- Insulin is an anabolic peptide hormone from pancreatic β cells (A chain 21 aa, B chain 30 aa, disulfide-linked).
- It is the only glucose-lowering and only anti-ketogenic hormone.
Structure, Synthesis and β-Cell Secretory Products
Products of the β-cell granule
- Insulin: about 95%.
- Proinsulin: about 3%, about 10% bioactivity.
- C peptide: about 7%, no bioactivity.
- Proinsulin and C peptide are secreted in equimolar ratio with insulin.
Steps of synthesis
- Insulin is synthesized as preproinsulin (110 amino acids).
- Its N-terminal signal peptide is cleaved to form proinsulin (86 amino acids: B chain, C peptide, A chain in sequence).
- Proinsulin enters the Golgi, where disulfide bridges fold it.
- Folded proinsulin is cleaved to insulin plus C peptide.
- The products are packaged with zinc into secretory granules.