Free SH group is present in
**Core Concept**
Free SH group, also known as a sulfhydryl group, is a functional group in biochemistry that contains a sulfur atom bonded to a hydrogen atom. This group is crucial in proteins, particularly in cysteine residues, where it plays a significant role in the structure and function of proteins.
**Why the Correct Answer is Right**
The presence of free SH groups in certain proteins allows them to form disulfide bonds, which are crucial for maintaining the tertiary structure of proteins. These bonds are formed between the SH groups of two cysteine residues, resulting in a covalent link that stabilizes the protein's 3D structure. This is particularly important in proteins that function in the extracellular environment, such as enzymes, hormones, and antibodies.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because free SH groups are not typically associated with carbohydrate molecules.
**Option B:** Incorrect because free SH groups are not a characteristic of lipid molecules.
**Option C:** Incorrect because free SH groups are not typically associated with nucleic acid molecules.
**Clinical Pearl / High-Yield Fact**
Free SH groups are susceptible to oxidation, which can lead to the formation of disulfide bonds. This process, known as disulfide bond formation, is essential for the proper folding and function of many proteins.
**Correct Answer:**
C. Cysteine