be part of a reversible inhibition mechanism?

    • Answer: The release of a prosthetic group; its release completely inactivates the enzyme, characterizing irreversible inhibition.