Proteins are probably the most important class of biochemical molecules, although of course lipids and carbohydrates are also essential for life. Proteins are the basis for the major structural components of animal and human tissue. Proteins are natural polymer molecules consisting of amino acid units. The number of amino acids in proteins may range from two to several thousand.
Although we are studying only about 20 amino acids, there are about six more found in the body. Many others are also known from a variety of sources. Amino acids are the building blocks used to make proteins and peptides. The different amino acids have interesting properties because they have a variety of structural parts which result in different polarities and solubilities.
Each amino acid has at least one amine and one acid functional group as the name implies. See graphic on the left. The different properties result from variations in the structures of different R groups. The R group is often referred to as the amino acid "side chain". Amino acids have special common names, however, a three letter abbreviation for the name is used most of the time. Consult the amino acid table on the next page for structure, names, and abbreviations.
Amino acid physical properties indicate a "salt-like" behavior. Amino acids are crystalline solids with relatively high melting points, and most are quite soluble in water and insoluble in non-polar solvents. In solution, the amino acid molecule appears to have a charge which changes with pH. An intramolecular neutralization reaction leads to a salt-like ion called a zwitterion. The accepted practice is to show the amino acids in the zwitterion form:
- The carboxyl group can lose a hydrogen ion to become negatively charged.
- The amine group can accept a hydrogen ion to become positively charged.
- Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook