(i) Peptide linkage:

An amide formed between the -COOH group of one amino acid and -NH₂ group of another amino acid. In this process water is eliminated as shown in the reaction below. This bond formed is called the peptide linkage.

(ii) Primary structure:

The primary structure of a protein refers to the sequence amino acids present in the protein. The sequence in which amino acids are arranged is different in each protein. A change in the sequence creates a different protein.

(iii) Denaturation:

A protein has a unique 3-dimensional structurewhich confers its biological activity. In such a situation, the protein is called native protein. However, when the native protein is subjected to physical changes such as change in temperature or chemical changes such as change in pH, its H-bonds are disturbed and they are broken. This disturbance leads to unfolding and opening up of the protein. As a result, the protein loses its biological activity. This condition of the protein is called denaturation. During denaturation, the secondary and the tertiary structures of the protein gets destroyed, but the primary structure remains unaltered as it is determined by the gene sequence.

Example of protein denaturation is the coagulation of egg white when an egg is boiled.