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Biology LibreTexts

B3. Levels of Protein Structure

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)