Enzyme Regulation: Covalent Modification

Intro to Covalent Modification - Enzyme Makeovers

• Regulates enzyme activity by attaching/detaching chemical groups via covalent bonds.
  • A key form of post-translational modification (PTM).
• Alters enzyme's conformation (3D structure) and/or active site.
  • Leads to enzyme activation or inhibition.
• These modifications are enzyme-catalysed and typically reversible.
• Generally slower onset; effects are often more sustained/durable.

⭐ Covalent modification often leads to a more sustained regulatory effect compared to allosteric regulation, providing longer-term control over enzyme activity.

Phosphorylation/Dephosphorylation - Phospho-Play Power

• Phosphorylation: Covalent addition of phosphate ($PO_4^{3-}$) to specific amino acid residues.
  • Catalyzed by Protein Kinases (e.g., Ser/Thr kinases, Tyr kinases).
  • Reaction: $Protein + ATP \rightarrow Protein-PO_4 + ADP$.
• Dephosphorylation: Removal of $PO_4^{3-}$ from phosphoprotein.
  • Catalyzed by Protein Phosphatases.
  • Reaction: $Protein-PO_4 + H_2O \rightarrow Protein + P_i$.
• 📌 Mnemonic: Kinases 'Kindle' (add P), Phosphatases 'Pluck' (remove P).
• Mechanism: Alters protein conformation & thereby its activity (activation or inactivation).
• Key sites on protein: Hydroxyl groups of Serine (Ser), Threonine (Thr), Tyrosine (Tyr) residues.
• Significance: Most common, rapid, and reversible post-translational modification for enzyme regulation. Crucial in signal transduction pathways.

⭐ In glycogen metabolism, glycogen phosphorylase is activated by phosphorylation, while glycogen synthase is inactivated by phosphorylation, demonstrating reciprocal regulation.

Other Covalent Modifications - Tag Team Champs

• ADP-Ribosylation: $NAD^+$ is donor.
  • Targets: Arg, Glu, Cys.
  • Functions: DNA repair, signaling.
  • Toxins: Cholera (↑Gsα), Pertussis (↓Giα), Diphtheria (↓eEF-2).

  ⭐ Diphtheria toxin catalyzes the ADP-ribosylation of elongation factor 2 (eEF-2), thereby inhibiting protein synthesis in eukaryotes.

• Methylation: SAM is donor.
  • Targets: Histone Lys/Arg; DNA Cytosine.
  • Function: Epigenetics.
• Acetylation: Acetyl-CoA is donor.
  • Targets: Histone Lys.
  • Function: Gene activation (HATs vs HDACs).
• Hydroxylation: Needs Vit C.
  • Targets: Collagen Pro/Lys.
  • Function: Collagen stability.
• Ubiquitination: Adds ubiquitin.
  • Function: Proteasomal degradation, signaling.
• SUMOylation: Adds SUMO protein.
  • Function: Nuclear processes, gene regulation.

Proteolytic Cleavage (Zymogens) - The Big Snip!

• Zymogens (Proenzymes): Inactive enzyme precursors requiring proteolytic cleavage for activation. 📌 Zymogens are 'Lazy-gens' until activated.
• Mechanism: Irreversible covalent modification. Specific peptide bonds are cleaved, exposing the active site.
  • A "one-way switch" - once activated, cannot be readily inactivated by reforming the bond.
• Significance:
  • Prevents autodigestion (e.g., pancreas by its own proteases).
  • Enables rapid enzyme deployment upon stimulus.
  • Crucial for cascades (e.g., blood clotting, complement).
• Key Examples:
  • Digestive Enzymes:
    • Pepsinogen → Pepsin (stomach)
    • Trypsinogen → Trypsin (pancreas)
    • Chymotrypsinogen → Chymotrypsin (pancreas)
    • Procarboxypeptidase → Carboxypeptidase (pancreas)
    • Proelastase → Elastase (pancreas)
  • Blood Clotting Factors: e.g., Prothrombin → Thrombin.
  • Apoptosis: Procaspases → Caspases.
  • Hormones: Proinsulin → Insulin.

⭐ Enteropeptidase (formerly enterokinase), secreted by the duodenal mucosa, is the key enzyme that initiates the activation of pancreatic zymogens by converting trypsinogen to active trypsin.

High‑Yield Points - ⚡ Biggest Takeaways

• Covalent modification: Reversible attachment/removal of chemical groups alters enzyme activity.
• Phosphorylation (by kinases) and dephosphorylation (by phosphatases) are most common.
• This changes enzyme conformation and thus its catalytic efficiency.
• Regulation speed: Slower than allosteric, but faster than enzyme synthesis/degradation.
• ATP typically serves as the phosphate group donor for kinases.
• Examples: Glycogen phosphorylase (activated by phosphorylation), pyruvate dehydrogenase (inactivated by phosphorylation).
• Other modifications: Adenylylation, ADP-ribosylation, methylation, acetylation.