Proteins are made up of amino acids, which are joined by peptide linkages. Although there are only 20 different naturally occurring amino acids, various combinations of these form the thousands of proteins used in metabolism.
All amino acids have a similar structure. There is a central carbon atom, called the alpha-carbon, which is bonded to an amino group on one side and a carboxyl group on the other. Also bonded to the alpha-carbon is a side chain—one of 20 different chemical groups—that gives each amino acid its unique identity and function. However, the backbone of an amino acid consists solely of the alpha-carbon, the amino group, and the carboxyl group, and this is the same for all amino acids.
The amino group consists of a nitrogen atom bonded to two hydrogenatoms: HNH. The carboxyl group is a carbon bonded to an alcohol group (—OH) and double bonded to an oxygen. This structure, O = COH, is called a carboxylic acid group.
Proteins are constructed from amino acids that are assembled by the formation of peptide bonds. The amino group of one amino acid bonds with the carboxyl group of another, eliminating one watermolecule (HOH). The bond between the two amino acids consists of a nitrogen with one hydrogen bonded to a carbon with a double bonded oxygen: HNC = O.
This simple structure, the peptide bond, is the basis of all the enzymes and proteins that make life possible.