* Primary Structure: The linear sequence of amino acids in the insulin molecule.

* Secondary Structure: Insulin contains both alpha-helices and beta-sheets, which are formed by hydrogen bonding between amino acids.

* Tertiary Structure: The three-dimensional folding of a single polypeptide chain. Insulin has a complex tertiary structure that includes disulfide bonds.

* Quaternary Structure: The arrangement of multiple polypeptide chains. Insulin is composed of two polypeptide chains, called the A chain and the B chain, linked by disulfide bonds.

Therefore, insulin exhibits all four levels of protein structure, with its quaternary structure being the defining characteristic of its functional form.