Regulation of Enzyme Activity

Regulation of Enzyme Activity

  • Importance of Regulation

    • Enzymes need regulation to control their activity in different cellular states.
    • Example: Enzymes for mitosis must be inactive when cells enter the G0 phase.

  • Feedback Regulation

    • Regulation by products downstream in a metabolic pathway.
    • Feedback inhibition is common; prevents overproduction of a product by shutting down the pathway.
    • Feedback activation exists but is less frequent.
Feedback Inhibition
  • Mechanism
    • The final product binds to the enzyme's active site or to previous enzymes, inhibiting their activity.
    • Ensures homeostasis once enough product is produced.
Types of Reversible Inhibition

  1. Competitive Inhibition

    • Inhibitors occupy the active site, preventing substrate access.
    • Can be overcome by increasing substrate concentration.
    • Effect on Kinetics:
      • No change in Vmax.
      • Increases Km (Michaelis constant), requiring higher substrate concentration to achieve half Vmax.

  2. Noncompetititve Inhibition

    • Inhibitors bind to an allosteric site, altering the enzyme's conformation without competing for the active site.
    • Cannot be overcome by increasing substrate concentration.
    • Effect on Kinetics:
      • Decreases Vmax (less active enzyme available).
      • No change in Km (affinity for substrate remains the same).

  3. Mixed Inhibition

    • Inhibitor can bind to either free enzyme or enzyme-substrate complex but with differing affinitive properties.
    • Effect on Kinetics:
      • Decreases Vmax.
      • Km varies (increases if preferential for enzyme; decreases if preferential for enzyme-substrate complex).

  4. Uncompetitive Inhibition

    • Inhibitors bind only to the enzyme-substrate complex, preventing product release.
    • Effect on Kinetics:
      • Decreases both Km and Vmax (binding locking in the substrate increases affinity).
Irreversible Inhibition
  • Mechanism
    • Inhibitors permanently deactivate the enzyme or make the active site unavailable.
    • Example: Aspirin irreversibly modifies cyclooxygenase, blocking pain mediation pathways.
Regulated Enzymes

  • Allosteric Enzymes

    • Multiple binding sites (active site and at least one allosteric site).
    • Can exist in active and inactive forms; shifts caused by allosteric activators or inhibitors.
    • Effect on Kinetics:
      • Cooperativity results in sigmoidal curves on Michaelis-Menten plots.

  • Covalent Modification

    • Enzymes can be activated/deactivated through phosphorylation/dephosphorylation, or glycosylation (adding sugar moieties).
    • The outcome of these modifications often requires experimental analysis.

  • Zymogens

    • Inactive precursors of enzymes (e.g., trypsinogen) that require activation to prevent self-digestion in tissues.
    • Contain both catalytic and regulatory domains that must be modified for activity.
Conclusion
  • Key focus: Regulation mechanisms of enzyme activity important for cellular function and maintaining homeostasis.
  • Revisions included types of inhibition and their effects on enzyme kinetics, allosteric and covalent modifications, and the importance of controlling dangerous enzymes with zymogens.