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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 in prokaryotes (including N-Formylmethionine, mainly used to initiate protein synthesis and often removed afterward), but only 21 are encoded by the nuclear genes of eukaryotes. Of the 23, pyrrolysine (O/Pyl) is incorporated into proteins by distinct post-translational biosynthetic mechanisms; all the other 21 are directly encoded by the genetic code, including selenocysteine (U/Sec), that uses a special case of insertion during the translational incorporation, but that is not considered a post-translational modification . Humans can synthesize 11 of these 20 from each other or from other molecules of intermediary metabolism. The other nine must be consumed (usually as their protein derivatives), and so they are called essential amino acids. The essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (i.e. H I L K M F T W V). The word "proteinogenic" means "protein creating". 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, nonproteinogenic 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 post-translational 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 autoaminoacylation systems. Thus, nonproteinogenic 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 nonproteinogenic 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 can be mistakenly incorporated into proteins, such as the arginine analog canavanine. Nonproteinogenic amino acids are incorporated in nonribosomal peptides, which are not produced by the ribosome during translation. == Structures == The following illustrates the structures and abbreviations of the 21 amino acids that are directly encoded for protein synthesis by the genetic code of eukaryotes. The structures given below are standard chemical structures, not the typical zwitterion forms that exist in aqueous solutions. image:L-alanine-skeletal.png|L-Alanine (Ala / A) image:L-arginine-skeletal-(tall).png|L-Arginine (Arg / R) image:L-asparagine-skeletal.png|L-Asparagine (Asn / N) image:L-aspartic-acid-skeletal.png|L-Aspartic acid (Asp / D) image:L-cysteine-skeletal.png|L-Cysteine (Cys / C) image:L-glutamic-acid-skeletal.png|L-Glutamic acid (Glu / E) image:L-glutamine-skeletal.png|L-Glutamine (Gln / Q) image:Glycine-skeletal.png|Glycine (Gly / G) image:L-histidine-skeletal.png|L-Histidine (His / H) image:L-isoleucine-skeletal.svg|L-Isoleucine (Ile / I) image:L-leucine-skeletal.png|L-Leucine (Leu / L) image:L-lysine-skeletal.png|L-Lysine (Lys / K) image:L-methionine-skeletal.png|L-Methionine (Met / M) image:L-phenylalanine-skeletal.png|L-Phenylalanine (Phe / F) image:L-proline-skeletal.png|L-Proline (Pro / P) image:L-serine-skeletal.png|L-Serine (Ser / S) image:L-threonine-skeletal.png|L-Threonine (Thr / T) image:L-tryptophan-skeletal.png|L-Tryptophan (Trp / W) image:L-tyrosine-skeletal.png|L-Tyrosine (Tyr / Y) image:L-valine-skeletal.png|L-Valine (Val / V) IUPAC/IUBMB now also recommends standard abbreviations for the following two amino acids: image:L-selenocysteine-2D-skeletal.png|L-Selenocysteine (Sec / U) image:Pyrrolysine.svg|L-Pyrrolysine (Pyl / O) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「proteinogenic amino acid」の詳細全文を読む スポンサード リンク
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