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.

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.