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Amino acid
Amino acid
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 are found in all forms of life, many non-natural amino acids have vital roles in technology and industry. For example, the chelating agents EDTA and nitriloacetic acid are alpha amino acids that are important in the chemical industry.
Overview
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.
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
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
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:
· Tryptophan is a precursor of the neurotransmitter serotonin
· Glycine is a precursor of porphyrins such as heme
· Arginine is a precursor of nitric oxide
· Carnitine is used in lipid transport within a cell,
· Ornithine and S-adenosylmethionine are precursors of polyamines,
· Homocysteine is an intermediate in S-adenosylmethionine recycling
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.
Uses in technology
Amino acid derivative and use in industry
Aspartame (aspartyl-phenylalanine-1-methyl ester) as Low-calorie artificial sweetener
5-HTP (5-hydroxytryptophan) used in Treatment for depression and the neurological problems of phenylketonuria.
L-DOPA (L-dihydroxyphenylalanine) used in Treatment for Parkinsonism.
Monosodium glutamate as Food additive that enhances flavor. Confers the taste umami.
Nutritional importance
Of the 20 standard proteinogenic amino acids, 8 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food.[23] However, the situation is a little more complicated since cysteine, tyrosine, histidine and arginine are semiessential amino acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed.[24] The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids.
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.[25][26]
Several common mnemonics have evolved for remembering the ten amino acids often described as essential. PVT TIM HALL ("Private Tim Hall") uses the first letter of each of these amino acids.[27] Another mnemonic that frequently occurs in student practice materials beneath "TV TILL Past Midnight", which just includes the 8 essential amino acids, is "These ten valuable amino acids have long preserved life in man".[28]
References and notes
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. PMID 14144484.
3. ^ Brosnan J (2000). "Glutamate, at the interface between amino acid and carbohydrate metabolism". J Nutr 130 (4S Suppl): 988S-90S. PMID 10736367.
4. ^ Young V, Ajami A (2001). "Glutamine: the emperor or his clothes?". J Nutr 131 (9 Suppl): 2449S-59S; discussion 2486S-7S. PMID 11533293.
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. PMID 14534753.
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. PMID 12324528.
7. ^ Llorca J (2004). "Organic matter in meteorites.". Int Microbiol 7 (4): 239-48. PMID 15666244.
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. PMID 15853325.
10. ^ van Heijenoort J (2001). "Formation of the glycan chains in the synthesis of bacterial peptidoglycan.". Glycobiology 11 (3): 25R-36R. PMID 11320055.
11. ^ Ibba M, Söll D (2001). "The renaissance of aminoacyl-tRNA synthesis". EMBO Rep 2 (5): 382-7. PMID 11375928.
12. ^ Lengyel P, Söll D (1969). "Mechanism of protein biosynthesis". Bacteriol Rev 33 (2): 264-301. PMID 4896351.
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. PMID 14988435.
14. ^ Meister A (1988). "Glutathione metabolism and its selective modification". J Biol Chem 263 (33): 17205–8. PMID 3053703.
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. PMID 12524431.
19. ^ Krzycki J (2005). "The direct genetic encoding of pyrrolysine.". Curr Opin Microbiol 8 (6): 706-12. PMID 16256420.
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. PMID 16082501.
21. ^ Brosnan J, Brosnan M (2006). "The sulfur-containing amino acids: an overview". J Nutr 136 (6 Suppl): 1636S-1640S. PMID 16702333.
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. PMID 9559057.
23. ^ Young VR (1994). "Adult amino acid requirements: the case for a major revision in current recommendations". J. Nutr. 124 (8 Suppl): 1517S–1523S. PMID 8064412.
24. ^ Imura K, Okada A (1998). "Amino acid metabolism in pediatric patients". Nutrition 14 (1): 143-8. PMID 9437700.
25. ^ Fürst P, Stehle P (2004). "What are the essential elements needed for the determination of amino acid requirements in humans?". J. Nutr. 134 (6 Suppl): 1558S–1565S. PMID 15173430.
26. ^ Reeds PJ (2000). "Dispensable and indispensable amino acids for humans". J. Nutr. 130 (7): 1835S–40S. PMID 10867060.
27. ^ Chapter 39 PVT TIM HALL. Retrieved on 2007-09-29.
28. ^ Memory aids for medical biochemistry. http://mednote.co.kr/Yellownote/BIOCHMNEMON.htm Access date 25 February 2006