Step 1: Understand what pro-insulin is.
In the human pancreas, insulin is first produced in an inactive precursor form called pro-insulin. This molecule is synthesized in the beta cells of the pancreas and contains three peptide chains. These chains are known as the A-chain, B-chain, and C-peptide. The C-peptide connects the A and B chains and helps the molecule fold properly during its formation.
Step 2: Conversion of pro-insulin into mature insulin.
Before insulin becomes functional, the C-peptide is removed by enzymatic cleavage. This processing occurs inside the secretory vesicles of the pancreatic beta cells. Once the connecting peptide is removed, only the A-chain and B-chain remain. These two chains are then linked together by disulfide bonds, forming the active insulin hormone.
Step 3: Structure of mature insulin.
The mature insulin molecule consists of only two peptide chains — the A-chain and the B-chain. These chains are held together by two disulfide bonds, while an additional disulfide bond may be present within the A-chain itself. This structure makes insulin biologically active and capable of regulating blood glucose levels in the body.
Step 4: Key difference between pro-insulin and mature insulin.
The main difference lies in the presence or absence of the C-peptide.
Pro-insulin contains the A-chain, B-chain, and C-peptide, making it a larger inactive precursor molecule. In contrast, mature insulin is formed after the C-peptide is removed, leaving only the A and B chains that function as the active hormone.
Final Answer:
Pro-insulin is an inactive precursor consisting of three peptide chains (A, B, and C-peptide).
Mature insulin is the active hormone formed after the removal of the C-peptide, leaving only the A-chain and B-chain linked by disulfide bonds.