Post-Translational Modifications

Introduction to PTMs - Protein Polish & Purpose

• Covalent chemical alterations to proteins after their synthesis on ribosomes.
• "Protein polish": Critical for transforming nascent polypeptides into mature, functional proteins.
• Vastly expand proteome complexity; >200 distinct PTM types identified.
• Essential for:
  • Regulating protein activity (e.g., enzyme kinetics).
  • Determining subcellular localization.
  • Modulating protein-protein/protein-ligand interactions.
  • Controlling protein half-life and degradation pathways.
• Occur in various cellular locations like ER, Golgi, cytoplasm.

⭐ PTMs are fundamental to cellular signaling, allowing rapid responses to stimuli.

Major PTMs I - Kinase Kisses & Sugar Coats

Phosphorylation: Reversible addition of phosphate ($PO_4^{3-}$)

• Enzymes: Kinases (add $PO_4^{3-}$ from ATP), Phosphatases (remove $PO_4^{3-}$).
• Targets: -OH of Ser, Thr, Tyr (key in eukaryotes).
• Functions: Regulates protein activity (on/off), signaling (MAPK), metabolism.

  ⭐ Protein kinases constitute one of the largest enzyme families, phosphorylating ~30% of all human proteins.

Glycosylation: Covalent attachment of oligosaccharides (sugars).

• Functions: Protein folding, stability, cell adhesion, immunity, signaling.
• Types:
  • N-linked:
    • To: Asn in Asn-X-Ser/Thr (X ≠ Pro).
    • Location: ER (core, on dolichol-P) → Golgi (modify).
    • Example: Immunoglobulins.
  • O-linked:
    • To: Ser, Thr (-OH).
    • Location: Golgi, cytoplasm, nucleus.
    • Example: ABO antigens, mucins. 📌 Mnemonic: N-linked to AsparagiNe. O-linked to -OH of Ser/Thr.

Major PTMs II - Tag, Tuck & Tweak

• Ubiquitination: Attaching ubiquitin protein to targets.
  • Pathway: 📌 E1 (activates Ub), E2 (conjugates Ub), E3 (ligase, specificity).

-   **Functions**: K48-polyUb → proteasomal degradation. MonoUb/K63-polyUb → signaling, DNA repair, endocytosis.
> ⭐ Bortezomib, a proteasome inhibitor, treats multiple myeloma by preventing degradation of pro-apoptotic factors.

• Acetylation: Adds acetyl ($CH_3CO-$) to lysine.

  • Enzymes: Histone Acetyltransferases (HATs) add; Histone Deacetylases (HDACs) remove.
  • Effect: Neutralizes lysine's (+) charge → loosens chromatin (euchromatin) → ↑ gene transcription.
  • 

• Methylation: Adds methyl ($-CH_3$) to lysine/arginine.

  • Enzymes: Methyltransferases add; Demethylases remove.
  • Effect: Context-dependent; H3K4me3 → active transcription; H3K9me3/H3K27me3 → repressive. DNA methylation (cytosine) → gene silencing.

PTMs in Disease & Therapy - Clinical Connections

• Pathogenesis:
  • Cancer: Dysregulated phosphorylation (kinases like EGFR), ubiquitination (p53, cyclins), histone PTMs (acetylation, methylation).
  • Neurodegeneration: Tau hyperphosphorylation (Alzheimer's), α-synuclein PTMs (Parkinson's), huntingtin PTMs (Huntington's).
  • Diabetes: Non-enzymatic glycosylation → Advanced Glycation End-products (AGEs) → complications.
  • Cystic Fibrosis: CFTR protein misfolding (glycosylation defects).
  • Inflammation: Altered SUMOylation, nitrosylation.
• Biomarkers:
  • $HbA1c$: Glycated hemoglobin for diabetes monitoring (long-term glucose).
  • Phosphorylated STAT3 (pSTAT3): Cancer prognosis.
  • Circulating tumor DNA (ctDNA) PTMs: Emerging.
• Therapeutic Targets:
  • Kinase inhibitors: Imatinib (BCR-ABL in CML), erlotinib (EGFR in lung cancer).
  • Proteasome inhibitors: Bortezomib (multiple myeloma) - targets ubiquitination pathway.
  • HDAC inhibitors: Vorinostat (lymphoma) - targets acetylation.
  • Monoclonal antibodies against PTM-modified proteins.

  ⭐ Imatinib, a tyrosine kinase inhibitor targeting the BCR-ABL fusion protein (a result of PTM pathway dysregulation), revolutionized CML treatment. oka

High-Yield Points - ⚡ Biggest Takeaways

• Phosphorylation by kinases (vs. phosphatases) critically regulates enzyme activity and cell signaling.
• Glycosylation: N-linked (ER, Asn) and O-linked (Golgi, Ser/Thr) affect protein folding, stability, and targeting.
• Ubiquitination: Polyubiquitin signals proteasomal degradation; monoubiquitin has other regulatory roles.
• Histone acetylation by HATs activates gene transcription; HDACs reverse this.
• Collagen synthesis requires Vitamin C for proline/lysine hydroxylation.
• Clotting factor activation needs Vitamin K for gamma-carboxylation of glutamate.