Editing Amino acid (section)

==Occurrence and functions in biochemistry==
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| image5            = Protein primary structure.svg
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| caption5          = A [[polypeptide]] is an unbranched chain of amino acids.

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| caption7          = The amino acid [[selenocysteine]]
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===Proteinogenic amino acids===
{{main|Proteinogenic amino acid}} {{See also|Protein primary structure|Posttranslational modification}}

Amino acids are the precursors to proteins.<ref name="NIGMS"/> They join by condensation reactions to form short polymer chains called peptides or longer chains called either polypeptides or proteins. These chains are linear and unbranched, with each amino acid residue within the chain attached to two neighboring amino acids. In nature, the process of making proteins encoded by RNA genetic material is called ''[[translation (biology)|translation]]'' and involves the step-by-step addition of amino acids to a growing protein chain by a [[ribozyme]] that is called a [[ribosome]].<ref>{{cite journal | vauthors = Rodnina MV, Beringer M, Wintermeyer W | title = How ribosomes make peptide bonds | journal = Trends in Biochemical Sciences | volume = 32 | issue = 1 | pages = 20–26 | date = January 2007 | pmid = 17157507 | doi = 10.1016/j.tibs.2006.11.007 }}</ref> The order in which the amino acids are added is read through the [[genetic code]] from an [[Messenger RNA|mRNA]] template, which is an [[RNA]] derived from one of the organism's [[gene]]s.

Twenty-two amino acids are naturally incorporated into polypeptides and are called [[proteinogenic]] or natural amino acids.<ref name="Creighton" /> Of these, 20 are encoded by the universal genetic code. The remaining 2, [[selenocysteine]] and [[pyrrolysine]], are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a [[SECIS element]], which causes the UGA codon to encode selenocysteine instead of a stop codon.<ref>{{cite journal | vauthors = Driscoll DM, Copeland PR | title = Mechanism and regulation of selenoprotein synthesis | journal = Annual Review of Nutrition | volume = 23 | issue = 1 | pages = 17–40 | year = 2003 | pmid = 12524431 | doi = 10.1146/annurev.nutr.23.011702.073318 }}</ref> [[Pyrrolysine]] is used by some [[methanogen]]ic [[archaea]] in enzymes that they use to produce [[methane]]. It is coded for with the codon UAG, which is normally a stop codon in other organisms.<ref>{{cite journal | vauthors = Krzycki JA | title = The direct genetic encoding of pyrrolysine | journal = Current Opinion in Microbiology | volume = 8 | issue = 6 | pages = 706–712 | date = December 2005 | pmid = 16256420 | doi = 10.1016/j.mib.2005.10.009 }}</ref>

Several independent evolutionary studies have suggested that Gly, Ala, Asp, Val, Ser, Pro, Glu, Leu, Thr may belong to a group of amino acids that constituted the early genetic code, whereas Cys, Met, Tyr, Trp, His, Phe may belong to a group of amino acids that constituted later additions of the genetic code.<ref>{{cite journal | vauthors = Wong JT | title = A co-evolution theory of the genetic code | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 72 | issue = 5 | pages = 1909–1912 | date = May 1975 | pmid = 1057181 | pmc = 432657 | doi = 10.1073/pnas.72.5.1909 | doi-access = free | bibcode = 1975PNAS...72.1909T }}</ref><ref>{{cite journal | vauthors = Trifonov EN | title = Consensus temporal order of amino acids and evolution of the triplet code | journal = Gene | volume = 261 | issue = 1 | pages = 139–151 | date = December 2000 | pmid = 11164045 | doi = 10.1016/S0378-1119(00)00476-5 }}</ref><ref>{{cite journal | vauthors = Higgs PG, Pudritz RE | title = A thermodynamic basis for prebiotic amino acid synthesis and the nature of the first genetic code | journal = Astrobiology | volume = 9 | issue = 5 | pages = 483–490 | date = June 2009 | pmid = 19566427 | doi = 10.1089/ast.2008.0280 | arxiv = 0904.0402 | s2cid = 9039622 | bibcode = 2009AsBio...9..483H }}</ref>

===Standard vs nonstandard amino acids===

The 20 amino acids that are encoded directly by the codons of the universal genetic code are called ''standard'' or ''canonical'' amino acids. A modified form of methionine ([[N-Formylmethionine|''N''-formylmethionine]]) is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria and [[plastid]]s (including chloroplasts). Other amino acids are called ''nonstandard'' or ''non-canonical''. Most of the nonstandard amino acids are also non-proteinogenic (i.e. they cannot be incorporated into proteins during translation), but two of them are proteinogenic, as they can be incorporated translationally into proteins by exploiting information not encoded in the universal genetic code.

The two nonstandard proteinogenic amino acids are selenocysteine (present in many non-eukaryotes as well as most eukaryotes, but not coded directly by DNA) and [[pyrrolysine]] (found only in some [[archaea]] and at least one [[bacterium]]). The incorporation of these nonstandard amino acids is rare. For example, 25 human proteins include selenocysteine in their primary structure,<ref>{{cite journal | vauthors = Kryukov GV, Castellano S, Novoselov SV, Lobanov AV, Zehtab O, Guigó R, Gladyshev VN | title = Characterization of mammalian selenoproteomes | journal = Science | volume = 300 | issue = 5624 | pages = 1439–1443 | date = May 2003 | pmid = 12775843 | doi = 10.1126/science.1083516 | s2cid = 10363908 | bibcode = 2003Sci...300.1439K }}</ref> and the structurally characterized enzymes (selenoenzymes) employ selenocysteine as the catalytic [[moiety (chemistry)|moiety]] in their active sites.<ref>{{cite journal | vauthors = Gromer S, Urig S, Becker K | title = The thioredoxin system--from science to clinic | journal = Medicinal Research Reviews | volume = 24 | issue = 1 | pages = 40–89 | date = January 2004 | pmid = 14595672 | doi = 10.1002/med.10051 | s2cid = 1944741 }}</ref> Pyrrolysine and selenocysteine are encoded via variant codons. For example, selenocysteine is encoded by stop codon and [[SECIS element]].<ref name="Tjong">{{cite thesis |url= https://diginole.lib.fsu.edu/islandora/object/fsu%3A175939 | vauthors = Tjong H |title=Modeling Electrostatic Contributions to Protein Folding and Binding|date=2008|publisher=Florida State University|type=PhD thesis|page=1 footnote|access-date=28 January 2020|archive-date=28 January 2020|archive-url=https://web.archive.org/web/20200128234717/https://diginole.lib.fsu.edu/islandora/object/fsu:175939|url-status=live}}</ref><ref name="VoJw6fIISSkC p.299">{{cite journal| vauthors = Stewart L, Burgin AB |journal=Frontiers in Drug Design & Discovery |date=2005|title=Whole Gene Synthesis: A Gene-O-Matic Future|url=https://books.google.com/books?id=VoJw6fIISSkC&pg=PA299|publisher=[[Bentham Science Publishers]]|volume=1|page=299|doi=10.2174/1574088054583318|isbn=978-1-60805-199-1|issn=1574-0889|access-date=5 January 2016|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414224011/https://books.google.com/books?id=VoJw6fIISSkC&pg=PA299|url-status=live}}</ref><ref name="url_The_Genetic_Codes_NCBI">{{cite web|date=7 April 2008|title=The Genetic Codes|url=https://www.ncbi.nlm.nih.gov/Taxonomy/Utils/wprintgc.cgi?mode=c|access-date=10 March 2010|publisher=National Center for Biotechnology Information (NCBI)|vauthors=Elzanowski A, Ostell J|archive-date=20 August 2016|archive-url=https://web.archive.org/web/20160820125755/http://130.14.29.110/Taxonomy/Utils/wprintgc.cgi?mode=c|url-status=live}}</ref>

[[N-Formylmethionine|''N''-formylmethionine]] (which is often the initial amino acid of proteins in bacteria, [[Mitochondrion|mitochondria]], and [[chloroplast]]s) is generally considered as a form of [[methionine]] rather than as a separate proteinogenic amino acid. Codon–[[transfer RNA|tRNA]] combinations not found in nature can also be used to [[Expanded genetic code|"expand" the genetic code]] and form novel proteins known as [[alloprotein]]s incorporating [[non-proteinogenic amino acid]]s.<ref name="pmid16260173">{{cite journal | vauthors = Xie J, Schultz PG | title = Adding amino acids to the genetic repertoire | journal = Current Opinion in Chemical Biology | volume = 9 | issue = 6 | pages = 548–554 | date = December 2005 | pmid = 16260173 | doi = 10.1016/j.cbpa.2005.10.011 }}</ref><ref name="pmid19318213">{{cite journal | vauthors = Wang Q, Parrish AR, Wang L | title = Expanding the genetic code for biological studies | journal = Chemistry & Biology | volume = 16 | issue = 3 | pages = 323–336 | date = March 2009 | pmid = 19318213 | pmc = 2696486 | doi = 10.1016/j.chembiol.2009.03.001 }}</ref><ref name="isbn0-387-22046-1">{{cite book | vauthors = Simon M | title = Emergent computation: emphasizing bioinformatics | url = https://archive.org/details/emergentcomputat00simo_754 | url-access = limited | publisher = AIP Press/Springer Science+Business Media | location = New York | year = 2005 | pages = [https://archive.org/details/emergentcomputat00simo_754/page/n116 105–106] | isbn = 978-0-387-22046-8 }}</ref>

===Non-proteinogenic amino acids===
{{main|Non-proteinogenic amino acids}}

Aside from the 22 [[proteinogenic amino acid]]s, many ''non-proteinogenic'' amino acids are known. Those either are not found in proteins (for example [[carnitine]], [[Gamma-aminobutyric acid|GABA]], [[levothyroxine]]) or are not produced directly and in isolation by standard cellular machinery. For example, [[hydroxyproline]], is synthesised from [[proline]]. Another example is [[selenomethionine]]).

Non-proteinogenic amino acids that are found in proteins are formed by [[post-translational modification]]. Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a [[phospholipid]] membrane.<ref>{{cite journal | vauthors = Blenis J, Resh MD | title = Subcellular localization specified by protein acylation and phosphorylation | journal = Current Opinion in Cell Biology | volume = 5 | issue = 6 | pages = 984–989 | date = December 1993 | pmid = 8129952 | doi = 10.1016/0955-0674(93)90081-Z }}</ref> Examples:
*the [[carboxylation]] of [[glutamate]] allows for better binding of [[calcium in biology|calcium cations]],<ref>{{cite journal | vauthors = Vermeer C | title = Gamma-carboxyglutamate-containing proteins and the vitamin K-dependent carboxylase | journal = The Biochemical Journal | volume = 266 | issue = 3 | pages = 625–636 | date = March 1990 | pmid = 2183788 | pmc = 1131186 | doi = 10.1042/bj2660625 }}</ref> 
*[[Hydroxyproline]], generated by [[hydroxylation]] of [[proline]], is a major component of the [[connective tissue]] [[collagen]].<ref>{{cite journal | vauthors = Bhattacharjee A, Bansal M | title = Collagen structure: the Madras triple helix and the current scenario | journal = IUBMB Life | volume = 57 | issue = 3 | pages = 161–172 | date = March 2005 | pmid = 16036578 | doi = 10.1080/15216540500090710 | s2cid = 7211864 }}</ref>
* [[Hypusine]] in the [[Eukaryotic initiation factor|translation initiation factor]] [[EIF5A]], contains a modification of lysine.<ref>{{cite journal | vauthors = Park MH | title = The post-translational synthesis of a polyamine-derived amino acid, hypusine, in the eukaryotic translation initiation factor 5A (eIF5A) | journal = Journal of Biochemistry | volume = 139 | issue = 2 | pages = 161–169 | date = February 2006 | pmid = 16452303 | pmc = 2494880 | doi = 10.1093/jb/mvj034 }}</ref>

Some non-proteinogenic amino acids are not found in proteins. Examples include [[2-aminoisobutyric acid]] and the neurotransmitter [[gamma-aminobutyric acid]]. Non-proteinogenic amino acids often occur as intermediates in the [[metabolic pathway]]s for standard amino acids&nbsp;– for example, [[ornithine]] and [[citrulline]] occur in the [[urea cycle]], part of amino acid [[catabolism]] (see below).<ref>{{cite journal | vauthors = Curis E, Nicolis I, Moinard C, Osowska S, Zerrouk N, Bénazeth S, Cynober L | title = Almost all about citrulline in mammals | journal = Amino Acids | volume = 29 | issue = 3 | pages = 177–205 | date = November 2005 | pmid = 16082501 | doi = 10.1007/s00726-005-0235-4 | s2cid = 23877884 }}</ref> A rare exception to the dominance of α-amino acids in biology is the β-amino acid [[beta alanine]] (3-aminopropanoic acid), which is used in plants and microorganisms in the synthesis of [[pantothenic acid]] (vitamin B<sub>5</sub>), a component of [[coenzyme A]].<ref>{{cite journal | vauthors = Coxon KM, Chakauya E, Ottenhof HH, Whitney HM, Blundell TL, Abell C, Smith AG | title = Pantothenate biosynthesis in higher plants | journal = Biochemical Society Transactions | volume = 33 | issue = Pt 4 | pages = 743–746 | date = August 2005 | pmid = 16042590 | doi = 10.1042/BST0330743 }}</ref>

===In mammalian nutrition===
[[File:Amino acids in food and blood.png|class=skin-invert-image|thumb|right|upright=1.75 |Share of amino acid in various human diets and the resulting mix of amino acids in human blood serum. Glutamate and glutamine are the most frequent in food at over 10%, while alanine, glutamine, and glycine are the most common in blood.|alt=Diagram showing the relative occurrence of amino acids in blood serum as obtained from diverse diets.]]
{{Main|Essential amino acid}}
{{further|Protein (nutrient)|Amino acid synthesis}}

Animals ingest amino acids in the form of protein. The protein is broken down into its constituent amino acids in the process of digestion. The amino acids are then used to synthesize new proteins and other [[nitrogen|nitrogenous]] biomolecules, or they are further [[catabolism|catabolized]] through [[oxidation]] to provide a source of energy.<ref>{{cite journal | vauthors = Sakami W, Harrington H | title = Amino Acid Metabolism | journal = Annual Review of Biochemistry | volume = 32 | issue = 1 | pages = 355–398 | year = 1963 | pmid = 14144484 | doi = 10.1146/annurev.bi.32.070163.002035 }}</ref> The oxidation pathway starts with the removal of the amino group by a [[transaminase]]; the amino group is then fed into the [[urea cycle]]. The other product of transamidation is a [[keto acid]] that enters the [[citric acid cycle]].<ref>{{cite journal | vauthors = Brosnan JT | title = Glutamate, at the interface between amino acid and carbohydrate metabolism | journal = The Journal of Nutrition | volume = 130 | issue = 4S Suppl | pages = 988S–990S | date = April 2000 | pmid = 10736367 | doi = 10.1093/jn/130.4.988S | doi-access = free }}</ref> [[Glucogenic amino acid]]s can also be converted into glucose, through [[gluconeogenesis]].<ref>{{cite journal | vauthors = Young VR, Ajami AM | title = Glutamine: the emperor or his clothes? | journal = The Journal of Nutrition | volume = 131 | issue = 9 Suppl | pages = 2449S–2459S, 2486S–2487S | date = September 2001 | pmid = 11533293 | doi = 10.1093/jn/131.9.2449S | doi-access = free }}</ref>

Of the 20 standard amino acids, nine ([[Histidine|His]], [[Isoleucine|Ile]], [[Leucine|Leu]], [[Lysine|Lys]], [[Methionine|Met]], [[Phenylalanine|Phe]], [[Threonine|Thr]], [[Tryptophan|Trp]] and [[Valine|Val]]) are called [[essential amino acid]]s because the [[human body]] cannot [[biosynthesis|synthesize]] them from other compounds at the level needed for normal growth, so they must be obtained from food.<ref>{{cite journal | vauthors = Young VR | title = Adult amino acid requirements: the case for a major revision in current recommendations | journal = The Journal of Nutrition | volume = 124 | issue = 8 Suppl | pages = 1517S–1523S | date = August 1994 | pmid = 8064412 | doi = 10.1093/jn/124.suppl_8.1517S | doi-access = free }}</ref><ref>{{cite journal | vauthors = Fürst P, Stehle P | title = What are the essential elements needed for the determination of amino acid requirements in humans? | journal = The Journal of Nutrition | volume = 134 | issue = 6 Suppl | pages = 1558S–1565S | date = June 2004 | pmid = 15173430 | doi = 10.1093/jn/134.6.1558S | doi-access = free }}</ref><ref>{{cite journal | vauthors = Reeds PJ | title = Dispensable and indispensable amino acids for humans | journal = The Journal of Nutrition | volume = 130 | issue = 7 | pages = 1835S–1840S | date = July 2000 | pmid = 10867060 | doi = 10.1093/jn/130.7.1835S | doi-access = free }}</ref>

====Semi-essential and conditionally essential amino acids, and juvenile requirements====
In addition, cysteine, [[tyrosine]], and [[arginine]] are considered semiessential amino acids, and [[taurine]] a semi-essential aminosulfonic acid in children. Some amino acids are [[Essential amino acid#Essentiality in humans|conditionally essential]] for certain ages or medical conditions. Essential amino acids may also vary from [[species]] to species.{{efn|For example, [[ruminant]]s such as cows obtain a number of amino acids via [[microbe]]s in the [[reticulorumen|first two stomach chambers]].}} The metabolic pathways that synthesize these monomers are not fully developed.<ref>{{cite journal | vauthors = Imura K, Okada A | title = Amino acid metabolism in pediatric patients | journal = Nutrition | volume = 14 | issue = 1 | pages = 143–148 | date = January 1998 | pmid = 9437700 | doi = 10.1016/S0899-9007(97)00230-X }}</ref><ref>{{cite journal | vauthors = Lourenço R, Camilo ME | title = Taurine: a conditionally essential amino acid in humans? An overview in health and disease | journal = Nutricion Hospitalaria | volume = 17 | issue = 6 | pages = 262–270 | year = 2002 | pmid = 12514918 }}</ref>

===Non-protein functions===
{{Catecholamine and trace amine biosynthesis|align=right|caption=[[Catecholamine]]s and [[trace amine]]s are synthesized from phenylalanine and tyrosine in humans.}}
{{Further|Amino acid neurotransmitter}}

Many proteinogenic and non-proteinogenic amino acids have biological functions beyond being precursors to proteins and peptides. In humans, amino acids also have important roles in diverse biosynthetic pathways. [[Plant defense against herbivory|Defenses against herbivores]] in plants sometimes employ amino acids.<ref name="Hylin1969">{{Cite journal| vauthors = Hylin JW |year=1969 |title=Toxic peptides and amino acids in foods and feeds |journal=Journal of Agricultural and Food Chemistry |volume=17 |issue=3 |pages=492–496 |doi=10.1021/jf60163a003|bibcode=1969JAFC...17..492H }}</ref> Examples:

====Standard amino acids====
* [[Tryptophan]] is a precursor of the neurotransmitter [[serotonin]].<ref>{{cite journal | vauthors = Savelieva KV, Zhao S, Pogorelov VM, Rajan I, Yang Q, Cullinan E, Lanthorn TH | title = Genetic disruption of both tryptophan hydroxylase genes dramatically reduces serotonin and affects behavior in models sensitive to antidepressants | journal = PLOS ONE | volume = 3 | issue = 10 | pages = e3301 | year = 2008 | pmid = 18923670 | pmc = 2565062 | doi = 10.1371/journal.pone.0003301 | veditors = Bartolomucci A | doi-access = free | bibcode = 2008PLoSO...3.3301S }}</ref>
* [[Tyrosine]] (and its precursor phenylalanine) are precursors of the [[catecholamine]] [[neurotransmitter]]s [[dopamine]], [[epinephrine]] and [[norepinephrine]] and various [[trace amine]]s.
* [[Phenylalanine]] is a precursor of [[phenethylamine]] and tyrosine in humans. In plants, it is a precursor of various [[phenylpropanoid]]s, which are important in plant metabolism.
* [[Glycine]] is a precursor of [[porphyrin]]s such as [[heme]].<ref>{{cite journal | vauthors = Shemin D, Rittenberg D | title = The biological utilization of glycine for the synthesis of the protoporphyrin of hemoglobin | journal = The Journal of Biological Chemistry | volume = 166 | issue = 2 | pages = 621–625 | date = December 1946 | pmid = 20276176 | doi = 10.1016/S0021-9258(17)35200-6 | doi-access = free }}</ref>
* [[Arginine]] is a precursor of [[nitric oxide]].<ref>{{cite journal | vauthors = Tejero J, Biswas A, Wang ZQ, Page RC, Haque MM, Hemann C, Zweier JL, Misra S, Stuehr DJ | title = Stabilization and characterization of a heme-oxy reaction intermediate in inducible nitric-oxide synthase | journal = The Journal of Biological Chemistry | volume = 283 | issue = 48 | pages = 33498–33507 | date = November 2008 | pmid = 18815130 | pmc = 2586280 | doi = 10.1074/jbc.M806122200 | doi-access = free }}</ref>
* [[Ornithine]] and [[S-Adenosyl methionine|''S''-adenosylmethionine]] are precursors of [[polyamine]]s.<ref>{{cite journal | vauthors = Rodríguez-Caso C, Montañez R, Cascante M, Sánchez-Jiménez F, Medina MA | title = Mathematical modeling of polyamine metabolism in mammals | journal = The Journal of Biological Chemistry | volume = 281 | issue = 31 | pages = 21799–21812 | date = August 2006 | pmid = 16709566 | doi = 10.1074/jbc.M602756200 | hdl-access = free | doi-access = free | bibcode = 2006JBiCh.28121799R | hdl = 10630/32289 }}</ref>
* [[Aspartate]], [[glycine]], and [[glutamine]] are precursors of [[nucleotide]]s.<ref name="Stryer_2002">{{cite book | vauthors = Stryer L, Berg JM, Tymoczko JL |title=Biochemistry |url=https://archive.org/details/biochemistry200100jere |url-access=registration |date=2002 |publisher=W.H. Freeman |location=New York |isbn=978-0-7167-4684-3 |edition=5th |pages=[https://archive.org/details/biochemistry200100jere/page/693 693–698]}}</ref>

====Roles for nonstandard amino acids====
*[[Carnitine]] is used in [[lipid|lipid transport]]. 
*[[gamma-aminobutyric acid]] is a neurotransmitter.<ref>{{cite journal | vauthors = Petroff OA | title = GABA and glutamate in the human brain | journal = The Neuroscientist | volume = 8 | issue = 6 | pages = 562–573 | date = December 2002 | pmid = 12467378 | doi = 10.1177/1073858402238515 | s2cid = 84891972 }}</ref> 
*[[5-HTP]] (5-hydroxytryptophan) is used for experimental treatment of depression.<ref>{{cite journal | vauthors = Turner EH, Loftis JM, Blackwell AD | title = Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan | journal = Pharmacology & Therapeutics | volume = 109 | issue = 3 | pages = 325–338 | date = March 2006 | pmid = 16023217 | doi = 10.1016/j.pharmthera.2005.06.004 | s2cid = 2563606 | url = https://escholarship.org/uc/item/58h866d5 }}</ref> 
*[[L-DOPA|<small>L</small>-DOPA]] (<small>L</small>-dihydroxyphenylalanine) for [[Parkinson's]] treatment,<ref>{{cite journal | vauthors = Kostrzewa RM, Nowak P, Kostrzewa JP, Kostrzewa RA, Brus R | title = Peculiarities of L: -DOPA treatment of Parkinson's disease | journal = Amino Acids | volume = 28 | issue = 2 | pages = 157–164 | date = March 2005 | pmid = 15750845 | doi = 10.1007/s00726-005-0162-4 | s2cid = 33603501 }}</ref> 
*[[Eflornithine]] inhibits [[ornithine decarboxylase]] and used in the treatment of [[African trypanosomiasis|sleeping sickness]].<ref>{{cite journal | vauthors = Heby O, Persson L, Rentala M | title = Targeting the polyamine biosynthetic enzymes: a promising approach to therapy of African sleeping sickness, Chagas' disease, and leishmaniasis | journal = Amino Acids | volume = 33 | issue = 2 | pages = 359–366 | date = August 2007 | pmid = 17610127 | doi = 10.1007/s00726-007-0537-9 | s2cid = 26273053 }}</ref>
*[[Canavanine]], an analogue of [[arginine]] found in many [[legume]]s is an [[antifeedant]], protecting the plant from predators.<ref>{{cite journal | vauthors = Rosenthal GA | title = L-Canavanine: a higher plant insecticidal allelochemical | journal = Amino Acids | volume = 21 | issue = 3 | pages = 319–330 | year = 2001 | pmid = 11764412 | doi = 10.1007/s007260170017 | s2cid = 3144019 }}</ref>
*[[Mimosine]] found in some legumes, is another possible [[antifeedant]].<ref>{{cite journal | vauthors = Hammond AC | title = Leucaena toxicosis and its control in ruminants | journal = Journal of Animal Science | volume = 73 | issue = 5 | pages = 1487–1492 | date = May 1995 | pmid = 7665380 | doi = 10.2527/1995.7351487x }}</ref> This compound is an analogue of [[tyrosine]] and can poison animals that graze on these plants.
However, not all of the functions of other abundant nonstandard amino acids are known.