Proteins are vital components of all forms of life, and are important to living cells both structurally and functionally. They are made up of amino acids linked together in a chain by a sequence of peptide bonds. A peptide bond links the amino group (-NH ₂) of one amino acid with the carboxyl group (-COOH) of the next amino acid in the chain. Most simple proteins contain from 100 to 300 amino acid units and have molecular weights ranging from 12,000 to 36,000. Elaborate proteins with more than one polypeptide chain may have molecular weights of several hundred thousand.

The central role of proteins in living organisms has been recognized for nearly 200 years. They make up about 90% of the dry weight of blood, 80% of the dry weight of muscle, and lesser amounts of other animal tissues and organs. They are also important constituents of plant and bacterial cells. Although they are important quantitatively...