**Question:** Folding of nascent polypeptide chains is the function of

A. Protein Disulfide Isomerase (PDI)
B. Ribosome
C. Chaperone Proteins
D. Calcium Binding Proteins

**Core Concept:**

Nascent polypeptide chains are the newly synthesized polypeptide chains that are still attached to the ribosome during translation. After synthesis, these chains need to fold into their correct 3D structure for proper function or interaction with other molecules. This process is crucial for proteins to carry out their biological roles accurately.

**Why the Correct Answer is Right:**

Protein folding is essential for protein stability, function, and interaction with other molecules. In the case of C, chaperone proteins play a critical role in this process. They prevent aggregation of nascent polypeptide chains and facilitate their proper folding. This ensures that the proteins reach their functional conformation without any adverse effects.

**Why Each Wrong Option is Incorrect:**

A. Protein Disulfide Isomerase (PDI): PDI is involved in the formation, reduction, isomerization, and disulfide bond formation during protein folding, particularly for secretory proteins with disulfide bonds. However, it does not directly fold nascent polypeptide chains but aids in the folding process.

B. Ribosome: The ribosome is the site of protein synthesis during translation, translating the genetic information into a polypeptide chain. Although the ribosome synthesizes the polypeptide chain, it does not directly fold the nascent polypeptide chains.

D. Calcium Binding Proteins: These proteins bind to calcium ions, and their function is unrelated to the folding of nascent polypeptide chains. They do not directly facilitate protein folding but are involved in calcium ion binding.

**Clinical Pearl:**

Understanding protein folding is crucial for understanding diseases like cystic fibrosis, which results from mutations in the CFTR gene encoding the cystic fibrosis transmembrane conductance regulator protein. Mutations in this protein lead to abnormal protein folding and impaired function, causing the clinical manifestations of cystic fibrosis.

In summary, chaperone proteins like Hsp70 and Hsp90 play a vital role in protein folding, ensuring that proteins reach their functional conformation without causing adverse effects.