Question

A\(^1\) and A\(^2\) milk differ in the amino acid residue at 67\(^{th}\) position in bovine \(\beta\)-CN. In the following options which is the correct statement regarding amino acid residues at 67\(^{th}\) position?

• A. A\(^2\) - Histidine; A\(^1\) - Proline
• B. A\(^2\) - Proline; A\(^1\) - Histidine
• C. A\(^2\) - Methionine; A\(^1\) - Proline
• D. A\(^2\) - Histidine; A\(^1\) - Methionine

Hint:

Remember:
- A2 = Proline (ancestral, safe, no BCM-7 release).
- A1 = Histidine (mutated, allows enzymatic cleavage to release BCM-7).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Bovine beta-casein (\(\beta\)-CN) is a major milk protein consisting of 209 amino acids.
Due to genetic mutations over time, several variants of beta-casein have emerged, with A1 and A2 being the most common.
These two variants differ by a single amino acid substitution at position 67 of the polypeptide chain.

Step 2: Detailed Explanation:
Let us identify the specific amino acid residues at position 67:
- A2 Milk Variant: The original, ancestral form of beta-casein contains the amino acid Proline at the 67th position.
Proline forms a strong peptide bond with the adjacent amino acid, preventing enzymatic cleavage at this site during digestion.
- A1 Milk Variant: A genetic mutation led to the substitution of proline with Histidine at the 67th position.
Because histidine forms a weaker peptide bond, digestive enzymes (such as elastase) can cleave the protein chain at this site, releasing a bioactive 7-amino-acid peptide called beta-casomorphin-7 (BCM-7).
Therefore, the correct pair is: A\(^2\) - Proline; A\(^1\) - Histidine.

Step 3: Final Answer:
At the 67th position of beta-casein, A\(^2\) contains Proline, and A\(^1\) contains Histidine.