A protein can be considered to have primary, secondary, tertiary, and quaternary structures.
- primary structure: the linear amino acid sequence of a protein
- secondary structure: regular repeating structures arising when hydrogen bonds between the peptide backbone amide hydrogens and carbonyl oxygens occur at regular intervals within a given linear sequence (strand) of a protein (as in the alpha helix) or between two adjacent strands (as in beta sheets and reverse turns)
Figure: Secondary Structure (purple -alpha helices, yellow - beta strands. Image made with VMD)
- tertiary structure: the overall three dimensional shape of a protein, often represented by a backbone trace
Figure: tertiary structure (calmodulin - image made with VMD)
- quaternary structure: oligomeric structure of a multisubunit protein in which separate proteins chains associate to form dimers, trimers, tetramers, and other oligomers. The different chains in the oligomers may be the same protein (homooligomers) or a combination of different protein chains (heteroliogomers). The different chains within the oligomer may be held together by noncovalent intermolecular forces or may also contain covalent interchain disulfides.
Figure: Quaternary structure (4 chains of hemoglobin - Image made with VMD)