Human peptide biosynthesis occurs through two primary mechanisms, each suited to different biological needs.
Ribosomal Synthesis (Pre-pro-peptide Pathway)
Most bioactive peptides begin life as large precursor proteins called prepropeptides. This pathway involves:
- **Translation** — mRNA is translated into a large precursor protein
- **ER Targeting** — Signal peptide directs the nascent chain to the Endoplasmic Reticulum
- **Signal Cleavage** — The signal sequence is removed, forming a propeptide
- **Golgi Processing** — The propeptide is sorted into secretory granules
- **Proteolytic Activation** — Prohormone Convertases (PC1/3, PC2) cleave the propeptide at specific sites
- **Post-translational Modifications** — Amidation, sulfation, and other modifications complete the active peptide
Example: Insulin Production Proinsulin (86 amino acids) is cleaved to release insulin (51 AA) and C-peptide (31 AA).
Non-Ribosomal Peptide Synthesis
Some peptides bypass the ribosomal machinery entirely:
Glutathione (γ-Glu-Cys-Gly) - Synthesized by specific ATP-dependent ligases - Features a unique γ-peptide bond resistant to proteases - Present at millimolar concentrations in cells
Carnosine (β-Ala-His) - Synthesized by carnosine synthase - Functions as a pH buffer in muscle tissue - Contains β-alanine, a non-proteinogenic amino acid
Why This Matters
- **One gene, multiple peptides** — POMC produces ACTH, β-endorphin, and MSH
- **Spatial and temporal control** — Processing occurs only in specific cells
- **Storage efficiency** — Inactive precursors can be stockpiled safely