Question

Explain the following in relation to proteins:
(i) Peptide bond
(ii) Denaturation

Hint:

Peptide bond = amide \( (-\text{CO}-\text{NH}-) \) linkage formed by loss of water between \( -\text{COOH} \) and \( -\text{NH}_2 \). Denaturation = loss of biological activity by breaking 2°/3° structure while primary structure stays intact.

Answer 1

Part (i): Peptide bond
Step 1 (What it is): A peptide bond is the amide linkage \( (-\text{CO}-\text{NH}-) \) that joins two amino acids in a protein.
Step 2 (How it forms): It is formed by a condensation reaction between the carboxyl group \( (-\text{COOH}) \) of one amino acid and the amino group \( (-\text{NH}_2) \) of the next amino acid, with the elimination of one water molecule.
\[ \text{H}_2\text{N-CHR-COOH} + \text{H}_2\text{N-CHR'-COOH} \rightarrow \text{H}_2\text{N-CHR-CO-NH-CHR'-COOH} + \text{H}_2\text{O} \]
Step 3 (Significance): The product of two amino acids is a dipeptide; many such peptide bonds link amino acids into long polypeptide chains that make up proteins.

Part (ii): Denaturation
Step 1 (Definition): Denaturation is the process in which a protein loses its biological activity when it is subjected to a physical change (such as heating) or a chemical change (such as adding acid, alkali, or heavy metal salts).
Step 2 (What changes): During denaturation the hydrogen bonds and other weak forces that maintain the secondary and tertiary structures are broken, so the protein uncoils. The native three-dimensional shape is destroyed, but the primary structure (the sequence of amino acids linked by peptide bonds) remains intact.
Step 3 (Examples): Coagulation of egg white (soluble globular protein) into a solid on boiling, and the curdling of milk, are common examples. Denaturation is generally irreversible.