Protein

The basic units from which proteins are made.Chemically compounds with an amino group (—NH2) and a carboxyl group (—COOH) attached to the same carbon atom. Thirteen of the amino acids involved in proteins can be synthesised in the body, and so are called non-essential or dispensable amino acids, since they do not have to be provided in the diet. They are alanine, arginine, aspartic acid, asparagine, cysteine, cystine, glutamic acid, glutamine, glycine, hydroxyproline, proline, serine and tyrosine. Nine amino acids cannot be synthesised in the body at all and so must be provided in the diet; they are called the essential or indispensable amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Arginine may be essential for infants, since their requirement is greater than their ability to synthesise it.

Two of the non-essential amino acids are made in the body from essential amino acids: cysteine (and cystine) from methionine, and tyrosine from phenylalanine. A number of other amino acids also occur in proteins, including hydroxyproline, hydroxylysine, γ-carboxyglutamate and methylhistidine, but are nutritionally unimportant since they cannot be reutilised for protein synthesis. Other amino acids occur as intermediates in metabolic pathways, but are not required for protein synthesis and are nutritionally unimportant, although they may occur in foods. These include homocysteine, citrulline and ornithine. Some of the non-protein amino acids that occur in plants are toxic.

The amino acids are sometimes classified by the chemical nature of the side chain. Two are acidic: glutamic acid (glutamate) and aspartic acid (aspartate) with a carboxylic acid (—COOH) group in the side chain. Three, lysine, arginine and histidine, have basic groups in the side chain. Three, phenylalanine, tyrosine and tryptophan, have an aromatic group in the side chain. Three, leucine, isoleucine and valine, have a branched chain structure.Two, methionine and cysteine, contain sulphur in the side chain; although cysteine is not an essential amino acid, it can be synthesised only from methionine, and it is conventional to consider the sum of methionine plus cysteine (the sulphur amino acids) in consideration of protein quality.