Which of the following amino acid contains free sulphydryl group –
**Core Concept**
The free sulphydryl group is a crucial functional group found in certain amino acids, which plays a significant role in various biochemical reactions, including enzyme activity and protein folding. This group is also known as a thiol (-SH) group.
**Why the Correct Answer is Right**
The correct answer is **Cysteine**. Cysteine contains a free sulphydryl group, which is essential for its function as a reducing agent in the body. The sulphydryl group in cysteine is responsible for its ability to form disulfide bonds with other cysteine residues, which helps in stabilizing the tertiary structure of proteins. This property makes cysteine an essential amino acid for various physiological processes.
**Why Each Wrong Option is Incorrect**
* **Option A:** Methionine is an amino acid that contains a sulphydryl group in its side chain, but it is not a free sulphydryl group.
* **Option B:** Lysine is a basic amino acid that does not contain a sulphydryl group.
* **Option D:** Histidine is an amino acid that contains an imidazole ring, but it does not have a free sulphydryl group.
**Clinical Pearl / High-Yield Fact**
Cysteine is an essential amino acid, and its deficiency can lead to various health problems, including decreased glutathione levels, which is a critical antioxidant in the body.
**Correct Answer:** C. Cysteine