A. Four Levels of Protein Structure
The correlation between form and function in proteins is an emergent property resulting from superimposed levels of protein structure:
1. Primary structure.
2. Secondary structure.
3. Tertiary structure.
When a protein has two or more polypeptide chains, it also has:
4. Quaternary structure.
1. Primary Structure
Primary structure = Unique sequence of amino acids in a protein.
a. Determination of amino acid composition by complete acid hydrolysis of peptide bonds and separation of resulting amino acids by chromatography. Using these techniques, Sanger identified the amino acids and determined the relative proportions of each.
b. Determination of amino acid sequence by partial hydrolysis with enzymes and other catalysts to break only specific peptide bonds. Sanger deductively reconstructed the primary structure from fragments with overlapping segments.
2. Secondary Structure
Secondary structure = Regular, repeated coiling and folding of a proteinís polypeptide backbone.
a. Alpha (a) Helix
Alpha (a) helix = Secondary structure of a polypeptide that is a helical coil stabilized by hydrogen bonding between every fourth peptide bond (3.6 amino acids per turn).
b. Beta (b) Pleated sheet
Beta (b) pleated sheet = Secondary protein structure which is a sheet of antiparallel chains folded into accordion pleats.
3. Tertiary Structure
Tertiary structure = Irregular contortions of a protein due to bonding between side chains (R groups); third level of protein structure superimposed upon primary and secondary structure.
Types of bonds contributing to tertiary structure are weak interactions and covalent linkage (both may occur in the same protein).
a. Weak Interactions Protein shape is stabilized by the cumulative effect of weak interactions. These weak interactions include:
Hydrophobic interactions = (Hydro = water; phobos = fear) The clustering of hydrophobic molecules as a result of their mutual exclusion from water.
b. Covalent Linkage
Disulfide bridges form between two cysteine monomers brought together by folding of the protein. This is a strong bond that reinforces conformation.
4. Quaternary Structure
Quaternary structure = Structure that results from the interaction among several polypeptides (subunits) in a single protein.