In human insulin is synthesised as a prohormone, which contains an extra stretch called :-

Options

A. A-peptide
B. C-peptide
C. B-peptide
D. `alpha` -peptide

Answer

To answer the question regarding the extra stretch present in proinsulin, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding Proinsulin**:
- Proinsulin is the precursor to insulin. It is synthesized in the pancreas and consists of three chains: the A chain, the B chain, and an additional stretch known as the C chain.

2. **Identifying the Chains**:
- The structure of proinsulin can be described as follows:
- **A Chain**: One of the two chains that make up mature insulin.
- **B Chain**: The second chain that also forms part of mature insulin.
- **C Chain**: This is the extra stretch that is present in proinsulin but is not included in the final, mature insulin structure.

3. **Cleavage of C Chain**:
- During the maturation process, the C chain is cleaved off from proinsulin. This cleavage results in the formation of mature insulin, which consists only of the A and B chains linked by disulfide bonds.

4. **Conclusion**:
- The question asks for the name of the extra stretch present in proinsulin. Since the C chain is the only part that is removed to form mature insulin, the correct answer is **C-peptide**.

### Final Answer:
The extra stretch in proinsulin is called **C-peptide**.

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