Proteinogenic amino acid

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Template:Short description

Proteinogenic amino acids are a small fraction of all amino acids

Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation from RNA. The word "proteinogenic" means "protein creating". Throughout known life, there are 22 genetically encoded (proteinogenic) amino acids, 20 in the standard genetic code and an additional 2 (selenocysteine and pyrrolysine) that can be incorporated by special translation mechanisms.<ref>Template:Cite journal</ref>

In contrast, non-proteinogenic amino acids are amino acids that are either not incorporated into proteins (like GABA, L-DOPA, or triiodothyronine), misincorporated in place of a genetically encoded amino acid, or not produced directly and in isolation by standard cellular machinery (like hydroxyproline). The latter often results from post-translational modification of proteins. Some non-proteinogenic amino acids are incorporated into nonribosomal peptides which are synthesized by non-ribosomal peptide synthetases.

Both eukaryotes and prokaryotes can incorporate selenocysteine into their proteins via a nucleotide sequence known as a SECIS element, which directs the cell to translate a nearby UGA codon as selenocysteine (UGA is normally a stop codon). In some methanogenic prokaryotes, the UAG codon (normally a stop codon) can also be translated to pyrrolysine.<ref name="LobanovTuranov2010">Template:Cite journal</ref>

In eukaryotes, there are only 21 proteinogenic amino acids, the 20 of the standard genetic code, plus 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 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).<ref>Template:Cite journal</ref>

The proteinogenic amino acids have been found to be related to the set of amino acids that can be recognized by ribozyme autoaminoacylation systems.<ref>Template:Cite journal</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, 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.

The evolutionary selection of certain proteinogenic amino acids from the primordial soup has been suggested to be because of their better incorporation into a polypeptide chain as opposed to non-proteinogenic amino acids.<ref>Template:Cite journal</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 zwitterion forms that exist in aqueous solutions.

Structure of the 21 proteinogenic amino acids with 3 and 1 letters codes, grouped by side chain functionality

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IUPAC/IUBMB now also recommends standard abbreviations for the following two amino acids:

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 isotopes at their natural abundances. Forming a peptide bond results in elimination of a molecule of 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

Amino acid Short Template:Abbr Avg. mass (Da) Template:Abbr pK1
(α-COO−) 		pK2
(α-NH3+)
scope="row" Template:Rh2 | Alanine A Ala 89.09404 6.01 2.35 9.87
scope="row" Template:Rh2 | Cysteine C Cys 121.15404 5.05 1.92 10.70
scope="row" Template:Rh2 | Aspartic acid D Asp 133.10384 2.85 1.99 9.90
scope="row" Template:Rh2 | Glutamic acid E Glu 147.13074 3.15 2.10 9.47
scope="row" Template:Rh2 | Phenylalanine F Phe 165.19184 5.49 2.20 9.31
scope="row" Template:Rh2 | Glycine G Gly 75.06714 6.06 2.35 9.78
scope="row" Template:Rh2 | Histidine H His 155.15634 7.60 1.80 9.33
scope="row" Template:Rh2 | Isoleucine I Ile 131.17464 6.05 2.32 9.76
scope="row" Template:Rh2 | Lysine K Lys 146.18934 9.60 2.16 9.06
scope="row" Template:Rh2 | Leucine L Leu 131.17464 6.01 2.33 9.74
scope="row" Template:Rh2 | Methionine M Met 149.20784 5.74 2.13 9.28
scope="row" Template:Rh2 | Asparagine N Asn 132.11904 5.41 2.14 8.72
scope="row" Template:Rh2 | Pyrrolysine O Pyl 255.31 Template:Dunno Template:Dunno Template:Dunno
scope="row" Template:Rh2 | Proline P Pro 115.13194 6.30 1.95 10.64
scope="row" Template:Rh2 | Glutamine Q Gln 146.14594 5.65 2.17 9.13
scope="row" Template:Rh2 | Arginine R Arg 174.20274 10.76 1.82 8.99
scope="row" Template:Rh2 | Serine S Ser 105.09344 5.68 2.19 9.21
scope="row" Template:Rh2 | Threonine T Thr 119.12034 5.60 2.09 9.10
scope="row" Template:Rh2 | Selenocysteine U Sec 168.053 5.47 1.91 10
scope="row" Template:Rh2 | Valine V Val 117.14784 6.00 2.39 9.74
scope="row" Template:Rh2 | Tryptophan W Trp 204.22844 5.89 2.46 9.41
scope="row" Template:Rh2 | Tyrosine Y Tyr 181.19124 5.64 2.20 9.21

Side-chain properties

Amino acid Short Template:Abbr Side chain Hydro-
phobic 		Template:Abbr Polar pH Small Tiny Aromatic
or Aliphatic 		van der Waals
volume (Å3)
scope="row" Template:Rh2 | Alanine A Ala -CH3 Template:Ya - Template:Na - Template:Ya Template:Ya Aliphatic 67
scope="row" Template:Rh2 | Cysteine C Cys -CH2SH Template:Ya 8.55 Template:Ya acidic Template:Ya Template:Ya - 86
scope="row" Template:Rh2 | Aspartic acid D Asp -CH2COOH Template:Na 3.67 Template:Ya acidic Template:Ya Template:Na - 91
scope="row" Template:Rh2 | Glutamic acid E Glu -CH2CH2COOH Template:Na 4.25 Template:Ya acidic Template:Na Template:Na - 109
scope="row" Template:Rh2 | Phenylalanine F Phe -CH2C6H5 Template:Ya - Template:Na - Template:Na Template:Na Aromatic 135
scope="row" Template:Rh2 | Glycine G Gly -H Template:Ya - Template:Na - Template:Ya Template:Ya - 48
scope="row" Template:Rh2 | Histidine H His -CH2-C3H3N2 Template:Na 6.54 Template:Ya weak basic Template:Na Template:Na Aromatic 118
scope="row" Template:Rh2 | Isoleucine I Ile -CH(CH3)CH2CH3 Template:Ya - Template:Na - Template:Na Template:Na Aliphatic 124
scope="row" Template:Rh2 | Lysine K Lys -(CH2)4NH2 Template:Na 10.40 Template:Ya basic Template:Na Template:Na - 135
scope="row" Template:Rh2 | Leucine L Leu -CH2CH(CH3)2 Template:Ya - Template:Na - Template:Na Template:Na Aliphatic 124
scope="row" Template:Rh2 | Methionine M Met -CH2CH2SCH3 Template:Ya - Template:Na - Template:Na Template:Na Aliphatic 124
scope="row" Template:Rh2 | Asparagine N Asn -CH2CONH2 Template:Na - Template:Ya - Template:Ya Template:Na - 96
scope="row" Template:Rh2 | Pyrrolysine O Pyl -(CH2)4NHCOC4H5NCH3 Template:Na Template:Abbr Template:Ya weak basic Template:Na Template:Na - Template:Dunno
scope="row" Template:Rh2 | Proline P Pro -CH2CH2CH2- Template:Ya - Template:Na - Template:Ya Template:Na - 90
scope="row" Template:Rh2 | Glutamine Q Gln -CH2CH2CONH2 Template:Na - Template:Ya - Template:Na Template:Na - 114
scope="row" Template:Rh2 | Arginine R Arg -(CH2)3NH-C(NH)NH2 Template:Na 12.3 Template:Ya strongly basic Template:Na Template:Na - 148
scope="row" Template:Rh2 | Serine S Ser -CH2OH Template:Na - Template:Ya - Template:Ya Template:Ya - 73
scope="row" Template:Rh2 | Threonine T Thr -CH(OH)CH3 Template:Na - Template:Ya - Template:Ya Template:Na - 93
scope="row" Template:Rh2 | Selenocysteine U Sec -CH2SeH Template:Na 5.43 Template:Na acidic Template:Ya Template:Ya - Template:Dunno
scope="row" Template:Rh2 | Valine V Val -CH(CH3)2 Template:Ya - Template:Na - Template:Ya Template:Na Aliphatic 105
scope="row" Template:Rh2 | Tryptophan W Trp -CH2C8H6N Template:Ya - Template:Na - Template:Na Template:Na Aromatic 163
scope="row" Template:Rh2 | Tyrosine Y Tyr -CH2-C6H4OH Template:Na 9.84 Template:Ya weak acidic Template:Na Template:Na Aromatic 141

§: Only ionizable residues have a meaningful pKa. Values for Asp, Cys, Glu, His, Lys & Tyr were determined using the amino acid residue placed centrally in an alanine pentapeptide.<ref>Template:Cite journal</ref> The value for Arg is from Pace et al. (2009).<ref>Template:Cite journal</ref> The value for Sec is from Byun & Kang (2011).<ref>Template:Cite journal</ref>

Note: the pKa value of an amino-acid residue in a small peptide is typically slightly different when it is inside a protein. 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

Amino acid Short Template:Abbr Codon(s) Occurrence Essential‡ in humans
in Archaean proteins
(%)& 		in Bacteria proteins
(%)& 		in Eukaryote proteins
(%)& 		in human proteins
(%)&
scope="row" Template:Rh2 | Alanine A Ala GCU, GCC, GCA, GCG 8.2 10.06 7.63 7.01 Template:No
scope="row" Template:Rh2 | Cysteine C Cys UGU, UGC 0.98 0.94 1.76 2.3 Template:Sometimes
scope="row" Template:Rh2 | Aspartic acid D Asp GAU, GAC 6.21 5.59 5.4 4.73 Template:No
scope="row" Template:Rh2 | Glutamic acid E Glu GAA, GAG 7.69 6.15 6.42 7.09 Template:Sometimes
scope="row" Template:Rh2 | Phenylalanine F Phe UUU, UUC 3.86 3.89 3.87 3.65 Template:Yes
scope="row" Template:Rh2 | Glycine G Gly GGU, GGC, GGA, GGG 7.58 7.76 6.33 6.58 Template:Sometimes
scope="row" Template:Rh2 | Histidine H His CAU, CAC 1.77 2.06 2.44 2.63 Template:Yes
scope="row" Template:Rh2 | Isoleucine I Ile AUU, AUC, AUA 7.03 5.89 5.1 4.33 Template:Yes
scope="row" Template:Rh2 | Lysine K Lys AAA, AAG 5.27 4.68 5.64 5.72 Template:Yes
scope="row" Template:Rh2 | Leucine L Leu UUA, UUG, CUU, CUC, CUA, CUG 9.31 10.09 9.29 9.97 Template:Yes
scope="row" Template:Rh2 | Methionine M Met AUG 2.35 2.38 2.25 2.13 Template:Yes
scope="row" Template:Rh2 | Asparagine N Asn AAU, AAC 3.68 3.58 4.28 3.58 Template:No
scope="row" Template:Rh2 | Pyrrolysine O Pyl UAG* 0 0 0 0 Template:No
scope="row" Template:Rh2 | Proline P Pro CCU, CCC, CCA, CCG 4.26 4.61 5.41 6.31 Template:No
scope="row" Template:Rh2 | Glutamine Q Gln CAA, CAG 2.38 3.58 4.21 4.77 Template:No
scope="row" Template:Rh2 | Arginine R Arg CGU, CGC, CGA, CGG, AGA, AGG 5.51 5.88 5.71 5.64 Template:Sometimes
scope="row" Template:Rh2 | Serine S Ser UCU, UCC, UCA, UCG, AGU, AGC 6.17 5.85 8.34 8.33 Template:No
scope="row" Template:Rh2 | Threonine T Thr ACU, ACC, ACA, ACG 5.44 5.52 5.56 5.36 Template:Yes
scope="row" Template:Rh2 | Selenocysteine U Sec UGA** 0 0 0 >0 Template:No
scope="row" Template:Rh2 | Valine V Val GUU, GUC, GUA, GUG 7.8 7.27 6.2 5.96 Template:Yes
scope="row" Template:Rh2 | Tryptophan W Trp UGG 1.03 1.27 1.24 1.22 Template:Yes
scope="row" Template:Rh2 | Tyrosine Y Tyr UAU, UAC 3.35 2.94 2.87 2.66 Template:Sometimes
scope="row" Template:Rh2 | Stop codon† - Term UAA, UAG, UGA†† Template:Dunno Template:Dunno Template:Dunno Template:N/a Template:N/a

* UAG is normally the 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>Template:Cite journal</ref>
** UGA is normally the opal (or umber) stop codon, but encodes selenocysteine if a SECIS element is present.
† The stop codon is not an amino acid, but is included for completeness.
†† UAG and UGA do not always act as stop codons (see above).
‡ 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.
& Occurrence of amino acids is based on 135 Archaea, 3775 Bacteria, 614 Eukaryota proteomes and human proteome (21 006 proteins) respectively.<ref>Template:Cite journal</ref>

Mass spectrometry

In 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 water (Monoisotopic mass = 18.01056 Da; average mass = 18.0153 Da). The residue masses are calculated from the tabulated chemical formulas and atomic weights.<ref>Template:Cite web</ref> In mass spectrometry, ions may also include one or more protons (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 Hydron (chemistry))

Amino acid Short Template:Abbr Formula Mon. mass§ (Template:Abbr) Template:Abbr (Template:Abbr)
scope="row" Template:Rh2 | Alanine A Ala C3H5NO 71.03711 71.0779
scope="row" Template:Rh2 | Cysteine C Cys C3H5NOS 103.00919 103.1429
scope="row" Template:Rh2 | Aspartic acid D Asp C4H5NO3 115.02694 115.0874
scope="row" Template:Rh2 | Glutamic acid E Glu C5H7NO3 129.04259 129.1140
scope="row" Template:Rh2 | Phenylalanine F Phe C9H9NO 147.06841 147.1739
scope="row" Template:Rh2 | Glycine G Gly C2H3NO 57.02146 57.0513
scope="row" Template:Rh2 | Histidine H His C6H7N3O 137.05891 137.1393
scope="row" Template:Rh2 | Isoleucine I Ile C6H11NO 113.08406 113.1576
scope="row" Template:Rh2 | Lysine K Lys C6H12N2O 128.09496 128.1723
scope="row" Template:Rh2 | Leucine L Leu C6H11NO 113.08406 113.1576
scope="row" Template:Rh2 | Methionine M Met C5H9NOS 131.04049 131.1961
scope="row" Template:Rh2 | Asparagine N Asn C4H6N2O2 114.04293 114.1026
scope="row" Template:Rh2 | Pyrrolysine O Pyl C12H19N3O2 237.14773 237.2982
scope="row" Template:Rh2 | Proline P Pro C5H7NO 97.05276 97.1152
scope="row" Template:Rh2 | Glutamine Q Gln C5H8N2O2 128.05858 128.1292
scope="row" Template:Rh2 | Arginine R Arg C6H12N4O 156.10111 156.1857
scope="row" Template:Rh2 | Serine S Ser C3H5NO2 87.03203 87.0773
scope="row" Template:Rh2 | Threonine T Thr C4H7NO2 101.04768 101.1039
scope="row" Template:Rh2 | Selenocysteine U Sec C3H5NOSe 150.95364 150.0489
scope="row" Template:Rh2 | Valine V Val C5H9NO 99.06841 99.1311
scope="row" Template:Rh2 | Tryptophan W Trp C11H10N2O 186.07931 186.2099
scope="row" Template:Rh2 | Tyrosine Y Tyr C9H9NO2 163.06333 163.1733

§ 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>Template:Cite book</ref> The abundance of amino acids includes amino acids in free form and in polymerization form (proteins).

Amino acid Short Template:Abbr Abundance
(# of molecules (×108)
per E. coli cell) 		ATP cost in synthesis
Aerobic
conditions 		Anaerobic
conditions
scope="row" Template:Rh2 | Alanine A Ala 2.9 −1 1
scope="row" Template:Rh2 | Cysteine C Cys 0.52 11 15
scope="row" Template:Rh2 | Aspartic acid D Asp 1.4 0 2
scope="row" Template:Rh2 | Glutamic acid E Glu 1.5 −7 −1
scope="row" Template:Rh2 | Phenylalanine F Phe 1.1 −6 2
scope="row" Template:Rh2 | Glycine G Gly 3.5 −2 2
scope="row" Template:Rh2 | Histidine H His 0.54 1 7
scope="row" Template:Rh2 | Isoleucine I Ile 1.7 7 11
scope="row" Template:Rh2 | Lysine K Lys 2.0 5 9
scope="row" Template:Rh2 | Leucine L Leu 2.6 −9 1
scope="row" Template:Rh2 | Methionine M Met 0.88 21 23
scope="row" Template:Rh2 | Asparagine N Asn 1.4 3 5
scope="row" Template:Rh2 | Pyrrolysine O Pyl - - -
scope="row" Template:Rh2 | Proline P Pro 1.3 −2 4
scope="row" Template:Rh2 | Glutamine Q Gln 1.5 −6 0
scope="row" Template:Rh2 | Arginine R Arg 1.7 5 13
scope="row" Template:Rh2 | Serine S Ser 1.2 −2 2
scope="row" Template:Rh2 | Threonine T Thr 1.5 6 8
scope="row" Template:Rh2 | Selenocysteine U Sec - - -
scope="row" Template:Rh2 | Valine V Val 2.4 −2 2
scope="row" Template:Rh2 | Tryptophan W Trp 0.33 −7 7
scope="row" Template:Rh2 | Tyrosine Y Tyr 0.79 −8 2

Remarks

Amino acid Template:Abbr Remarks
scope="row" Template:Rh2 | 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" Template:Rh2 | Asparagine or aspartic acid B Asx A placeholder when either amino acid may occupy a position
scope="row" Template:Rh2 | Cysteine C Cys The sulfur atom bonds readily to heavy metal ions. Under oxidizing conditions, two cysteines can join in a disulfide bond to form the amino acid cystine. When cystines are part of a protein, insulin for example, the tertiary structure is stabilized, which makes the protein more resistant to denaturation; therefore, disulfide bonds are common in proteins that have to function in harsh environments including digestive enzymes (e.g., pepsin and chymotrypsin) and structural proteins (e.g., keratin). Disulfides are also found in peptides too small to hold a stable shape on their own (e.g. insulin).
scope="row" Template:Rh2 | 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" Template:Rh2 | Glutamic acid E Glu Glu behaves similarly to aspartic acid, and has a longer, slightly more flexible side chain.
scope="row" Template:Rh2 | Phenylalanine F Phe Essential for humans, phenylalanine, tyrosine, and tryptophan contain a large, rigid 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" Template:Rh2 | 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 collagen. As too much flexibility is usually not desired, as a structural component, it is less common than alanine.
scope="row" Template:Rh2 | Histidine H His His is essential for humans. In even slightly acidic conditions, 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 endosome or lysosome, enforcing conformation change in enzymes. However, only a few histidines are needed for this, so it is comparatively scarce.
scope="row" Template:Rh2 | 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" Template:Rh2 | Leucine or isoleucine J Xle A placeholder when either amino acid may occupy a position
scope="row" Template:Rh2 | 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" Template:Rh2 | Leucine L Leu Leu is essential for humans, and behaves similarly to isoleucine and valine.
scope="row" Template:Rh2 | 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" Template:Rh2 | Asparagine N Asn Similar to aspartic acid, Asn contains an amide group where Asp has a carboxyl.
scope="row" Template:Rh2 | Pyrrolysine O Pyl Similar to lysine, but it has a pyrroline ring attached.
scope="row" Template:Rh2 | 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 collagen, it often undergoes a post-translational modification to hydroxyproline.
scope="row" Template:Rh2 | Glutamine Q Gln Similar to glutamic acid, Gln contains an amide group where Glu has a carboxyl. Used in proteins and as a storage for ammonia, it is the most abundant amino acid in the body.
scope="row" Template:Rh2 | Arginine R Arg Functionally similar to lysine.
scope="row" Template:Rh2 | 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" Template:Rh2 | Threonine T Thr Essential for humans, Thr behaves similarly to serine.
scope="row" Template:Rh2 | Selenocysteine U Sec The selenium analog of cysteine, in which selenium replaces the sulfur atom.
scope="row" Template:Rh2 | Valine V Val Essential for humans, Val behaves similarly to isoleucine and leucine.
scope="row" Template:Rh2 | Tryptophan W Trp Essential for humans, Trp behaves similarly to phenylalanine and tyrosine. It is a precursor of serotonin and is naturally fluorescent.
scope="row" Template:Rh2 | Unknown X Xaa Placeholder when the amino acid is unknown or unimportant.
scope="row" Template:Rh2 | Tyrosine Y Tyr Tyr behaves similarly to phenylalanine (precursor to tyrosine) and tryptophan, and is a precursor of melanin, epinephrine, and thyroid hormones. Naturally fluorescent, its fluorescence is usually quenched by energy transfer to tryptophans.
scope="row" Template:Rh2 | Glutamic acid or glutamine Z Glx A placeholder when either amino acid may occupy a position
Amino acid catabolism

Catabolism

Amino acids can be classified according to the properties of their main products:<ref>Template:Cite book</ref>

  • Glucogenic, with the products having the ability to form glucose by gluconeogenesis
  • Ketogenic, with the products not having the ability to form glucose: These products may still be used for ketogenesis or lipid synthesis.
  • Amino acids catabolized into both glucogenic and ketogenic products

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See also

References

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General references

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