Passage VI

A polypeptide molecule is a chain of amino acids. A protein consists of 1 or more polypeptides. A protein’s shape is described by 3 or 4 levels of structure.

1. The primary structure of a protein is the sequence of amino acids in each polypeptide.

2. The secondary structure of a protein is the local folding patterns within short segments of each polypeptide due to hydrogen bonding (weak chemical bonds).

3. The tertiary structure is the folding patterns that result from interactions between amino acid side chains (parts of an amino acid) in each polypeptide. These folding patterns generally occur across greater distances than those associated with the secondary structure.

4. The quaternary structure is the result of the clustering between more than 1 folded polypeptide.

A protein can adopt different shapes, and each shape has a relative energy. Lower-energy shapes are more stable than higher-energy shapes, and a protein with a relatively high-energy shape may denature (unfold) and then renature (refold), adopting a more stable shape. A protein that is almost completely denatured is called a random coil. Random coils are unstable because they are high-energy shapes; however, some can renature, adopting more stable shapes.

Two scientists discuss protein shape.

Scientist 1

The active shape (the biologically functional shape) of a protein is always identical to the protein’s lowest-energy shape. Any other shape would be unstable. Because a protein’s lowest-energy shape is determined by its primary structure, its active shape is determined by its primary structure.

Scientist 2

The active shape of a protein is dependent upon its primary structure. However, a protein’s active shape may also depend on its process of synthesis, the order (in time) in which the amino acids were bonded together. As synthesis occurs, stable, local structures form within short segments of the polypeptide chain due to hydrogen bonding. These local structures may be different than the local structures associated with the protein’s lowest-energy shape. .After synthesis, these structures persist, trapping the protein in an active shape that has more energy than its lowest-energy shape.