The four structural levels of proteins are primary structure, secondary structure, tertiary structure and quaternary structure.
Primary structure
Primary structure is the number and sequence of amino acids or peptides. An example of primary structure is a polypeptide.
Secondary structure
Secondary structure involves coiling of a polypeptide chain in a helical or parallel structure which is stabilised by hydrogen bonds. An example of secondary structure is a fibrous protein.
Tertiary structure
Tertiary structure involves folding of polypeptide into a more compact, globular structure which is stabilised by disulphide, ionic and hydrogen bonds. An example of tertiary structure is a globular protein.
Quaternary structure
Quaternary structure involves combining two or more polypeptide chains to form a larger, more complex molecule which is stabilised by disulphide, ionic and hydrogen bonds. Non protein molecules are usually integrated onto this complex, forming a conjugated protein. An example of quaternary structure is haemoglobin.