Chemically, which of the following hormones is a polypeptide?

Endorphin

The correct answer is option 2. Endorphin.

To provide a detailed explanation, let’s break down the chemical nature of each hormone and clarify why Endorphin is the polypeptide among the options:

1. Androgens

Androgens are steroid hormones. Steroid hormones have a characteristic structure consisting of four fused carbon rings: three six-membered rings and one five-membered ring.

Androgens such as testosterone are involved in the development of male characteristics and reproductive activity.

Examples: Testosterone and dihydrotestosterone (DHT) are classic examples of androgens.

Conclusion: Since androgens are steroids and not composed of amino acids, they are not polypeptides.

2. Endorphin

Endorphins are peptides, which are short chains of amino acids linked together by peptide bonds. They are classified as polypeptides if they contain more than ten amino acids, and endorphins fit this criterion. For example, β-Endorphin consists of 31 amino acids.

Endorphins act as neurotransmitters and are involved in pain relief, mood regulation, and pleasure. They bind to opioid receptors in the brain to reduce pain and produce feelings of euphoria.

Examples: β-Endorphin, enkephalins, and dynorphins are well-known endorphins.

Conclusion: Endorphins are polypeptides due to their amino acid chain structure.

3. Epinephrine

Epinephrine (also known as adrenaline) is a catecholamine derived from the amino acid tyrosine. Its structure features a benzene ring with two hydroxyl groups and an amine group.

Epinephrine is involved in the fight-or-flight response. It increases heart rate, muscle strength, and glucose metabolism.

Examples: Epinephrine is commonly used in emergency medicine to treat severe allergic reactions and cardiac arrest.

Conclusion: Epinephrine is a small organic molecule and not a polypeptide.

4. Norepinephrine

Norepinephrine (noradrenaline) is structurally similar to epinephrine. It also derives from tyrosine and features a benzene ring with hydroxyl groups and an amine group.

Norepinephrine is a neurotransmitter and hormone involved in arousal, alertness, and the stress response.

Examples: Norepinephrine is used in clinical settings to treat low blood pressure and in the management of certain psychiatric disorders.

Conclusion: Norepinephrine is a small organic molecule, not a polypeptide.

Summary

Endorphins are the only hormones among the options that are polypeptides. They consist of chains of amino acids, making them distinct from:

Steroid Hormones like Androgens, which have a steroid ring structure.

Small Organic Molecules like Epinephrine and Norepinephrine, which are derived from tyrosine and have aromatic ring structures.

Endorphins function as neurotransmitters and hormones by binding to specific receptors in the body to modulate pain and mood. Their polypeptide nature is crucial for their biological activity and interaction with these receptors.

Therefore, the correct answer is: 2. Endorphin.