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Phenylalanine is one of the standard amino acids.

In chemistry, an amino acid is a molecule that contains both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent.[1] In the alpha amino acids, the amino and carboxylate groups are attached to the same carbon, which is called the α–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. They can vary in size from just a hydrogen atom in glycine, through a methyl group in alanine, to a large heterocyclic group in tryptophan.

Beyond the amino acids that recur throughout biochemistry (see below), many non-natural amino acids are also important.

Overview[edit | edit source]

Alpha-amino acids are the building blocks of proteins. A protein forms via the condensation of amino acids to form a chain of amino acid "residues" linked by peptide bonds. Proteins are defined by their unique sequence of amino acid residues; this sequence is the primary structure of the protein. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a huge variety of proteins.

About twenty standard amino acids are used by cells in protein biosynthesis, and these are specified by the general genetic code. These twenty amino acids are biosynthesized from other molecules, but organisms differ in which ones they can synthesize and which ones must be provided in their diet. The ones that cannot be synthesized by an organism are called essential amino acids.

Functions in proteins[edit | edit source]

A polypeptide is a chain of amino acids.

Amino acids are the basic structural building units of proteins. They form short polymer chains called peptides or longer chains either called polypeptides or proteins. The process of such formation from an mRNA template is known as translation which is part of protein biosynthesis. Twenty amino acids are encoded by the standard genetic code and are called proteinogenic or standard amino acids. Other amino acids contained in proteins are usually formed by post-translational modification, which is modification after translation in protein synthesis. These modifications are often essential for the function or regulation of a protein; for example, the carboxylation of glutamate allows for better binding of calcium cations, and the hydroxylation of proline is critical for maintaining connective tissues and responding to oxygen starvation. 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.

Non-protein functions[edit | edit source]

The twenty standard amino acids are either used to synthesize proteins and other biomolecules, or oxidized to urea and carbon dioxide as a source of energy.[2] 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.[3] Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.[4]

Hundreds of types of non-protein amino acids have been found in nature and they have multiple functions in living organisms. Microorganisms and plants can produce uncommon amino acids. In microbes, examples include 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged alanine dimer. Both these amino acids are both found in peptidic lantibiotics such as alamethicin.[5] While in plants, 1-Aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene.[6]

In humans, non-protein amino acids also have biologically-important roles. Glycine, gamma-aminobutyric acid and glutamate are neurotransmitters and many amino acids are used to synthesize other molecules, for example:

Also present are hydroxyproline, hydroxylysine, and sarcosine. The thyroid hormones are also alpha-amino acids.

Some amino acids have even been detected in meteorites, especially in a type known as carbonaceous chondrites.[7] This observation has prompted the suggestion that life may have arrived on earth from an extraterrestrial source.

General structure[edit | edit source]

Further information: List of standard amino acids

The general structure of an α-amino acid, with the amino group on the left and the carboxyl group on the right.

In the structure shown to the right, the R represents a side chain specific to each amino acid. The central carbon atom called Cα is a chiral central carbon atom (with the exception of glycine) to which the two termini and the R-group are attached. Amino acids are usually classified by the properties of the side chain into four groups. The side chain can make them behave like a weak acid, a weak base, a hydrophile if they are polar, and hydrophobe if they are nonpolar. The chemical structures of the 20 standard amino acids, along with their chemical properties, are cataloged in the list of standard amino acids.

The phrase "branched-chain amino acids" or BCAA is sometimes used to refer to the amino acids having aliphatic side-chains that are non-linear, these are leucine, isoleucine and valine. Proline is the only proteinogenic amino acid whose side group links to the α-amino group, and thus is also the only proteinogenic amino acid containing a secondary amine at this position. Proline has sometimes been termed an imino acid, but this is not correct in the current nomenclature.[8]

The two optical isomers of alanine.

Isomerism[edit | edit source]

Most amino acids can exist in either of two optical isomers, called D and L. The L-amino acids represent the vast majority of amino acids found in proteins. D-amino acids are found in some proteins produced by exotic sea-dwelling organisms, such as cone snails.[9] They are also abundant components of the peptidoglycan cell walls of bacteria.[10]

The L and D conventions for amino acid configuration do not refer to the optical activity, but rather to the optical activity of the isomer of glyceraldehyde having the same stereochemistry as the amino acid. S-Glyceraldehyde is levorotary, and R-glyceraldehyde is dexterorotary, and so S-amino acids are called L- even if they are not levorotary, and R-amino acids are likewise called D- even if they are not dexterorotary.

There are two exceptions to these general rules of amino acid isomerism. Firstly, glycine, where R = H, no isomerism is possible because the alpha-carbon bears two identical groups (hydrogen). Secondly, in cysteine, the L = S and D = R assignment is reversed to L = R and D = S. Cysteine is structured similarly (with respect to glyceraldehyde) to the other amino acids but the sulfur atom alters the interpretation of the Cahn-Ingold-Prelog priority rule.

Reactions[edit | edit source]


Peptide bond formation[edit | edit source]

The condensation of two amino acids to form a peptide bond.

For more details on this topic, see Peptide bond.

As both the amine and carboxylic acid groups of amino acids can react to form amide bonds, one amino acid molecule can react with another and become joined through an amide linkage. This polymerization of amino acids is what creates proteins. This condensation reaction yields the newly formed peptide bond and a molecule of water. In cells, this reaction does not occur directly, instead the amino acid is activated by attachment to a transfer RNA molecule through an ester bond. This aminoacyl-tRNA is produced in an ATP-dependent reaction carried out by an aminoacyl tRNA synthetase.[11] This aminoacyl-tRNA is then a substrate for the ribosome, which catalyzes the attack of the amino group of the elongating protein chain on the ester bond.[12] As a result of this mechanism, all proteins are synthesized starting at their N-terminus and moving towards their C-terminus.

However, not all peptide bonds are formed in this way. In a few cases peptides are synthesized by specific enzymes. For example, the tripeptide glutathione is an essential part of the defenses of cells against oxidative stress. This peptide is synthesized in two steps from free amino acids.[13] In the first step gamma-glutamylcysteine synthetase condenses cysteine and glutamic acid through a peptide bond formed between the side-chain carboxyl of the glutamate (the gamma carbon of this side chain) and the amino group of the cysteine. This dipeptide is then condensed with glycine by glutathione synthetase to form glutathione.[14]


Zwitterions[edit | edit source]

An amino acid, in its (1) normal (unionized) and (2) zwitterionic forms.

As amino acids have both the active groups of an amine and a carboxylic acid they can be considered both acid and base (though their natural pH is usually influenced by the R group). At a certain pH known as the isoelectric point, the amine group gains a positive charge (is protonated) and the acid group a negative charge (is deprotonated). The exact value is specific to each different amino acid. This ion is known as a zwitterion, which comes from the German word Zwitter meaning "hybrid". A zwitterion can be extracted from the solution as a white crystalline structure with a very high melting point, due to its dipolar nature. Near-neutral physiological pH allows most free amino acids to exist as zwitterions.

Hydrophilic and hydrophobic amino acids[edit | edit source]

Depending on the polarity of the side chain, amino acids vary in their hydrophilic or hydrophobic character. These properties are important in protein structure and protein-protein interactions. The importance of the physical properties of the side chains comes from the influence this has on the amino acid residues' interactions with other structures, both within a single protein and between proteins. The distribution of hydrophilic and hydrophobic amino acids determines the tertiary structure of the protein, and their physical location on the outside structure of the proteins influences their quaternary structure. For example, soluble proteins have surfaces rich with polar amino acids like serine and threonine, while integral membrane proteins tend to have outer ring of hydrophobic amino acids that anchors them into the lipid bilayer, and proteins anchored to the membrane have a hydrophobic end that locks into the membrane. Similarly, proteins that have to bind to positively-charged molecules have surfaces rich with negatively charged amino acids like glutamate and aspartate, while proteins binding to negatively-charged molecules have surfaces rich with positively charged chains like lysine and arginine. Recently a new scale of hydrophobicity based on the free energy of hydrophobic association has been proposed[16]

Hydrophilic and hydrophobic interactions of the proteins do not have to rely only on the sidechains of amino acids themselves. By various posttranslational modifications other chains can be attached to the proteins, forming hydrophobic lipoproteins or hydrophilic glycoproteins.

Table of standard amino acid abbreviations and side chain properties[edit | edit source]

Main article: List of standard amino acids
Amino Acid 3-Letter 1-Letter Side chain polarity Side chain acidity or basicity Hydropathy index[17]
Alanine Ala A nonpolar neutral 1.8
Arginine Arg R polar basic (strongly) -4.5
Asparagine Asn N polar neutral -3.5
Aspartic acid Asp D polar acidic -3.5
Cysteine Cys C polar neutral 2.5
Glutamic acid Glu E polar acidic -3.5
Glutamine Gln Q polar neutral -3.5
Glycine Gly G nonpolar neutral -0.4
Histidine His H polar basic (weakly) -3.2
Isoleucine Ile I nonpolar neutral 4.5
Leucine Leu L nonpolar neutral 3.8
Lysine Lys K polar basic -3.9
Methionine Met M nonpolar neutral 1.9
Phenylalanine Phe F nonpolar neutral 2.8
Proline Pro P nonpolar neutral -1.6
Serine Ser S polar neutral -0.8
Threonine Thr T polar neutral -0.7
Tryptophan Trp W nonpolar neutral -0.9
Tyrosine Tyr Y nonpolar neutral -1.3
Valine Val V nonpolar neutral 4.2

In addition to the normal amino acid codes, placeholders were used historically in cases where chemical or crystallographic analysis of a peptide or protein could not completely establish the identity of a certain residue in a structure. The ones they could not resolve between are these pairs of amino-acids:

Ambiguous Amino Acids 3-Letter 1-Letter
Asparagine or aspartic acid Asx B
Glutamine or glutamic acid Glx Z
Leucine or Isoleucine Xle J
Unspecified or unknown amino acid Xaa X


Nonstandard amino acids[edit | edit source]

The amino acid selenocysteine.

Aside from the twenty standard amino acids, there is a vast number of "nonstandard amino acids". Two of these can be encoded in the genetic code, but are rather rare in proteins. Selenocysteine is incorporated into some proteins at a UGA codon, which is normally a stop codon.[18] Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane. It is coded for with the codon UAG.[19]

Examples of nonstandard amino acids that are not found in proteins include lanthionine, 2-aminoisobutyric acid, dehydroalanine and the neurotransmitter gamma-aminobutyric acid. Nonstandard amino acids often occur as intermediates in the metabolic pathways for standard amino acids - for example ornithine and citrulline occur in the urea cycle, part of amino acid catabolism.[20]

Nonstandard amino acids are usually formed through modifications to standard amino acids. For example, homocysteine is formed through the transsulfuration pathway or by the demethylation of methionine via the intermediate metabolite S-adenosyl methionine, [21] while dopamine is synthesized from l-DOPA, and hydroxyproline is made by a posttranslational modification of proline.[22]

Uses in technology[edit | edit source]

Amino acid derivative Use in industry
Aspartame (aspartyl-phenylalanine-1-methyl ester) Low-calorie artificial sweetener
5-HTP (5-hydroxytryptophan) Treatment for depression and the neurological problems of phenylketonuria.
L-DOPA (L-dihydroxyphenylalanine) Treatment for Parkinsonism.
Monosodium glutamate Food additive that enhances flavor. Confers the taste umami.

Nutritional importance[edit | edit source]

Further information: Protein in nutrition

Of the 20 standard proteinogenic amino acids, 10 are called essential amino acids because the human body cannot synthesize them from other compounds through chemical reactions, and they therefore must be obtained from food. Cysteine, tyrosine, histidine and arginine are considered as semiessential amino acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed.[23]

Essential Nonessential
Isoleucine Alanine
Leucine Asparagine
Lysine Aspartate
Methionine Cysteine
Phenylalanine Glutamate
Threonine Glutamine
Tryptophan Glycine
Valine Proline
Arginine* Serine
Histidine* Tyrosine

(*) Essential only in certain cases

Several common mnemonics have evolved for remembering the essential amino acids. PVT TIM HALL ("Private Tim Hall") uses the first letter of each essential amino acid, excluding arginine.[24] Another mnemonic that frequently occurs in student practice materials is "These ten valuable amino acids have long preserved life in man".[25]

See also[edit | edit source]

References and notes[edit | edit source]

  1. Proline is an exception to this general formula. It lacks the NH2 group because of the cyclization of the side chain.
  2. Sakami W, Harrington H. Amino acid metabolism. Annu Rev Biochem 32: 355-98.
  3. Brosnan J (2000). Glutamate, at the interface between amino acid and carbohydrate metabolism. J Nutr 130 (4S Suppl): 988S-90S.
  4. Young V, Ajami A (2001). Glutamine: the emperor or his clothes?. J Nutr 131 (9 Suppl): 2449S-59S; discussion 2486S-7S.
  5. Whitmore L, Wallace B (2004). Analysis of peptaibol sequence composition: implications for in vivo synthesis and channel formation.. Eur Biophys J 33 (3): 233-7.
  6. Alexander L, Grierson D (2002). Ethylene biosynthesis and action in tomato: a model for climacteric fruit ripening. J Exp Bot 53 (377): 2039-55.
  7. Llorca J (2004). Organic matter in meteorites.. Int Microbiol 7 (4): 239-48.
  8. Claude Liebecq (Ed) Biochemical Nomenclature and Related Documents, 2nd edition, Portland Press, 1992, pages 39-69 ISBN 978-1855780057
  9. Pisarewicz K, Mora D, Pflueger F, Fields G, Marí F (2005). Polypeptide chains containing D-gamma-hydroxyvaline.. J Am Chem Soc 127 (17): 6207-15.
  10. van Heijenoort J (2001). Formation of the glycan chains in the synthesis of bacterial peptidoglycan.. Glycobiology 11 (3): 25R-36R.
  11. Ibba M, Söll D (2001). The renaissance of aminoacyl-tRNA synthesis. EMBO Rep 2 (5): 382-7.
  12. Lengyel P, Söll D (1969). Mechanism of protein biosynthesis. Bacteriol Rev 33 (2): 264-301.
  13. Wu G, Fang Y, Yang S, Lupton J, Turner N (2004). Glutathione metabolism and its implications for health. J Nutr 134 (3): 489-92.
  14. Meister A (1988). Glutathione metabolism and its selective modification. J Biol Chem 263 (33): 17205–8.
  15. Carpino, L. A. (1992) 1-Hydroxy-7-azabenzotriazole. An efficient Peptide Coupling Additive. J. Am. Chem. Soc. 115, 4397-4398.
  16. Urry, D. W. (2004). The change in Gibbs free energy for hydrophobic association - Derivation and evaluation by means of inverse temperature transitions. Chemical Physics Letters 399 (1-3): 177-183.
  17. Kyte J & RF Doolittle (1982). A simple method for displaying the hydropathic character of a protein. J. Mol. Biol. (157): 105-132. PMID 7108955.
  18. Driscoll D, Copeland P. Mechanism and regulation of selenoprotein synthesis.. Annu Rev Nutr 23: 17-40.
  19. Krzycki J (2005). The direct genetic encoding of pyrrolysine.. Curr Opin Microbiol 8 (6): 706-12.
  20. Curis E, Nicolis I, Moinard C, Osowska S, Zerrouk N, Bénazeth S, Cynober L (2005). Almost all about citrulline in mammals. Amino Acids 29 (3): 177-205.
  21. Brosnan J, Brosnan M (2006). The sulfur-containing amino acids: an overview. J Nutr 136 (6 Suppl): 1636S-1640S.
  22. Kivirikko K, Pihlajaniemi T. Collagen hydroxylases and the protein disulfide isomerase subunit of prolyl 4-hydroxylases. Adv Enzymol Relat Areas Mol Biol 72: 325-98.
  23. Imura K, Okada A (1998). Amino acid metabolism in pediatric patients. Nutrition 14 (1): 143-8.
  24. Distance Medical Biochemistry course from the University of New England. http://www.faculty.une.edu/com/courses/bionut/distbio/obj-512/Chap39-pvttimhall.htm Access date 25 February 2006
  25. Memory aids for medical biochemistry. http://mednote.co.kr/Yellownote/BIOCHMNEMON.htm Access date 25 February 2006

Further reading[edit | edit source]


External links[edit | edit source]

Amino acids

Essential amino acid | Protein | Peptide | Genetic code