important class of organic compounds that contain both the amino (—NH2) and carboxyl (—COOH) groups. Of these acids, 20 serve as the building blocks of proteins (see PROTEIN,). Known as the standard, or alpha, amino acids, they comprise alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. All 20 are constructed according to a general formula:

As the formula shows, the amino and carboxyl groups are both attached to a single carbon atom, which is called the alpha carbon atom. Attached to the carbon atom is a variable group (R); it is in their R groups that the molecules of the 20 standard amino acids differ from one another. In the simplest of the acids, glycine, the R consists of a single hydrogen atom. Other amino acids have more complex R groups that contain carbon as well as hydrogen and may include oxygen, nitrogen, or sulfur, as well.

When a living cell makes protein, the carboxyl group of one amino acid is linked to the amino group of another to form a peptide bond. The carboxyl group of the second amino acid is similarly linked to the amino group of a third, and so on, until a long chain is produced. This chain-like molecule, which may contain from 50 to several hundred amino acid subunits, is called a polypeptide. A protein may be formed of a single polypeptide chain, or it may consist of several such chains held together by weak molecular bonds. Each protein is formed according to a precise set of instructions contained within the nucleic acid (see NUCLEIC ACIDS,), which is the genetic material of the cell. These instructions determine which of the 20 standard amino acids are to be incorporated into the protein, and in what sequence. The R groups of the amino acid subunits determine the final shape of the protein and its chemical properties; an extraordinary variety of proteins can be produced from the same 20 subunits.

The standard amino acids serve as raw materials for the manufacture of many other cellular products, including hormones (see HORMONE,) and pigments. In addition, several of these amino acids are key intermediates in cellular metabolism (see METABOLISM,).

Most plants and microorganisms are able to use inorganic compounds to make all the amino acids they require for normal growth. Animals, however, must obtain some of the standard amino acids from their diet in order to survive; these particular amino acids are called essential. Essential amino acids for humans include lysine, tryptophan, valine, histidine, leucine, isoleucine, phenylalanine, threonine, methionine, and arginine. They are found in adequate amounts in protein-rich foods from animal sources or in carefully chosen combinations of plant proteins.

In addition to the amino acids that form proteins, more than 150 other amino acids have been found in nature, including some that have the carboxyl and amino groups attached to separate carbon atoms. These unusually structured amino acids are most often found in fungi and higher plants.        M.L.H., MARY LYNN HENDRIX, B.A.

For further information on this topic, see the Bibliography, sections 415. Organic chemistry, 439. Biochemistry, 443. Cell–444. Nucleic acids.