Proteins

Glycine is optically inactive amino acid due to absence of chiral carbon atom.

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 N-CH -COOH. The -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 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 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).