View source for Chemistry:Proteinogenic amino acid

{{short description|Amino acid that is incorporated biosynthetically into proteins during translation}}
[[File:Nonproteinogenic AAs.svg|thumbnail|400 px|Proteinogenic amino acids are a small fraction of all amino acids]]
'''Proteinogenic amino acids''' are [[Chemistry:Amino acid|amino acid]]s that are incorporated biosynthetically into [[Biology:Protein|protein]]s during [[Biology:Translation|translation]]. The word "proteinogenic" means "protein creating". Throughout known life, there are 22 genetically encoded (proteinogenic) amino acids, 20 in the standard [[Biology:Genetic code|genetic code]] and an additional 2 ([[Chemistry:Selenocysteine|selenocysteine]] and [[Chemistry:Pyrrolysine|pyrrolysine]]) that can be incorporated by special translation mechanisms.<ref>{{cite journal | vauthors = Ambrogelly A, Palioura S, Söll D | title = Natural expansion of the genetic code | journal = Nature Chemical Biology | volume = 3 | issue = 1 | pages = 29–35 | date = January 2007 | pmid = 17173027 | doi = 10.1038/nchembio847 }}</ref>

In contrast, [[Chemistry:Non-proteinogenic amino acids|non-proteinogenic amino acids]] are amino acids that are either not incorporated into proteins (like GABA, [[Chemistry:L-DOPA|<small>L</small>-DOPA]], or [[Chemistry:Triiodothyronine|triiodothyronine]]), misincorporated in place of a genetically encoded amino acid, or not produced directly and in isolation by standard cellular machinery (like [[Chemistry:Hydroxyproline|hydroxyproline]]). The latter often results from [[Biology:Post-translational modification|post-translational modification]] of proteins. Some non-proteinogenic amino acids are incorporated into [[Biology:Nonribosomal peptide|nonribosomal peptide]]s which are synthesized by non-ribosomal peptide synthetases.

Both [[Biology:Eukaryote|eukaryote]]s and [[Biology:Prokaryote|prokaryote]]s can incorporate [[Chemistry:Selenocysteine|selenocysteine]] into their proteins via a nucleotide sequence known as a [[Biology:SECIS element|SECIS element]], which directs the cell to translate a nearby UGA codon as [[Chemistry:Selenocysteine|selenocysteine]] (UGA is normally a [[Biology:Stop codon|stop codon]]).  In some methanogenic prokaryotes, the UAG codon (normally a stop codon) can also be translated to [[Chemistry:Pyrrolysine|pyrrolysine]].<ref name="LobanovTuranov2010">{{cite journal | vauthors = Lobanov AV, Turanov AA, Hatfield DL, Gladyshev VN | title = Dual functions of codons in the genetic code | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 45 | issue = 4 | pages = 257–65 | date = August 2010 | pmid = 20446809 | pmc = 3311535 | doi = 10.3109/10409231003786094 }}</ref>

In eukaryotes, there are only 21 proteinogenic amino acids, the 20 of the standard genetic code, plus [[Chemistry:Selenocysteine|selenocysteine]]. Humans can synthesize 12 of these 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 [[Chemistry:Essential amino acid|essential amino acid]]s.  The essential amino acids are [[Chemistry:Histidine|histidine]], [[Chemistry:Isoleucine|isoleucine]], [[Chemistry:Leucine|leucine]], [[Chemistry:Lysine|lysine]], [[Chemistry:Methionine|methionine]], [[Chemistry:Phenylalanine|phenylalanine]], [[Chemistry:Threonine|threonine]], [[Chemistry:Tryptophan|tryptophan]], and [[Chemistry:Valine|valine]] (i.e. H, I, L, K, M, F, T, W, V).<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 | url = http://jn.nutrition.org/cgi/reprint/124/8_Suppl/1517S.pdf }}</ref>

The proteinogenic amino acids have been found to be related to the set of [[Chemistry:Amino acid|amino acid]]s that can be recognized by [[Biology:Ribozyme|ribozyme]] autoaminoacylation systems.<ref>{{cite journal | vauthors = Erives A | title = A model of proto-anti-codon RNA enzymes requiring L-amino acid homochirality | journal = Journal of Molecular Evolution | volume = 73 | issue = 1–2 | pages = 10–22 | date = August 2011 | pmid = 21779963 | pmc = 3223571 | doi = 10.1007/s00239-011-9453-4 | bibcode = 2011JMolE..73...10E }}</ref> 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, [[Chemistry:Ornithine|ornithine]] and [[Chemistry:Homoserine|homoserine]] cyclize against the peptide backbone and fragment the protein with relatively short [[Biology:Biological half-life|half-lives]], while others are toxic because they can be mistakenly incorporated into proteins, such as the arginine analog [[Chemistry:Canavanine|canavanine]].

The [[Biology:Evolution|evolution]]ary selection of certain proteinogenic amino acids from the [[Biology:Primordial soup|primordial soup]] has been suggested to be because of their better incorporation into a polypeptide chain as opposed to non-proteinogenic amino acids.<ref>{{Cite journal|last1=Frenkel-Pinter|first1=Moran|last2=Haynes|first2=Jay W.|last3=C|first3=Martin|last4=Petrov|first4=Anton S.|last5=Burcar|first5=Bradley T.|last6=Krishnamurthy|first6=Ramanarayanan|last7=Hud|first7=Nicholas V.|last8=Leman|first8=Luke J.|last9=Williams|first9=Loren Dean|date=2019-08-13|title=Selective incorporation of proteinaceous over nonproteinaceous cationic amino acids in model prebiotic oligomerization reactions|journal=Proceedings of the National Academy of Sciences|language=en|volume=116|issue=33|pages=16338–16346|doi=10.1073/pnas.1904849116|issn=0027-8424|pmc=6697887|pmid=31358633|bibcode=2019PNAS..11616338F |doi-access=free}}</ref>

== 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 [[Physics:Zwitterion|zwitterion]] forms that exist in aqueous solutions.

[[File:Proteinogenic Amino Acid Table.png|frame|left|Structure of the 21 proteinogenic amino acids with 3 and 1 letters codes, grouped by side chain functionality]]
{{Clear}}
<gallery>
image:L-alanine-skeletal.png|[[Chemistry:Alanine|<small>L</small>-Alanine]]<br>(Ala&nbsp;/&nbsp;A)
image:Arginina.svg|[[Chemistry:Arginine|<small>L</small>-Arginine]]<br>(Arg&nbsp;/&nbsp;R)
image:L-asparagine-skeletal.png|[[Chemistry:Asparagine|<small>L</small>-Asparagine]]<br>(Asn&nbsp;/&nbsp;N)
image:L-aspartic-acid-skeletal.png|[[Chemistry:Aspartic acid|<small>L</small>-Aspartic acid]]<br>(Asp&nbsp;/&nbsp;D)
image:L-cysteine-skeletal.png|[[Chemistry:Cysteine|<small>L</small>-Cysteine]]<br>(Cys&nbsp;/&nbsp;C)
image:L-glutamic-acid-skeletal.png|[[Chemistry:Glutamic acid|<small>L</small>-Glutamic acid]]<br>(Glu&nbsp;/&nbsp;E)
image:L-glutamine-skeletal.png|[[Chemistry:Glutamine|<small>L</small>-Glutamine]]<br>(Gln&nbsp;/&nbsp;Q)
image:Glycine-skeletal.png|[[Chemistry:Glycine|Glycine]]<br>(Gly&nbsp;/&nbsp;G)
image:L-histidine-skeletal.png|[[Chemistry:Histidine|<small>L</small>-Histidine]]<br>(His&nbsp;/&nbsp;H)
image:L-isoleucine-skeletal.svg|[[Chemistry:Isoleucine|<small>L</small>-Isoleucine]]<br>(Ile&nbsp;/&nbsp;I)
image:L-leucine-skeletal.png|[[Chemistry:Leucine|<small>L</small>-Leucine]]<br>(Leu&nbsp;/&nbsp;L)
image:L-lysine-skeletal.png|[[Chemistry:Lysine|<small>L</small>-Lysine]]<br>(Lys&nbsp;/&nbsp;K)
image:L-methionine-skeletal.png|[[Chemistry:Methionine|<small>L</small>-Methionine]]<br>(Met&nbsp;/&nbsp;M)
image:Fenyloalanina.svg|[[Chemistry:Phenylalanine|<small>L</small>-Phenylalanine]]<br>(Phe&nbsp;/&nbsp;F)
image:L-proline-skeletal.png|[[Chemistry:Proline|<small>L</small>-Proline]]<br>(Pro&nbsp;/&nbsp;P)
image:L-serine-skeletal.png|[[Chemistry:Serine|<small>L</small>-Serine]]<br>(Ser&nbsp;/&nbsp;S)
image:L-threonine-skeletal.png|[[Chemistry:Threonine|<small>L</small>-Threonine]]<br>(Thr&nbsp;/&nbsp;T)
image:L-tryptophan-skeletal.png|[[Chemistry:Tryptophan|<small>L</small>-Tryptophan]]<br>(Trp&nbsp;/&nbsp;W)
image:L-tyrosine-skeletal.png|[[Chemistry:Tyrosine|<small>L</small>-Tyrosine]]<br>(Tyr&nbsp;/&nbsp;Y)
image:L-valine-skeletal.png|[[Chemistry:Valine|<small>L</small>-Valine]]<br>(Val&nbsp;/&nbsp;V)
</gallery>

IUPAC/IUBMB now also recommends standard abbreviations for the following two amino acids:
<gallery>
image:L-selenocysteine-2D-skeletal.png|[[Chemistry:Selenocysteine|<small>L</small>-Selenocysteine]]<br>(Sec&nbsp;/&nbsp;U)
image:Pyrrolysine.svg|[[Chemistry:Pyrrolysine|<small>L</small>-Pyrrolysine]]<br>(Pyl&nbsp;/&nbsp;O)
</gallery>

== Chemical properties ==

Following is a table listing the one-letter symbols, the three-letter symbols, and the chemical properties of the side chains of the standard amino acids. The masses listed are based on weighted averages of the elemental [[Physics:Isotope|isotope]]s at their [[Chemistry:Natural abundance|natural abundance]]s. Forming a [[Biology:Peptide bond|peptide bond]] results in elimination of a molecule of [[Chemistry:Water (molecule)|water]]. Therefore, the protein's mass is equal to the mass of amino acids the protein is composed of minus 18.01524 Da per peptide bond.

===General chemical properties===
{| class="wikitable sortable" style="text-align: center"
|-
! scope="col" | Amino acid
! scope="col" | Short
! scope="col" | {{abbr|Abbrev.|Abbreviation}}
! scope="col" | Avg. mass (Da)
! scope="col" | {{abbr|[[Biology:Isoelectric point|pI]]|Isoelectric point}}
! scope="col" | [[Chemistry:Dissociation constant|pK]]<sub>1</sub><br>(α-COOH)
! scope="col" | pK<sub>2</sub><br>(α-<sup>+</sup>NH<sub>3</sub>)
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Alanine|Alanine]]
| A
| Ala
| 89.09404
| 6.01
| 2.35
| 9.87
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Cysteine|Cysteine]]
| C
| Cys
| 121.15404
| 5.05
| 1.92
| 10.70
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Aspartic acid|Aspartic acid]]
| D
| Asp
| 133.10384
| 2.85
| 1.99
| 9.90
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]]
| E
| Glu
| 147.13074
| 3.15
| 2.10
| 9.47
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Phenylalanine|Phenylalanine]]
| F
| Phe
| 165.19184
| 5.49
| 2.20
| 9.31
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glycine|Glycine]]
| G
| Gly
| 75.06714
| 6.06
| 2.35
| 9.78
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Histidine|Histidine]]
| H
| His
| 155.15634
| 7.60
| 1.80
| 9.33
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Isoleucine|Isoleucine]]
| I
| Ile
| 131.17464
| 6.05
| 2.32
| 9.76
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Lysine|Lysine]]
| K
| Lys
| 146.18934
| 9.60
| 2.16
| 9.06
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]]
| L
| Leu
| 131.17464
| 6.01
| 2.33
| 9.74
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Methionine|Methionine]]
| M
| Met
| 149.20784
| 5.74
| 2.13
| 9.28
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]]
| N
| Asn
| 132.11904
| 5.41
| 2.14
| 8.72
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Pyrrolysine|Pyrrolysine]]
| O
| Pyl
| 255.31
| {{dunno}}
| {{dunno}}
| {{dunno}}
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Proline|Proline]]
| P
| Pro
| 115.13194
| 6.30
| 1.95
| 10.64
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamine|Glutamine]]
| Q
| Gln
| 146.14594
| 5.65
| 2.17
| 9.13
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Arginine|Arginine]]
| R
| Arg
| 174.20274
| 10.76
| 1.82
| 8.99
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Serine|Serine]]
| S
| Ser
| 105.09344
| 5.68
| 2.19
| 9.21
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Threonine|Threonine]]
| T
| Thr
| 119.12034
| 5.60
| 2.09
| 9.10
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Selenocysteine|Selenocysteine]]
| U
| Sec
| 168.053
| 5.47
| 1.91
| 10
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Valine|Valine]]
| V
| Val
| 117.14784
| 6.00
| 2.39
| 9.74
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tryptophan|Tryptophan]]
| W
| Trp
| 204.22844
| 5.89
| 2.46
| 9.41
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Tyrosine|Tyrosine]]
| Y
| Tyr
| 181.19124
| 5.64
| 2.20
| 9.21
|}

===Side-chain properties===

{| class="wikitable sortable" style="text-align: center"
! scope="col" | Amino acid
! scope="col" | Short
! scope="col" | {{abbr|Abbrev.|Abbreviation}}
! scope="col" | Side chain
! scope="col" | [[Chemistry:Hydrophobic|Hydro-<br>phobic]]
! scope="col" | {{abbr|[[Chemistry:Acid dissociation constant|pKa]]<sup>§</sup>|Acid dissociation constant}}
! scope="col" | [[Chemistry:Chemical polarity|Polar]]
! scope="col" | [[Chemistry:PH|pH]]
! scope="col" | Small
! scope="col" | Tiny
! scope="col" | [[Chemistry:Aromaticity|Aromatic]]<br>or Aliphatic
! scope="col" | [[Chemistry:Van der Waals radius|van der Waals<br>volume]] (Å<sup>3</sup>)
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Alanine|Alanine]]
| A
| Ala
| -CH<sub>3</sub>
| {{ya}}
| -
| {{na}}
| -
| {{ya}}
| {{ya}}
| Aliphatic
| 67
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Cysteine|Cysteine]]
| C
| Cys
| -CH<sub>2</sub>[[Chemistry:Thiol|SH]]
| {{ya}}
| 8.55
| {{ya}}
| acidic
| {{ya}}
| {{ya}}
| -
| 86
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Aspartic acid|Aspartic acid]]
| D
| Asp
| -CH<sub>2</sub>COOH
| {{na}}
| 3.67
| {{ya}}
| acidic
| {{ya}}
| {{na}}
| -
| 91
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]]
| E
| Glu
| -CH<sub>2</sub>CH<sub>2</sub>COOH
| {{na}}
| 4.25
| {{ya}}
| acidic
| {{na}}
| {{na}}
| -
| 109
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Phenylalanine|Phenylalanine]]
| F
| Phe
| -CH<sub>2</sub>C<sub>6</sub>H<sub>5</sub>
| {{ya}}
| -
| {{na}}
| -
| {{na}}
| {{na}}
| Aromatic
| 135
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glycine|Glycine]]
| G
| Gly
| -H
| {{ya}}
| -
| {{na}}
| -
| {{ya}}
| {{ya}}
| -
| 48
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Histidine|Histidine]]
| H
| His
| -CH<sub>2</sub>-[[Chemistry:Imidazole|C<sub>3</sub>H<sub>3</sub>N<sub>2</sub>]]
| {{na}}
| 6.54
| {{ya}}
| weak basic
| {{na}}
| {{na}}
| Aromatic
| 118
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Isoleucine|Isoleucine]]
| I
| Ile
| -CH(CH<sub>3</sub>)CH<sub>2</sub>CH<sub>3</sub>
| {{ya}}
| -
| {{na}}
| -
| {{na}}
| {{na}}
| Aliphatic
| 124
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Lysine|Lysine]] 
| K
| Lys
| -(CH<sub>2</sub>)<sub>4</sub>NH<sub>2</sub>
| {{na}}
| 10.40
| {{ya}}
| basic
| {{na}}
| {{na}}
| -
| 135
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]]
| L
| Leu
| -CH<sub>2</sub>CH(CH<sub>3</sub>)<sub>2</sub>
| {{ya}}
| -
| {{na}}
| -
| {{na}}
| {{na}}
| Aliphatic
| 124
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Methionine|Methionine]]
| M
| Met
| -CH<sub>2</sub>CH<sub>2</sub>[[Chemistry:Sulfur|S]]CH<sub>3</sub>
| {{ya}}
| -
| {{na}}
| -
| {{na}}
| {{na}}
| Aliphatic
| 124
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]]
| N
| Asn
| -CH<sub>2</sub>CONH<sub>2</sub>
| {{na}}
| -
| {{ya}}
| -
| {{ya}}
| {{na}}
| -
| 96
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Pyrrolysine|Pyrrolysine]]
| O
| Pyl
| -(CH<sub>2</sub>)<sub>4</sub>NHCO[[Chemistry:Pyrroline|C<sub>4</sub>H<sub>5</sub>N]]CH<sub>3</sub>
| {{na}}
|N.D.
| {{ya}}
| weak basic
| {{na}}
| {{na}}
| -
| {{dunno}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Proline|Proline]]
| P
| Pro
| -CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>-
| {{ya}}
| -
| {{na}}
| -
| {{ya}}
| {{na}}
| -
| 90
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamine|Glutamine]]
| Q
| Gln
| -CH<sub>2</sub>CH<sub>2</sub>CONH<sub>2</sub>
| {{na}}
| -
| {{ya}}
| -
| {{na}}
| {{na}}
| -
| 114
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Arginine|Arginine]]
| R
| Arg
| -(CH<sub>2</sub>)<sub>3</sub>NH-C(NH)NH<sub>2</sub>
| {{na}}
| 12.3
| {{ya}}
| strongly basic
| {{na}}
| {{na}}
| -
| 148
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Serine|Serine]]
| S
| Ser
| -CH<sub>2</sub>OH
| {{na}}
| -
| {{ya}}
| -
| {{ya}}
| {{ya}}
| -
| 73
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Threonine|Threonine]]
| T
| Thr
| -CH(OH)CH<sub>3</sub>
| {{na}}
| -
| {{ya}}
| -
| {{ya}}
| {{na}}
| -
| 93
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Selenocysteine|Selenocysteine]]
| U
| Sec
| -CH<sub>2</sub>[[Chemistry:Selenol|SeH]]
| {{na}}
| 5.43
| {{na}}
| acidic
| {{ya}}
| {{ya}}
| -
| {{dunno}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Valine|Valine]]
| V
| Val
| -CH(CH<sub>3</sub>)<sub>2</sub>
| {{ya}}
| -
| {{na}}
| -
| {{ya}}
| {{na}}
| Aliphatic
| 105
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Tryptophan|Tryptophan]]
| W
| Trp
| -CH<sub>2</sub>[[Chemistry:Indole|C<sub>8</sub>H<sub>6</sub>N]]
| {{ya}}
| -
| {{na}}
| -
| {{na}}
| {{na}}
| Aromatic
| 163
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Tyrosine|Tyrosine]]
| Y
| Tyr
| -CH<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>OH
| {{na}}
| 9.84
| {{ya}}
| weak acidic
| {{na}}
| {{na}}
| Aromatic
| 141
|}

§: Values for Asp, Cys, Glu, His, Lys & Tyr were determined using the amino acid residue placed centrally in an alanine pentapeptide.<ref>{{cite journal | vauthors = Thurlkill RL, Grimsley GR, Scholtz JM, Pace CN | title = pK values of the ionizable groups of proteins | journal = Protein Science | volume = 15 | issue = 5 | pages = 1214–8 | date = May 2006 | pmid = 16597822 | pmc = 2242523 | doi = 10.1110/ps.051840806 }}</ref> The value for Arg is from Pace ''et al.'' (2009).<ref>{{cite journal | vauthors = Pace CN, Grimsley GR, Scholtz JM | title = Protein ionizable groups: pK values and their contribution to protein stability and solubility | journal = The Journal of Biological Chemistry | volume = 284 | issue = 20 | pages = 13285–9 | date = May 2009 | pmid = 19164280 | pmc = 2679426 | doi = 10.1074/jbc.R800080200 | doi-access = free }}</ref> The value for Sec is from Byun & Kang (2011).<ref>{{cite journal | vauthors = Byun BJ, Kang YK | title = Conformational preferences and pK(a) value of selenocysteine residue | journal = Biopolymers | volume = 95 | issue = 5 | pages = 345–53 | date = May 2011 | pmid = 21213257 | doi = 10.1002/bip.21581 | s2cid = 11002236 }}</ref>

N.D.: The pKa value of Pyrrolysine has not been reported.

Note: The pKa value of an amino-acid residue in a small peptide is typically slightly different when it is inside a protein. [[Biology:Protein pKa calculations|Protein pKa calculations]] are sometimes used to calculate the change in the pKa value of an amino-acid residue in this situation.

===Gene expression and biochemistry===
{| class="wikitable sortable" style="text-align: center"
! scope="col" rowspan="2" | Amino acid
! scope="col" rowspan="2" | Short
! scope="col" rowspan="2" | {{abbr|Abbrev.|Abbreviation}}
! scope="col" rowspan="2" | Codon(s)
! scope="col" colspan="4" | Occurrence 
! scope="col" rowspan="2" | Essential‡ in humans
|-
! scope="col" | in Archaean proteins<br>(%)&
! scope="col" | in Bacteria proteins<br>(%)&
! scope="col" | in Eukaryote proteins<br>(%)&
! scope="col" | Occurrence<br>in human proteins<br>(%)&
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Alanine|Alanine]]
| A
| Ala
| GCU, GCC, GCA, GCG
|8.2
|10.06
|7.63
| 7.01
| {{No}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Cysteine|Cysteine]]
| C
| Cys
| UGU, UGC
|0.98
|0.94
|1.76
| 2.3
| {{sometimes|Conditionally}}
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Aspartic acid|Aspartic acid]]
| D
| Asp
| GAU, GAC
|6.21
|5.59
|5.4
| 4.73
| {{No}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]]
| E
| Glu
| GAA, GAG
|7.69
|6.15
|6.42
| 7.09 
| {{sometimes|Conditionally}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Phenylalanine|Phenylalanine]]
| F
| Phe
| UUU, UUC 
|3.86
|3.89
|3.87
| 3.65
| {{Yes}}
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Glycine|Glycine]]
| G
| Gly
| GGU, GGC, GGA, GGG 
|7.58
|7.76
|6.33
| 6.58
| {{sometimes|Conditionally}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Histidine|Histidine]]
| H 
| His
| CAU, CAC
|1.77
|2.06
|2.44
| 2.63
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Isoleucine|Isoleucine]]
| I
| Ile
| AUU, AUC, AUA
|7.03
|5.89
|5.1
| 4.33
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Lysine|Lysine]] 
| K
| Lys
| AAA, AAG
|5.27
|4.68
|5.64
| 5.72
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]]
| L
| Leu
| UUA, UUG, CUU, CUC, CUA, CUG
|9.31
|10.09
|9.29
| 9.97
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Methionine|Methionine]]
| M 
| Met
| AUG
|2.35
|2.38
|2.25
| 2.13
| {{Yes}}
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]]
| N
| Asn
| AAU, AAC
|3.68
|3.58
|4.28
| 3.58
| {{No}}
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Pyrrolysine|Pyrrolysine]]
| O
| Pyl
| UAG*
|0
|0
|0
| 0
| {{No}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Proline|Proline]]
| P
| Pro
| CCU, CCC, CCA, CCG 
|4.26
|4.61
|5.41
| 6.31
| {{No}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamine|Glutamine]]
| Q
| Gln
| CAA, CAG
|2.38
|3.58
|4.21
| 4.77
| {{No}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Arginine|Arginine]]
| R
| Arg
| CGU, CGC, CGA, CGG, AGA, AGG
|5.51
|5.88
|5.71
| 5.64
| {{sometimes|Conditionally}} 
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Serine|Serine]]
| S
| Ser
| UCU, UCC, UCA, UCG, AGU, AGC
|6.17
|5.85
|8.34
| 8.33
| {{No}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Threonine|Threonine]]
| T
| Thr
| ACU, ACC, ACA, ACG 
|5.44
|5.52
|5.56
| 5.36
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Selenocysteine|Selenocysteine]]
| U
| Sec
| UGA**
|0
|0
|0
| >0
| {{No}}
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Valine|Valine]]
| V
| Val
| GUU, GUC, GUA, GUG 
|7.8
|7.27
|6.2
| 5.96
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tryptophan|Tryptophan]]
| W
| Trp
| UGG 
|1.03
|1.27
|1.24
| 1.22
| {{Yes}}
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tyrosine|Tyrosine]]
| Y
| Tyr
| UAU, UAC 
|3.35
|2.94
|2.87
| 2.66
| {{sometimes|Conditionally}}
|- 
! scope="row" {{rh2|align=right}} | Stop codon†
| -
| Term
| UAA, UAG, UGA††
| {{dunno}}
| {{dunno}}
| {{dunno}}
| {{n/a}}
| {{n/a}}
|}
<span id="pyrronote">*</span> UAG is normally the [[Biology:Stop codon|amber stop codon]], but in organisms containing the biological machinery encoded by the pylTSBCD cluster of genes the amino acid pyrrolysine will be incorporated.<ref>{{cite journal | vauthors = Rother M, Krzycki JA | title = Selenocysteine, pyrrolysine, and the unique energy metabolism of methanogenic archaea | journal = Archaea | volume = 2010 | pages = 1–14 | date = August 2010 | pmid = 20847933 | pmc = 2933860 | doi = 10.1155/2010/453642 | doi-access = free }}</ref><br />
<span id="selenonote">**</span> UGA is normally the opal (or umber) stop codon, but encodes selenocysteine if a [[Biology:SECIS element|SECIS element]] is present.<br />
<span id="stopnote">†</span> The [[Biology:Stop codon|stop codon]] is not an amino acid, but is included for completeness.<br />
<span id="stopcodonnote">††</span> UAG and UGA do not always act as stop codons (see above).<br />
<span id="essentialnote">‡</span> An essential amino acid cannot be synthesized in humans and must, therefore, be supplied in the diet. Conditionally essential amino acids are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts.<br />
<span id="occurrencenote">&</span> Occurrence of amino acids is based on 135 Archaea, 3775 Bacteria, 614 Eukaryota proteomes and human proteome (21 006 proteins) respectively.<ref>{{cite journal | vauthors = Kozlowski LP | title = Proteome-pI: proteome isoelectric point database | journal = Nucleic Acids Research | volume = 45 | issue = D1 | pages = D1112–D1116 | date = January 2017 | pmid = 27789699 | pmc = 5210655 | doi = 10.1093/nar/gkw978 }}</ref>

===Mass spectrometry===
In [[Physics:Mass spectrometry|mass spectrometry]] of peptides and proteins, knowledge of the masses of the residues is useful. The mass of the peptide or protein is the sum of the residue masses plus the mass of [[Chemistry:Water (molecule)|water]] ([[Chemistry:Monoisotopic mass|Monoisotopic mass]] = 18.01056 Da; average mass = 18.0153 Da). The residue masses are calculated from the tabulated chemical formulas and atomic weights.<ref>{{cite web|url=http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl |title=Atomic Weights and Isotopic Compositions for All Elements |access-date=2016-12-12 |publisher=NIST }}</ref> In [[Physics:Mass spectrometry|mass spectrometry]], ions may also include one or more protons ([[Chemistry:Monoisotopic mass|Monoisotopic mass]] = 1.00728 Da; average mass* = 1.0074 Da). *Protons cannot have an average mass, this confusingly infers to Deuterons as a valid isotope, but they should be a different species (see [[Chemistry:Hydron|Hydron]])

{| class="wikitable sortable" style="text-align: center"
|-
! scope="col" | Amino acid
! scope="col" | Short
! scope="col" | {{abbr|Abbrev.|Abbreviation}}
! scope="col" | Formula
! scope="col" | Mon. mass§ ({{abbr|Da|Daltons}})
! scope="col" | {{abbr|Avg. mass|Average mass}} ({{abbr|Da|Daltons}})
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Alanine|Alanine]]
| A
| Ala
| C<sub>3</sub>H<sub>5</sub>NO
| 71.03711
| 71.0779
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Cysteine|Cysteine]]
| C
| Cys
| C<sub>3</sub>H<sub>5</sub>NOS
| 103.00919
| 103.1429
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Aspartic acid|Aspartic acid]]
| D
| Asp
| C<sub>4</sub>H<sub>5</sub>NO<sub>3</sub>
| 115.02694
| 115.0874
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]]
| E
| Glu
| C<sub>5</sub>H<sub>7</sub>NO<sub>3</sub>
| 129.04259
| 129.1140
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Phenylalanine|Phenylalanine]]
| F
| Phe
| C<sub>9</sub>H<sub>9</sub>NO
| 147.06841
|147.1739
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glycine|Glycine]]
| G
| Gly
| C<sub>2</sub>H<sub>3</sub>NO
| 57.02146
| 57.0513
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Histidine|Histidine]]
| H
| His
| C<sub>6</sub>H<sub>7</sub>N<sub>3</sub>O
| 137.05891
| 137.1393
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Isoleucine|Isoleucine]]
| I
| Ile
| C<sub>6</sub>H<sub>11</sub>NO
| 113.08406
| 113.1576
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Lysine|Lysine]]
| K
| Lys
| C<sub>6</sub>H<sub>12</sub>N<sub>2</sub>O
| 128.09496
| 128.1723
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]]
| L
| Leu
| C<sub>6</sub>H<sub>11</sub>NO
| 113.08406
| 113.1576
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Methionine|Methionine]]
| M
| Met
| C<sub>5</sub>H<sub>9</sub>NOS
| 131.04049
| 131.1961
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]]
| N
| Asn
| C<sub>4</sub>H<sub>6</sub>N<sub>2</sub>O<sub>2</sub>
| 114.04293
| 114.1026
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Pyrrolysine|Pyrrolysine]]
| O
| Pyl
| C<sub>12</sub>H<sub>19</sub>N<sub>3</sub>O<sub>2</sub>
| 237.14773
| 237.2982
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Proline|Proline]]
| P
| Pro
| C<sub>5</sub>H<sub>7</sub>NO
| 97.05276
| 97.1152
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamine|Glutamine]]
| Q
| Gln
| C<sub>5</sub>H<sub>8</sub>N<sub>2</sub>O<sub>2</sub>
| 128.05858
| 128.1292
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Arginine|Arginine]]
| R
| Arg
| C<sub>6</sub>H<sub>12</sub>N<sub>4</sub>O
| 156.10111
| 156.1857
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Serine|Serine]]
| S
| Ser
| C<sub>3</sub>H<sub>5</sub>NO<sub>2</sub>
| 87.03203
| 87.0773
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Threonine|Threonine]]
| T
| Thr
| C<sub>4</sub>H<sub>7</sub>NO<sub>2</sub>
| 101.04768
| 101.1039
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Selenocysteine|Selenocysteine]]
| U
| Sec
| C<sub>3</sub>H<sub>5</sub>NOSe
| 150.95364
| 150.0489
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Valine|Valine]]
| V
| Val
| C<sub>5</sub>H<sub>9</sub>NO
| 99.06841
| 99.1311
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tryptophan|Tryptophan]]
| W
| Trp
| C<sub>11</sub>H<sub>10</sub>N<sub>2</sub>O
| 186.07931
| 186.2099
|- 
! scope="row" {{rh2|align=right}} | [[Chemistry:Tyrosine|Tyrosine]]
| Y
| Tyr
| C<sub>9</sub>H<sub>9</sub>NO<sub>2</sub>
| 163.06333
| 163.1733
|}
<span id="monmassnote">§</span> [[Chemistry:Monoisotopic mass|Monoisotopic mass]]

===Stoichiometry and metabolic cost in cell===
The table below lists the abundance of amino acids in ''E.coli'' cells and the metabolic cost (ATP) for synthesis of the amino acids. Negative numbers indicate the metabolic processes are energy favorable and do not cost net ATP of the cell.<ref>{{cite book | vauthors = Phillips R, Kondev J, Theriot J, Garcia HG, Orme N |title=Physical biology of the cell |isbn=978-0-8153-4450-6 |edition=Second | publisher = Garland Science | page = 178 |year=2013 }}</ref> The abundance of amino acids includes amino acids in free form and in polymerization form (proteins).

{| class="wikitable sortable" style="text-align: center"
|-
! scope="col" rowspan="2" | Amino acid
! scope="col" rowspan="2" | Short
! scope="col" rowspan="2" | {{abbr|Abbrev.|Abbreviation}}
! scope="col" rowspan="2" | Abundance <br>(# of molecules (×10<sup>8</sup>)<br>per ''[[Biology:Escherichia coli|E. coli]]'' cell)
! scope="col" colspan="2" | ATP cost in synthesis
|-
! scope="col" | Aerobic<br>conditions
! scope="col" | Anaerobic<br>conditions
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Alanine|Alanine]]
| A
| Ala
| 2.9
| -1
| 1
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Cysteine|Cysteine]]
| C
| Cys
| 0.52
| 11
| 15
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Aspartic acid|Aspartic acid]]
| D
| Asp
| 1.4
| 0
| 2
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]]
| E
| Glu
| 1.5
| -7
| -1
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Phenylalanine|Phenylalanine]]
| F
| Phe
| 1.1
| -6
| 2
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glycine|Glycine]]
| G
| Gly
| 3.5
| -2
| 2
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Histidine|Histidine]]
| H
| His
| 0.54
| 1
| 7
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Isoleucine|Isoleucine]]
| I
| Ile
| 1.7
| 7
| 11
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Lysine|Lysine]]
| K
| Lys
| 2.0
| 5
| 9
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]]
| L
| Leu
| 2.6
| -9
| 1
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Methionine|Methionine]]
| M
| Met
| 0.88
| 21
| 23
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]]
| N
| Asn
| 1.4
| 3
| 5
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Pyrrolysine|Pyrrolysine]]
| O
| Pyl
| -
| -
| -
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Proline|Proline]]
| P
| Pro
| 1.3
| -2
| 4
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamine|Glutamine]]
| Q
| Gln
| 1.5
| -6
| 0
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Arginine|Arginine]]
| R
| Arg
| 1.7
| 5
| 13
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Serine|Serine]]
| S
| Ser
| 1.2
| -2
| 2
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Threonine|Threonine]]
| T
| Thr
| 1.5
| 6
| 8
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Selenocysteine|Selenocysteine]]
| U
| Sec
| -
| -
| -
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Valine|Valine]]
| V
| Val
| 2.4
| -2
| 2
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tryptophan|Tryptophan]]
| W
| Trp
| 0.33
| -7
| 7
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tyrosine|Tyrosine]]
| Y
| Tyr
| 0.79
| -8
| 2
|}

===Remarks===

{| class="wikitable"
! scope="col" | Amino acid
! scope="col" colspan="2" | {{abbr|Abbrev.|Abbreviations}}
! scope="col" | Remarks
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Alanine|Alanine]]
| A
| Ala
| Very abundant and very versatile, it is more stiff than glycine, but small enough to pose only small steric limits for the protein conformation. It behaves fairly neutrally, and can be located in both hydrophilic regions on the protein outside and the hydrophobic areas inside.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]] or [[Chemistry:Aspartic acid|aspartic acid]]
| B
| Asx
| A placeholder when either amino acid may occupy a position
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Cysteine|Cysteine]]
| C
| Cys
| The sulfur atom bonds readily to [[Chemistry:Heavy metals|heavy metal]] ions. Under oxidizing conditions, two cysteines can join in a disulfide bond to form the amino acid [[Chemistry:Cystine|cystine]]. When cystines are part of a protein, [[Biology:Insulin|insulin]] for example, the [[Biology:Tertiary structure|tertiary structure]] is stabilized, which makes the protein more resistant to [[Biology:Denaturation (biochemistry)|denaturation]]; therefore, disulfide bonds are common in proteins that have to function in harsh environments including digestive enzymes (e.g., [[Biology:Pepsin|pepsin]] and [[Biology:Chymotrypsin|chymotrypsin]]) and structural proteins (e.g., [[Biology:Keratin|keratin]]). Disulfides are also found in peptides too small to hold a stable shape on their own (e.g. [[Biology:Insulin|insulin]]).
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Aspartic acid|Aspartic acid]]
| D
| Asp
| Asp behaves similarly to glutamic acid, and carries a hydrophilic acidic group with strong negative charge. Usually, it is located on the outer surface of the protein, making it water-soluble. It binds to positively charged molecules and ions, and is often used in enzymes to fix the metal ion. When located inside of the protein, aspartate and glutamate are usually paired with arginine and lysine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]]
| E
| Glu
| Glu behaves similarly to aspartic acid, and has a longer, slightly more flexible side chain.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Phenylalanine|Phenylalanine]]
| F
| Phe
| Essential for humans, phenylalanine, tyrosine, and tryptophan contain a large, rigid [[Chemistry:Aromaticity|aromatic]] group on the side chain. These are the biggest amino acids. Like isoleucine, leucine, and valine, these are hydrophobic and tend to orient towards the interior of the folded protein molecule. Phenylalanine can be converted into tyrosine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glycine|Glycine]]
| G
| Gly
| Because of the two hydrogen atoms at the α carbon, glycine is not optically active. It is the smallest amino acid, rotates easily, and adds flexibility to the protein chain. It is able to fit into the tightest spaces, e.g., the triple helix of [[Biology:Collagen|collagen]]. As too much flexibility is usually not desired, as a structural component, it is less common than alanine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Histidine|Histidine]]
| H
| His
| His is essential for humans. In even slightly acidic conditions, [[Chemistry:Protonation|protonation]] of the nitrogen occurs, changing the properties of histidine and the polypeptide as a whole.  It is used by many proteins as a regulatory mechanism, changing the conformation and behavior of the polypeptide in acidic regions such as the late [[Biology:Endosome|endosome]] or [[Biology:Lysosome|lysosome]], enforcing conformation change in enzymes. However, only a few histidines are needed for this, so it is comparatively scarce.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Isoleucine|Isoleucine]]
| I
| Ile
| Ile is essential for humans. Isoleucine, leucine, and valine have large aliphatic hydrophobic side chains. Their molecules are rigid, and their mutual hydrophobic interactions are important for the correct folding of proteins, as these chains tend to be located inside of the protein molecule.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]] or [[Chemistry:Isoleucine|isoleucine]]
| J
| Xle
| A placeholder when either amino acid may occupy a position
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Lysine|Lysine]]
| K
| Lys
| Lys is essential for humans, and behaves similarly to arginine. It contains a long, flexible side chain with a positively charged end. The flexibility of the chain makes lysine and arginine suitable for binding to molecules with many negative charges on their surfaces. E.g., DNA-binding proteins have their active regions rich with arginine and lysine. The strong charge makes these two amino acids prone to be located on the outer hydrophilic surfaces of the proteins; when they are found inside, they are usually paired with a corresponding negatively charged amino acid, e.g., aspartate or glutamate.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Leucine|Leucine]]
| L
| Leu
| Leu is essential for humans, and behaves similarly to isoleucine and valine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Methionine|Methionine]]
| M
| Met
| Met is essential for humans. Always the first amino acid to be incorporated into a protein, it is sometimes removed after translation. Like cysteine, it contains sulfur, but with a methyl group instead of hydrogen. This methyl group can be activated, and is used in many reactions where a new carbon atom is being added to another molecule.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Asparagine|Asparagine]]
| N
| Asn
| Similar to aspartic acid, Asn contains an [[Chemistry:Amide|amide]] group where Asp has a carboxyl.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Pyrrolysine|Pyrrolysine]]
| O
| Pyl
| Similar to [[Chemistry:Lysine|lysine]], but it has a [[Chemistry:Pyrroline|pyrroline]] ring attached.  
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Proline|Proline]]
| P
| Pro
| Pro contains an unusual ring to the N-end amine group, which forces the CO-NH amide sequence into a fixed conformation. It can disrupt protein folding structures like α helix or β sheet, forcing the desired kink in the protein chain. Common in [[Biology:Collagen|collagen]], it often undergoes a [[Biology:Post-translational modification|post-translational modification]] to [[Chemistry:Hydroxyproline|hydroxyproline]].
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamine|Glutamine]]
| Q
| Gln
| Similar to glutamic acid,  Gln contains an [[Chemistry:Amide|amide]] group where Glu has a carboxyl. Used in proteins and as a storage for [[Chemistry:Ammonia|ammonia]], it is the most abundant amino acid in the body.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Arginine|Arginine]]
| R
| Arg
| Functionally similar to lysine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Serine|Serine]]
| S
| Ser
| Serine and threonine have a short group ended with a hydroxyl group. Its hydrogen is easy to remove, so serine and threonine often act as hydrogen donors in enzymes. Both are very hydrophilic, so the outer regions of soluble proteins tend to be rich with them.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Threonine|Threonine]]
| T
| Thr
| Essential for humans, Thr behaves similarly to serine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Selenocysteine|Selenocysteine]]
| U
| Sec
| The [[Chemistry:Selenium|selenium]] analog of cysteine, in which [[Chemistry:Selenium|selenium]] replaces the [[Chemistry:Sulfur|sulfur]] atom.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Valine|Valine]]
| V
| Val
| Essential for humans, Val behaves similarly to isoleucine and leucine.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tryptophan|Tryptophan]]
| W
| Trp
| Essential for humans, Trp behaves similarly to phenylalanine and tyrosine. It is a precursor of [[Chemistry:Serotonin|serotonin]] and is naturally fluorescent.
|-
! scope="row" {{rh2|align=right}} | Unknown
| X
| Xaa
| Placeholder when the amino acid is unknown or unimportant.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Tyrosine|Tyrosine]]
| Y
| Tyr
| Tyr behaves similarly to phenylalanine (precursor to tyrosine) and tryptophan, and is a precursor of [[Chemistry:Melanin|melanin]], epinephrine, and thyroid hormones. Naturally fluorescent, its fluorescence is usually quenched by energy transfer to tryptophans.
|-
! scope="row" {{rh2|align=right}} | [[Chemistry:Glutamic acid|Glutamic acid]] or [[Chemistry:Glutamine|glutamine]]
| Z
| Glx
| A placeholder when either amino acid may occupy a position
|}[[File:Amino acid catabolism revised.png|thumb|400px|Amino acid catabolism]]

===Catabolism===
Amino acids can be classified according to the properties of their main products:<ref>{{cite book | vauthors = Ferrier DR | chapter = Chapter 20: Amino Acid Degradation and Synthesis | veditors = Champe PC, Harvey RA, Ferrier DR |title=Lippincott's Illustrated Reviews: Biochemistry (Lippincott's Illustrated Reviews) |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |isbn=978-0-7817-2265-0 | year = 2005 }}</ref>

* Glucogenic, with the products having the ability to form [[Chemistry:Glucose|glucose]] by [[Biology:Gluconeogenesis|gluconeogenesis]]
* Ketogenic, with the products not having the ability to form glucose: These products may still be used for [[Biology:Ketogenesis|ketogenesis]] or lipid synthesis.
* Amino acids catabolized into both glucogenic and ketogenic products
{{clear}}

== See also ==
* [[Chemistry:Glucogenic amino acid|Glucogenic amino acid]]
* [[Chemistry:Ketogenic amino acid|Ketogenic amino acid]]

== References ==
{{Reflist}}

==General references==
{{refbegin}}
* {{cite book |last=Nelson |first=David L. |author2=Cox, Michael M. |title=Lehninger Principles of Biochemistry |edition=3rd |year=2000 |publisher=Worth Publishers |isbn=978-1-57259-153-0 |url-access=registration |url=https://archive.org/details/lehningerprincip01lehn }}
* {{cite journal | vauthors = Kyte J, Doolittle RF | title = A simple method for displaying the hydropathic character of a protein | journal = Journal of Molecular Biology | volume = 157 | issue = 1 | pages = 105–32 | date = May 1982 | pmid = 7108955 | doi = 10.1016/0022-2836(82)90515-0 | citeseerx = 10.1.1.458.454 }}
* {{cite book |last=Meierhenrich |first=Uwe J. |title=Amino acids and the asymmetry of life |edition=1st |year=2008 |publisher=Springer |isbn=978-3-540-76885-2 }}
* {{cite book |last=Biochemistry |first=Harpers |title=Harpers Illustrated Biochemistry |edition=30st |year=2015 |publisher=Lange |isbn=978-0-07-182534-4 }}
{{refend}}

== External links ==
* [http://www.biology.arizona.edu/biochemistry/problem_sets/aa/Dayhoff.html The origin of the single-letter code for the amino acids]

{{Amino acids}}

[[Category:Alpha-Amino acids]]
[[Category:Proteinogenic amino acids| ]]
[[Category:Nitrogen cycle]]

{{Sourceattribution|Proteinogenic amino acid}}