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.