**Question:** Folding of protein chain is due to:

A. Hydrogen bonding
B. Ionic interactions
C. Hydrophobic interactions
D. Electrostatic interactions

**Core Concept:** Protein folding is the process by which a linear polypeptide chain is converted into its functional three-dimensional structure. This process is crucial for the proper functioning and interaction of proteins with other molecules. Proteins can fold spontaneously due to non-covalent interactions between amino acid residues.

**Why the Correct Answer is Right:**
Protein folding is primarily driven by hydrophobic interactions and hydrogen bonding. Amino acids have hydrophilic (water-loving) side chains and hydrophobic (water-fearing) side chains. In the native protein structure, hydrophobic amino acids are sequestered within the protein core, while hydrophilic side chains face the solvent (water). This arrangement minimizes the contact between the hydrophobic amino acids and the solvent, thus reducing energy expenditure and stabilizing the protein structure. Hydrogen bonding between amino acids also contributes to protein folding and stability.

**Why Each Wrong Option is Incorrect:**

A. Hydrogen bonding alone is not sufficient for protein folding, although it plays a role in stabilizing the folded structure.
B. Ionic interactions, although present in proteins, are not the primary driving force behind protein folding.
C. Hydrophobic interactions are the primary driving force behind protein folding.
D. Electrostatic interactions, while important for some specific proteins and conditions, are not the main force responsible for protein folding.

**Clinical Pearl:**
Understanding protein folding is essential for understanding protein function, structure, and stability. Knowledge of the role of different factors in protein folding is crucial for understanding various protein diseases, such as cystic fibrosis, caused by mutations in cystic fibrosis transmembrane conductance regulator (CFTR) protein folding and interactions with chaperone proteins.

**Correct Answer:** C. Hydrophobic interactions

**Why Each Wrong Option is Incorrect:**
A. Hydrogen bonding is important for protein folding, but hydrophobic interactions are the primary driving force.
B. Ionic interactions are not the main driving force behind protein folding.
C. Hydrophobic interactions are the primary driving force behind protein folding.
D. Electrostatic interactions are not the main force responsible for protein folding.