Primary structure of proteins refer to the sequence in which the amino acids are arranged in them. Proteins have specific sequence of amino acids which are essential for a particular biological activity. The change in even one amino acid can alter the biological activity.

Secondary structures of protein refer to the arrangement of the polypeptide chain giving rise to a particular shape, which arises due to hydrogen bonding.

The two common secondary structures of proteins are α-helix and β-pleated sheet structure.

An α helix is a right-handed helix that is held together by hydrogen bonding. In this structure the hydrogen bond is formed between the N-H on one amino acid and the C=O on another amino acid 4 residues away.

The β sheet is formed when beta strands are linked together by hydrogen bonds, forming a pleated sheet of amino acid residues. Again, the hydrogen bonds are between the N-H group of one amino acid and the C=O group of another.

Structures of α-helix and β-pleated sheet.