Problem 105 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

The two polypeptide chains present in insulin contain the following number of amino acids: [KCET 1997] (a) 18,12 (b) 20,12 (c) 21,30 (d) 25,16

Step-by-Step Solution

Answer

The correct answer is (c) 21,30.

Step 1: Understand Insulin Structure

Insulin is a hormone composed of two polypeptide chains, commonly referred to as the A-chain and B-chain. These chains are covalently linked by disulfide bonds and have specific numbers of amino acids.

Step 2: Recall Amino Acid Composition

In humans, the insulin protein consists of an A-chain with 21 amino acids and a B-chain with 30 amino acids. This specific combination plays a crucial role in its biological function.

Step 3: Match with Given Options

Review the given options for the number of amino acids in the two polypeptide chains: (a) 18,12, (b) 20,12, (c) 21,30, (d) 25,16. Compare these options with the known composition of human insulin.

Step 4: Identify the Correct Answer

The correct answer must match the known composition of the A-chain and B-chain of human insulin. From the options, (c) 21,30 matches exactly with 21 amino acids in the A-chain and 30 in the B-chain.

Key Concepts

Polypeptide Chains

Insulin, a vital hormone in regulating blood glucose levels, is constructed from two polypeptide chains known as the A-chain and B-chain. Each of these chains is a sequence of amino acids arranged end-to-end, forming a linear structure.
These chains are crucial because they dictate insulin's functionality by determining how insulin interacts with other molecules in the body. The A-chain in insulin consists of 21 amino acids, while the B-chain has 30 amino acids.
This arrangement isn't arbitrary—it's specifically tailored to support insulin's role in facilitating glucose uptake in cells.
The linear arrangement of amino acids in these chains allows for the intricate folding necessary for insulin's biological activity. Therefore, understanding the concept of polypeptide chains is fundamental when studying proteins like insulin.

• A-chain: composed of 21 amino acids
• B-chain: composed of 30 amino acids

These numbers are specific to human insulin and are essential for its proper biological function.

Amino Acid Composition

Amino acids are the building blocks of proteins, and the particular sequence of amino acids in a protein chain like insulin determines its structure and function. Insulin's precise sequence is vital for its activity as a hormone, which involves controlling blood sugar levels.
The sequence of amino acids in protein chains is unique and influences how the protein folds, its stability, and its biological interactions. In the case of insulin, the combination of 21 amino acids in the A-chain and 30 amino acids in the B-chain is essential for it to function correctly.

• Each amino acid has a specific role, whether structural or functional, contributing to insulin's ability to signal cells to absorb glucose.
• Any alteration in this sequence can lead to dysfunction, which is why the composition is highly conserved across individuals.

Understanding the amino acid composition helps grasp how insulin performs its role in the body.

Disulfide Bonds

Disulfide bonds play a pivotal role in stabilizing the structure of insulin by linking its two polypeptide chains. These bonds are strong covalent connections formed between sulfur atoms in the amino acid cysteine.
In insulin, there are three critical disulfide bonds—two interconnecting the A and B chains and one intra-chain bond within the A-chain.
These bonds are essential because they provide structural integrity to insulin, ensuring it maintains a correct shape and can effectively bind to its receptor on the cell surface to regulate blood sugar.

• Two of these bonds link the A-chain to the B-chain, reinforcing the overall structure.
• One additional bond within the A-chain helps maintain its specific folding pattern.

Without these disulfide bonds, insulin would not function properly, highlighting their importance in protein chemistry and hormone biology.