Figure 2 Zwitterionic structure of an amino acid.

acids, and creatine; it acts as a neurotransmitter and as a conjugating substance that aids the excretion of xenobiotics by making them more water-soluble. Alanine is the transamination product of pyruvic acid and is thus closely associated with the metabolism of carbohydrates, acting as a major precursor for gluconeogenesis.

The a-carbon atom is asymmetric so that amino acids show stereoisomerism; the exception to this is glycine, in which the R group is a second hydrogen atom. Most of the amino acids found in nature are in the l form, and only l-amino acids can be used for protein synthesis in higher organisms. However, d-amino acids may be ingested from bacterial sources, and if high concentrations accumulate they may be toxic. The human body has a d-amino acid oxidase enzyme, found in the liver and the kidney, that disposes of these molecules by oxidative deamination.

The most important common chemical property of the amino acids is their ability to form peptide bonds with one another. The a-amino group of one amino acid reacts with the a-carboxyl group of another to form a peptide bond with the elimination of water (Figure 3). The results of this process are conventionally known as peptides or oligopeptides if they contain 2-20 amino acid residues or as polypeptides, which may contain 21 to several thousand amino acid residues. The polypeptides may undergo further processing, including chemical modification, before taking up their final conformation as proteins.

Each amino acid also has specific chemical properties that depend on the nature of the R group. This affects the behavior of the free amino acids and the corresponding amino acid residues in peptides and polypeptides. For convenience, the amino acids may be considered in groups according to some common properties.

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