Proteins

Scientists have traditionally addressed protein structure at four levels: primary, secondary, tertiary, and quaternary. Primary structure is simply the linear sequence of amino acids in the peptide chain. Secondary and tertiary structure both refer to the three-dimensional shape into which a protein chain folds. The distinction is partly historical: secondary structure refers to certain highly regular arrangements of amino acids that scientists could detect as long ago as the 1950s, while tertiary structure refers to the complete three-dimensional shape. Determining a protein's tertiary structure can be difficult even today, although researchers have made major strides within the past decade.

The tertiary structure of many proteins shows a "string of beads" organization. The protein includes several compact regions known as domains, separated by short stretches where the protein chain assumes an extended, essentially random configuration. Some scientists believe that domains were originally separate proteins that, over the course of evolution, have come together to perform their functions more efficiently.

Quaternary structure refers to the way in which protein chains—either identical or different—associate with each other. For example, a complete molecule of the oxygen-carrying protein hemoglobin includes four protein chains of two slightly different types. Simple laboratory tests usually allow scientists to determine how many chains make up a complete protein molecule.