Question

Find the correct statement related to proteins.

• A. \(\beta\)-sheet structure of protein is more stable than \(\alpha\)-helical structure
• B. \(\beta\)-casein and \(\alpha\)-casein don't possess proline
• C. \(\beta\)-casein and \(\alpha_{s1}\)-casein have only secondary structure
• D. \(\alpha\)-helical structured proteins are not amphiphilic in nature

Hint:

Because caseins lack a rigid tertiary structure, they exist in milk as highly stable colloidal aggregates called casein micelles, held together by calcium phosphate bridges.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Caseins (\(\alpha_{s1}\), \(\alpha_{s2}\), \(\beta\), and \(\kappa\)) are the major class of proteins in milk.
Their unique structural properties differ significantly from globular proteins.

Step 2: Detailed Explanation:
- Caseins contain an exceptionally high number of proline residues distributed throughout their polypeptide chains.
- Proline is a cyclic amino acid that introduces sharp kinks, preventing the formation of tightly folded \(\alpha\)-helices and \(\beta\)-sheets.
- As a result, \(\beta\)-casein and \(\alpha_{s1}\)-casein lack a well-defined, folded tertiary structure.
- They are open, highly flexible, and "intrinsically disordered proteins," exhibiting only loose secondary/random coil structures.
This open structure explains why caseins are highly heat-stable (they do not denature like globular proteins) and are highly accessible to digestive proteases.

Step 3: Final Answer:
The correct statement is that \(\beta\)-casein and \(\alpha_{s1}\)-casein have only secondary structure, which corresponds to option (C).