Question

Incorrect statement about \(\alpha\)-amino acids of proteins among the following is

• A. Methionine is an essential amino acid
• B. Glycine doesn't exhibit enantiomerism
• C. Glycylalanylglutamine has three amide linkages
• D. Zwitterion of valine exhibits amphoteric behaviour

Hint:

For a polypeptide chain, the number of peptide (amide) linkages is always one less than the number of amino acid residues. A dipeptide has one linkage, a tripeptide has two, a tetrapeptide has three, and so on. Number of linkages = n - 1, where n is the number of amino acids.

Answer 1

The Correct Option is C

Step 1: Evaluate each statement.
(A) Methionine is an essential amino acid: Essential amino acids are those that cannot be synthesized by the human body and must be obtained from the diet. Methionine is one of the nine essential amino acids. This statement is correct.
(B) Glycine doesn't exhibit enantiomerism: Enantiomerism (optical activity) occurs in molecules that have a chiral center - a carbon atom bonded to four different groups. The structure of glycine is H\(_2\)N-CH\(_2\)-COOH. The \(\alpha\)-carbon is bonded to two hydrogen atoms, one amino group, and one carboxyl group. Since it is not bonded to four *different* groups, it is not chiral. Therefore, glycine does not exhibit enantiomerism. This statement is correct.
(C) Glycylalanylglutamine has three amide linkages: This is a tripeptide, meaning it is formed from three amino acids: Glycine, Alanine, and Glutamine. A peptide bond (which is an amide linkage) is formed between each pair of adjacent amino acids.

• A bond forms between Glycine and Alanine. (1st amide linkage)
• A bond forms between Alanine and Glutamine. (2nd amide linkage)

A tripeptide has two peptide (amide) linkages connecting the three amino acid residues. Glutamine itself has an amide group in its side chain, but the question asks about the amide *linkages* forming the peptide backbone. The statement says there are three amide linkages, which is incorrect. This statement is incorrect.
(D) Zwitterion of valine exhibits amphoteric behaviour: A zwitterion is a molecule that has both a positive charge (on the -NH\(_3\)\(^+\) group) and a negative charge (on the -COO\(^-\) group). This structure can react with both acids (by accepting a proton at the -COO\(^-\) group) and bases (by donating a proton from the -NH\(_3\)\(^+\) group). A substance that can act as both an acid and a base is called amphoteric. This statement is correct.
Step 2: Final Answer.
The incorrect statement is (C).