Question

Two units of insulin bind through

• A. H-bond
• B. Peptide Bond
• C. Di-sulphide Bond
• D. None

Hint:

Disulfide bonds are crucial for the stability and proper folding of many proteins, especially those with multiple polypeptide chains or complex tertiary structures, like insulin and antibodies.

Answer 1

The Correct Option is C

Step 1: Understand the basic structure of insulin.
Insulin is a vital protein hormone involved in glucose metabolism. It is a small protein, typically composed of two distinct polypeptide chains:
An A-chain, which is 21 amino acids long.
A B-chain, which is 30 amino acids long. These two chains are synthesized separately within the cell and then joined together to form the functional insulin molecule. Step 2: Identify the type of bond that links protein chains.
Proteins exhibit different levels of structural organization, and various types of bonds stabilize these structures:
Peptide bonds: These are covalent bonds that link individual amino acids together to form a single polypeptide chain (primary structure). They are present \textit{within} each chain of insulin but do not connect the two separate chains.
Hydrogen bonds (H-bonds): These are weaker, non-covalent interactions that play a crucial role in stabilizing the secondary (e.g., alpha-helices, beta-sheets) and tertiary structures of proteins. While present in insulin's overall structure, they are not the primary covalent bonds linking the A and B chains.
Disulfide bonds (Di-sulphide bonds): These are strong covalent bonds formed between the sulfhydryl (-SH) groups of two cysteine amino acid residues. They are critical for stabilizing the tertiary structure of single polypeptide chains and, importantly, for covalently linking separate polypeptide chains. Step 3: Apply the knowledge to the specific case of insulin.
In the mature insulin molecule, the A and B chains are held together by two interchain disulfide bonds. Additionally, there is one intrachain disulfide bond within the A-chain itself, contributing to its stability. The question specifically asks how the "Two units of insulin bind through", referring to the linkage between the A and B chains. Therefore, the primary bonds linking the two units (polypeptide chains) of insulin are disulfide bonds.