Amino acid

An amino acid is a building block for proteins. Any molecule containing both an amino group and a carboxylic acid group is an amino acid in chemistry, however, the term typically refers to the group of amino acids that are used to make proteins. The name 'acid' refers to the carboxylic acid group only - amino acids can be bases if their side chain is basic.

Amino acids in human protein
There are 20 amino acids in human proteins. These are the proteinogenic amino acids. They are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid (glutamate), glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. These are the proteinogenic amino acids.

Selenocysteine is coded for in humans, however, it has a more complex coding system than the normal 20 amino acids. Selenocysteine's code is UGA, which is usually a 'stop' codon. Other features of the mRNA signal to the cell to insert a selenocysteine instead.

Non standard amino acids in protein
One additional amino acid is incorporated into the proteins of some microorganisms. Pyrrolysine is used by certain archaea, particularly in the enzymes that produce methane.

Other amino acids
Many amino acids are involved in human metabolism that are not incorporated into proteins. These include homocysteine, ornithine, and creatine. Taurine is often considered an amino acid, although it has a sulfonic acid group instead of a carboxylic acid.