Cysteine is a non-essential sulphur-containing non polar and hydrophobic amino acid. Disulfide bonds/bridges (S-S bonds) are covalent bonds formed by the oxidation of two thiol (sulphur) groups within cysteine residues which stabilizes the tertiary structure of proteins. Methionine being one of the most hydrophobic amino acid is usually found in the interior of the protein. Cysteine on other hand ionize to form thiolate anion, hence it forms S-S bonds.