The transformation from protein precursor to active peptide is one of biology's most elegant processing systems.
The Prohormone Convertase Family
Prohormone convertases (PCs) are a family of subtilisin-like serine proteases:
PC1/3 (PCSK1) - Primary converter of proinsulin, proglucagon, POMC - Expressed in neuroendocrine cells - Cleaves at **Arg-X-X-Arg↓** and **Lys-Arg↓** sites
PC2 (PCSK2) - Works with PC1/3 for complete processing - Often performs second cleavage - Requires 7B2 chaperone for activation
Furin (PCSK3) - Ubiquitously expressed (constitutive pathway) - Cleaves growth factors, receptors, viral proteins - Minimal motif: **Arg-X-Lys/Arg-Arg↓**
The Processing Pathway
1. Signal Peptide Removal - Occurs co-translationally in the ER - Signal peptidase cleaves ~20 AA leader sequence - **Prepropeptide → Propeptide**
2. Trafficking to Secretory Granules - Propeptide sorted in trans-Golgi network - Aggregation with chromogranins - Budding into immature secretory granules
3. Endoproteolytic Cleavage - Granules acidify as they mature (pH ~5.5) - PCs activate in acidic environment - Cleavage at paired basic residues
4. Carboxypeptidase E Trimming - Removes C-terminal basic residues left by PCs - **Peptide-Lys/Arg → Peptide**
5. Final Modifications - **PAM** — Amidation of C-terminal glycine - **Other PTMs** — Sulfation, acetylation, etc.
Case Study: POMC Processing
Pro-opiomelanocortin (POMC) demonstrates tissue-specific processing:
- POMC → ACTH + β-lipotropin
- ACTH → α-MSH + CLIP
- β-lipotropin → γ-lipotropin + β-endorphin
The same precursor produces different peptides in different tissues!
Clinical Significance
Genetic Disorders - **PC1/3 deficiency** — Obesity, hypoglycemia, intestinal dysfunction - **PC2 deficiency** — Hypoglycemia, processing defects
Therapeutic Targets - **PCSK9 inhibitors** — Lower LDL cholesterol - **Furin inhibitors** — Antiviral (prevent spike protein cleavage)
Why This System Evolved
- **Efficiency** — One gene produces multiple peptides
- **Control** — Processing only in specialized cells
- **Storage** — Inactive precursors can be stockpiled
- **Diversity** — Tissue-specific processing patterns
- **Regulation** — Processing can be modulated