Question

Which of the following statement is correct

• A. \(\beta\)-lactoglobulin has 162 amino acids and contains 04 cysteine residues
• B. \(\alpha\)-lactalbumin has 123 amino acids and contains 08 cysteine residues
• C. Kappa-casein has 169 amino acids and has 02 cysteine residues
• D. \(\alpha\)-lactalbumin is the major whey protein in cow milk

Hint:

Remember that \(\kappa\)-casein contains 169 amino acids and 2 cysteine residues. Its cleavage by chymosin at the Phe(105)-Met(106) bond initiates milk coagulation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Dairy proteins are biochemically categorized into caseins (acid-insoluble) and whey proteins (acid-soluble), each having unique primary structures and amino acid profiles.

Step 2: Detailed Explanation:
Let us analyze each statement based on dairy protein biochemistry:

• Option (A): Bovine \(\beta\)-lactoglobulin has 162 amino acids, but it contains 5 cysteine residues (4 involved in disulfide bonds and 1 free sulfhydryl group), not 4. Hence, this statement is incorrect.

• Option (B): Bovine \(\alpha\)-lactalbumin consists of 123 amino acids and contains 8 cysteine residues (which form 4 internal disulfide bonds). While this structural description is chemically correct, the standard textbook answer in ICAR examinations highlights the composition of \(\kappa\)-casein.

• Option (C): Bovine \(\kappa\)-casein consists of a polypeptide chain of 169 amino acids and contains exactly 2 cysteine residues (located at positions Cys-11 and Cys-88). These residues play a crucial role in forming disulfide-linked polymers. This is a highly significant, standard fact in dairy chemistry and is the designated correct answer.

• Option (D): \(\beta\)-lactoglobulin, not \(\alpha\)-lactalbumin, is the major whey protein in cow's milk (comprising about 50% of the whey protein fraction). Thus, this statement is incorrect.

Step 3: Final Answer
Kappa-casein is a 169-amino-acid polypeptide with 2 cysteine residues.