Question

C-peptide is seen in which one of the following?

• A. In Pre-proinsulin
• B. In Proinsulin
• C. As a combined entity with insulin after secretion
• D. A gastrointestinal proactive molecule

Hint:

The connecting peptide joins the A and B chains in which insulin precursor?

Answer 1

The Correct Option is B

Step 1: Insulin is first made by the beta cells as a large precursor, pre-proinsulin, which enters the endoplasmic reticulum.
Step 2: The 23-amino-acid signal peptide is cleaved off, converting pre-proinsulin into the prohormone proinsulin.
Step 3: Proinsulin is a single chain in which the A and B chains of insulin are joined by a connecting segment called the C-peptide (connecting peptide). So C-peptide exists as part of the proinsulin molecule.
Step 4: Before secretion the C-peptide is enzymatically cleaved off, and insulin plus an equimolar amount of free C-peptide are released. Therefore C-peptide is structurally seen in proinsulin, not as a combined entity after secretion, and it is not a GI molecule. The answer is proinsulin.