amino acids

amino acids 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.

Eleven of the amino acids involved in proteins can be synthesized 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, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.

Nine amino acids cannot be synthesized 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. In addition, arginine may be essential for infants, since their requirement is greater than their ability to synthesize 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.

The limiting amino acid of a protein is that essential amino acid present in least amount relative to the requirement for that amino acid. The ratio between the amount of the limiting amino acid in a protein and the requirement for that amino acid provides a chemical estimation of the nutritional value (protein quality) of that protein, termed chemical score. Most cereal proteins are limited by lysine, and most animal and other vegetable proteins by the sum of methionine + cysteine (the sulphur amino acids). In whole diets it is usually the sulphur amino acids that are limiting.

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 re‐utilized 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.

The amino acids can be 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 side‐chains. Three, phenylalanine, tyrosine, and tryptophan, have aromatic side‐chains. Three, leucine, isoleucine, and valine, have a branched side‐chain. These three have very similar metabolism, and a rare genetic disease affecting their metabolism results in maple syrup urine disease. Two, methionine and cysteine, contain sulphur in the side‐chain; although cysteine is not an essential amino acid, it can only be synthesized from methionine, and it is conventional to consider the sum of methionine plus cysteine (the sulphur amino acids) in respect to protein quality.

An alternative classification of the amino acids is by their metabolic fate; whether they can be utilized for glucose synthesis or not. Those that can give rise to glucose are termed glucogenic (or sometimes antiketogenic); those that give rise to ketones or acetate when they are metabolized are termed ketogenic. Only leucine and lysine are purely ketogenic; isoleucine, phenylalanine, tyrosine, and tryptophan give rise to both ketogenic and glucogenic fragments; the remainder are purely glucogenic.