Editing Protein folding (section)

=== Tertiary structure ===
{{Main|Protein tertiary structure}}

The α-Helices and β-Sheets are commonly amphipathic, meaning they have a hydrophilic and a hydrophobic portion. This ability helps in forming tertiary structure of a protein in which folding occurs so that the hydrophilic sides are facing the aqueous environment surrounding the protein and the hydrophobic sides are facing the hydrophobic core of the protein.<ref name="Fersht_1999" /> Secondary structure hierarchically gives way to tertiary structure formation. Once the protein's tertiary structure is formed and stabilized by the hydrophobic interactions, there may also be [[covalent bond]]ing in the form of [[disulfide bond|disulfide bridges]] formed between two [[cysteine]] residues. These non-covalent and covalent contacts take a specific [[circuit topology|topological]] arrangement in a native structure of a protein. Tertiary structure of a protein involves a single polypeptide chain; however, additional interactions of folded polypeptide chains give rise to quaternary structure formation.<ref>{{cite web | url = http://www.nature.com/scitable/topicpage/protein-structure-14122136 | title = Protein Structure | publisher = Nature Education | access-date = 2016-11-26 | work = Scitable }}</ref>