iGen 06-04

Four levels of Protein Structure

(a) The primary structure is the succession of amino acid residues, usually abbreviated by the 1- or 3-letter codes. (b) The secondary structure is the 3-D arrangement of the right-handed alpha helix (shown here), or alternative structures such as a beta-pleated sheet. (c) The tertiary structure is the 3-D folding of the alpha helix (show as a purple ribbon), shaped by structures such as proline corners, disulfide bridges between cysteine residues, and electrostic bonds. [The inorganic heme group is part of the beta-globin polypeptide, but is not a typical part of the tertiary structure]. (d) Where more than one protein chain contributes to the protein, the quaternary structure is the arrangement of these subunits. In the case of hemoglobin shown here, the quaternary structure comprises two alpha and two beta polypeptides, held together by elecrostatic bonds.

Figure after © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2010 by Steven M. Carr