Explain the concepts of primary, secondary, tertiary and quaternary

A protein's function is determined by its structure. The structure of a protein can be described in four different ways, the most basic of which is the primary structure.

Proteins are made up of polypeptide chains, and polypeptides are made up of amino acids linked by peptide bonds. The primary structure of a protein is simply the order of amino acids in the polypeptide. Hydrogen bonds are one type of interaction between amino acids, causing polypeptides to adopt a certain form of secondary structure, either alpha-helixes (where bonds are formed between neighbouring amino acids) or beta pleated sheets (where bonds are formed between amino acids in different areas of the polypeptide). Tertiary structure describes the 3D shape of the protein in space. It is determined by interactions between the residual groups on each amino acid, such as Van der Waals forces, disulphide bridges and ionic bonds. Often, proteins are made up of more than one polypeptide chain - either multiple copies of the same chain, or different polypeptides coming together. The number and arrangement of these subunits are described by the quaternary structure.