Question

The number of amino acid residues in glycomacropeptide (GMP) are

• A. 44
• B. 54
• C. 64
• D. 84

Hint:

To remember this quickly: $\kappa$-casein is 169 residues long. Rennet cleaves at the Phe105-Met106 bond. The C-terminal part released is GMP, which is $169 - 105 = 64$ residues long.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Glycomacropeptide (GMP), also known as caseinomacropeptide (CMP), is a biologically active sialic acid-containing phosphorylated glycopeptide.
It is released into the sweet whey during the enzymatic coagulation phase of cheese making.
This reaction is initiated by the action of chymosin (rennet) on milk $\kappa$-casein.
The enzyme specifically cleaves a single peptide bond within $\kappa$-casein, dividing it into two distinct parts: para-$\kappa$-casein and GMP.

Step 2: Detailed Explanation:
$\kappa$-Casein is a key protein in milk consisting of a polypeptide chain of 169 amino acid residues.
During the primary phase of rennet coagulation, chymosin highly specifically hydrolyzes the peptide bond between phenylalanine at position 105 (Phe105) and methionine at position 106 (Met106) of $\kappa$-casein.
This cleavage yields two main fragments:
Para-$\kappa$-casein, which contains amino acid residues 1 to 105 and remains associated with the curd micelle.
Glycomacropeptide (GMP), which contains the hydrophilic C-terminal residues from position 106 to 1
Calculating the difference:
\[ 169 - 106 + 1 = 64 \text{ residues} \]
Thus, the resulting glycomacropeptide consists of exactly 64 amino acid residues.
GMP is rich in branched-chain amino acids but completely lacks aromatic amino acids like phenylalanine, tryptophan, and tyrosine, making it highly valuable in specialized clinical diets (such as for phenylketonuria patients).

Step 3: Final Answer
The number of amino acid residues in glycomacropeptide (GMP) is