Post-translational modifications (PTMs) are essential chemical alterations that convert inactive peptide precursors into stable, bioactive molecules.
Proteolytic Processing
Most peptides are carved from larger precursors by specific enzymes:
Prohormone Convertases (PCs) - **PC1/3 and PC2** — Primary neuroendocrine processors - Cleave at paired basic residues (Lys-Arg, Arg-Arg) - Active in acidic secretory granules
Carboxypeptidase E (CPE) - Trims C-terminal basic residues after PC cleavage - Essential for proper peptide maturation
Example: POMC Processing Pro-opiomelanocortin (POMC) yields multiple hormones: - ACTH → Adrenal cortisol release - β-Endorphin → Pain modulation - α-MSH → Melanin production, appetite suppression
Terminal Modifications
C-Terminal Amidation **Over 50% of bioactive peptides are amidated**
- Peptide-Gly → Peptide-NH₂
- Neutralizes C-terminal charge
- Enhances receptor binding
- Blocks carboxypeptidase degradation
- Examples: Oxytocin, Vasopressin, GnRH
N-Terminal Pyroglutamate Cyclization of N-terminal glutamine forms pyroglutamate (pGlu): - Blocks aminopeptidase attack - Found in GnRH, TRH
Structural Modifications
Disulfide Bonds Covalent links between cysteine residues: - **Constrain structure** — Reduce conformational entropy - **Increase stability** — Resist unfolding - **Examples**: Insulin (2 bonds), Oxytocin (1 bond), Defensins (3 bonds)
Tyrosine Sulfation Sulfate addition to tyrosine residues: - Catalyzed by TPSTs (Tyrosylprotein Sulfotransferases) - Critical for receptor binding (CCK, Hirudin) - Affects protein-protein interactions
Other PTMs | Modification | Function | Example | |--------------|----------|---------| | **Phosphorylation** | Signaling regulation | Various | | **Glycosylation** | Stability, recognition | Erythropoietin | | **Acetylation** | Stability, activity | α-MSH | | **Palmitoylation** | Membrane anchoring | Ghrelin (octanoylation) |
Why PTMs Matter
- **Activation** — Convert inactive precursors to active peptides
- **Stability** — Protect against proteolytic degradation
- **Specificity** — Fine-tune receptor interactions
- **Regulation** — Enable tissue-specific processing