Question

Casein is rich in

• A. Tryptophan
• B. Phenylalanine
• C. Proline
• D. Glycine

Hint:

High proline content prevents casein from forming rigid secondary/tertiary structures, rendering it resistant to heat coagulation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Casein is the primary phosphoprotein family in bovine milk, comprising $\alpha_{s1}$-, $\alpha_{s2}$-, $\beta$-, and $\kappa$-caseins.

Step 2: Detailed Explanation:
Casein contains an exceptionally high proportion of proline residues (up to $17\%$ in $\beta$-casein and $\approx 10\text{--}12\%$ across all casein fractions).
The rigid cyclic pyrrolidine ring of proline prevents the polypeptide backbone from folding into regular $\alpha$-helices or $\beta$-pleated sheets.
This structural feature imparts an open, flexible, rheomorphic conformation that confers extraordinary stability against heat denaturation.

Step 3: Final Answer:
Therefore, casein is rich in proline.