What is correct about insulin ?

Options

A. Two short polypeptide chains
B. Polypeptide chains are linked together by disulphide bridges.
C. In mammals insulin is synthesized as a prohormone.
D. All of the above

Answer

### Step-by-Step Solution:

1. **Understanding Insulin Structure**:
- Insulin is a peptide hormone composed of two polypeptide chains known as the A chain and the B chain. These chains are linked together by disulfide bonds.

2. **Source of Insulin**:
- Insulin is synthesized in the pancreas of mammals. Initially, it is produced as a precursor molecule known as proinsulin, which contains an additional peptide called the C-peptide.

3. **Proinsulin Formation**:
- In mammals, insulin is synthesized as proinsulin, which consists of three parts: the A chain, the B chain, and the C-peptide. The C-peptide is removed during the maturation process to form active insulin.

4. **Disulfide Bridges**:
- The A and B chains of insulin are connected by disulfide bridges (bonds), which are crucial for the stability and functionality of the insulin molecule.

5. **Conclusion**:
- All the statements provided in the options are correct:
- Insulin consists of two polypeptide chains.
- The chains are linked by disulfide bridges.
- In mammals, insulin is synthesized as a pro-hormone.

### Final Answer:
All of the statements about insulin are correct.