proteinogenic amino acids are amino acids that are precursors to proteins, and are incorporated into proteins cotranslationally - that is, during translation. there are 23 proteinogenic amino acids, but only 20 are encoded by the nuclear genes of eukaryotes. of the 23, selenocysteine and pyrrolysine are incorporated into proteins by distinct post-translational biosynthetic mechanisms, and n-formylmethionine is often the initial amino acid of proteins in bacteria, mitochondria, and chloroplasts, but is often removed post-translationally. the other 20 are directly encoded by the genetic code. humans can synthesize 11 of these 20 from each other or from other molecules of intermediary metabolism. the other 9 must be consumed (usually as their protein derivatives) in the diet and so are thus called essential amino acids. the essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. the word proteinogenic means "protein building". proteinogenic amino acids can be condensed into a polypeptide (the subunit of a protein) through a process called translation (the second stage of protein biosynthesis, part of the overall process of gene expression). in contrast, non-proteinogenic amino acids are either not incorporated in proteins (like gaba, l-dopa, or triiodothyronine), or are not produced directly and in isolation by standard cellular machinery (like hydroxyproline and selenomethionine). the latter often results from posttranslational modification of proteins. the proteinogenic amino acids have been found to be related to the set of amino acids that can be recognized by ribozyme auto-aminoacylation systems. thus, non-proteinogenic amino acids would have been excluded by the contingent evolutionary success of nucleotide-based life forms. other reasons have been offered to explain why certain specific non-proteinogenic amino acids are not generally incorporated into proteins: for example, ornithine and homoserine cyclize against the peptide backbone and fragment the protein with relatively short half-lives, while others are toxic because they c
Nitrogen_metabolism Nutrition Proteinogenic_amino_acids