Proteinogenic amino acid

A proteinogenic amino acid is an amino acid that is incorporated into proteins. There are 20 standard proteinogenic amino acids: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid (glutamate), glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine.

Nonstandard proteinogenic amino acids
In some cases, a special configuration of mRNA causes selenocysteine to be used in place of a stop codon. Selenocysteine is capable of powerful redox reactions and is most often found serving this role for the enzymes it is found in.

Pyrrolysine is incorporated into the proteins of some microorganisms.

Other amino acids in proteins
Many amino acids are modified after they are incorporated into a protein. For example, some enzymes attach sugar groups to proteins (see glycosylation).

Other amino acids are sometimes incorporated into proteins. For example, canavanine is an amino acid that mimics arginine, making it poisonous to some worms who will incorporate it into their proteins. Use of canavanine instead of arginine creates a malformed protein that will not function properly. Certain legume plants store canavanine in their seeds as a defense mechanism against such worms.