**2. The properties of protein and peptide**

A peptide is made up of short polymers of ⍺-amino acid, which is around 20 to 50 amino acids. The function of small peptides depends on the functional group of various amino acids. Examples of active peptides are glutathione, bradykinin, angiotensin, vasopressin and oxytocin [4].

Protein is a macromolecular and high molecular weight polypeptide, which is made up of long-chain amino acids (more than 50) arranged in a linear chain through peptide bonds [50]. It can exist in four different structural conformations such as primary, secondary, tertiary, and quaternary. The formation of these structures is dependent on the intermolecular interaction between functional groups of amino acids [51], through covalent bonds or non-covalent bonds.

The covalent bonds are strong bonds which include peptide bonds and disulfide bonds [51]. Peptide bonds are interactions between two consecutive amino acids through amino and carboxyl groups. Meanwhile, disulfide bonds link two cysteine residues through sulphhydryl groups [52].

On the other hand, non-covalent bonds are weak bonds that include hydrogen, electrostatic and hydrophobic bonds. Hydrogen bonds link two different peptides with the hydrogen atom of the N-H group and oxygen of the carboxylic group. Hydrophobic bonds will occur if the hydrophobic nature between non-polar side chains of amino acid interacts with each other [51].
