Insulin is a protein hormone, crucial for maintaining blood sugar at appropriate levels. (Below) Chains of amino acids in a specific sequence (the primary structure) define a protein such as insulin. Amino acids close to one another within this sequence can fold into regular structures (the secondary structure), such as the α-helix. Entire chains fold into well-defined structures (the tertiary structure)—in this case, a single insulin molecule. Such structures assemble with other chains to form arrays such as the complex of six insulin molecules shown at the far right (the quaternary structure). These arrays can often be induced to form well-defined crystals (photograph at left), which allows a determination of these structures in detail.

[Photograph from Christo Nanev.]