coenzyme a. in human nutrition share of amino acid in various human diets and the resulting mix of amino acids in human blood serum. glutamate and glutamine are the most frequent in food at over 10%, while alanine, glutamine, and glycine are the most common in blood. further information: protein (nutrient) and amino acid synthesis when taken up into the human body from the diet, the 20 standard amino acids either are used to synthesize proteins, other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy. 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. glucogenic amino acids can also be converted into glucose, through gluconeogenesis. of the 20 standard amino acids, nine (his, ile, leu, lys, met, phe, thr, trp and val) 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. in addition, cysteine, tyrosine, and arginine are considered semiessential amino acids, and taurine a semiessential aminosulfonic acid in children. the metabolic pathways that synthesize these monomers are not fully developed. 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. dietary exposure to the nonstandard amino acid bmaa has been linked to human neurodegenerative diseases, including als. diagram of the molecular signaling cascades that are involved in myofibrillar muscle protein synthesis and mitochondrial biogenesis in response to physical exercise and specific amino acids or their derivatives (primarily l-leucine and hmb). many amino acids derived from food protein promote the activation of mtorc1 and increase protein synthesis by signaling through rag gtpases.abbreviations and representations: • pld: phospholipase d • pa: phosphatidic acid • mtor: mechanistic target of rapamycin • amp: adenosine monophosphate • atp: adenosine triphosphate • ampk: amp-activated protein kinase • pgc‐1α: peroxisome proliferator-activated receptor gamma coactivator-1α • s6k1: p70s6 kinase • 4ebp1: eukaryotic translation initiation factor 4e-binding protein 1 • eif4e: eukaryotic translation initiation factor 4e • rps6: ribosomal protein s6 • eef2: eukaryotic elongation factor 2 • re: resistance exercise; ee: endurance exercise • myo: myofibrillar; mito: mitochondrial • aa: amino acids • hmb: β-hydroxy β-methylbutyric acid • ↑ represents activation • τ represents inhibition resistance training stimulates muscle protein synthesis (mps) for a period of up to 48 hours following exercise (shown by lighter dotted line). ingestion of a protein-rich meal at any point during this period will augment the exercise-induced increase in muscle protein synthesis (shown by solid lines). non-protein functions biosynthetic pathways for catecholamines and trace amines in the human brain l-phenylalanine l-tyrosine l-dopa epinephrine phenethylamine p-tyramine dopamine norepinephrine n-methylphenethylamine n-methyltyramine p-octopamine synephrine 3-methoxytyramine aadc aadc aadc primarypathway pnmt pnmt pnmt pnmt aaah aaah braincyp2d6 minorpathway comt dbh dbh catecholamines and trace amines are synthesized from phenylalanine and tyrosine in humans. in humans, non-protein amino acids also have important roles as metabolic intermediates, such as in the biosynthesis of the neurotransmitter gamma-aminobutyric acid (gaba). many amino acids are used to synthesize other molecules, for example: tryptophan is a precursor of the neurotransmitter serotonin. tyrosine (and its precursor phenylalanine) are precursors of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine and various trace amines. phenylalanine is a precursor of phenethylamine and tyrosine in humans. in plants, it is a precursor of various phenylpropanoids, which are important in plant metabolism. glycine is a precursor of porphyrins such as heme. arginine is a precursor of nitric oxide. ornithine and s-adenosylmethionine are precursors of polyamines. aspartate, glycine, and glutamine are precursors of nucleotides. however, not all of the functions of other abundant nonstandard amino acids are known. some nonstandard amino acids are used as defenses against herbivores in plants. for example, canavanine is an analogue of arginine that is found in many legumes, and in particularly large amounts in canavalia gladiata (sword bean). this amino acid protects the plants from predators such as insects and can cause illness in people if some types of legumes are eaten without processing. the non-protein amino acid mimosine is found in other species of legume, in particular leucaena leucocephala. this compound is an analogue of tyrosine and can poison animals that graze on these plants. uses in industry fertilizer the chelating ability of amino acids is sometimes used in fertilizers to facilitate the delivery of minerals to plants in order to correct mineral deficiencies, such as iron chlorosis. these fertilizers are also used to prevent deficiencies from occurring and to improve the overall health of the plants. animal feed amino acids are sometimes added to animal feed because some of the components of these feeds, such as soybeans, have low levels of some of the essential amino acids, especially of lysine, methionine, threonine, and tryptophan. likewise amino acids are used to chelate metal cations in order to improve the absorption of minerals from feed supplements. food the food industry is a major consumer of amino acids, especially glutamic acid, which is used as a flavor enhancer, and aspartame (aspartylphenylalanine 1-methyl ester), which is used as an artificial sweetener. amino acids are sometimes added to food by manufacturers to alleviate symptoms of mineral deficiencies, such as anemia, by improving mineral absorption and reducing negative side effects from inorganic mineral supplementation. pharmaceuticals and cosmetics similarly, some amino acids derivatives are used in pharmaceutical industry. they include 5-htp (5-hydroxytryptophan) used for experimental treatment of depression, l-dopa (l-dihydroxyphenylalanine) for parkinson's treatment, and eflornithine drug that inhibits ornithine decarboxylase and used in the treatment of sleeping sickness. amino acids are used in the synthesis of some cosmetics. expanded genetic code since 2001, 40 non-natural amino acids have been added into protein by creating a unique codon (recoding) and a corresponding transfer-rna:aminoacyl – trna-synthetase pair to encode it with diverse physicochemical and biological properties in order to be used as a tool to exploring protein structure and function or to create novel or enhanced proteins. nullomers nullomers are codons that in theory code for an amino acid, however, in nature there is a selective bias against using this codon in favor of another, for example bacteria prefer to use cga instead of aga to code for arginine. this creates some sequences that do not appear in the genome. this characteristic can be taken advantage of and used to create new selective cancer-fighting drugs and to prevent cross-contamination of dna samples from crime-scene investigations. chemical building blocks amino acids are important as low-cost feedstocks. these compounds are used in chiral pool synthesis as enantiomerically pure building blocks. amino acids have been investigated as precursors chiral catalysts, such as for asymmetric hydrogenation reactions, although no commercial applications exist. biodegradable plastics amino acids have been considered as components of biodegradable polymers, which have applications as environmentally friendly packaging and in medicine in drug delivery and the construction of prosthetic implants. an interesting example of such materials is polyaspartate, a water-soluble biodegradable polymer that may have applications in disposable diapers and agriculture. due to its solubility and ability to chelate metal ions, polyaspartate is also being used as a biodegradable antiscaling agent and a corrosion inhibitor. in addition, the aromatic amino acid tyrosine has been considered as a possible replacement for phenols such as bisphenol a in the manufacture of polycarbonates. synthesis the strecker amino acid synthesis chemical synthesis the commercial production of amino acids usually relies on mutant bacteria that overproduce individual amino acids using glucose as a carbon source. some amino acids are produced by enzymatic conversions of synthetic intermediates. 2-aminothiazoline-4-carboxylic acid is an intermediate in one industrial synthesis of l-cysteine for example. aspartic acid is produced by the addition of ammonia to fumarate using a lyase. biosynthesis in plants, nitrogen is first assimilated into organic compounds in the form of glutamate, formed from alpha-ketoglutarate and ammonia in the mitochondrion. for other amino acids, plants use transaminases to move the amino group from glutamate to another alpha-keto acid. for example, aspartate aminotransferase converts glutamate and oxaloacetate to alpha-ketoglutarate and aspartate. other organisms use transaminases for amino acid synthesis, too. 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-adenosylmethionine, while hydroxyproline is made by a post translational modification of proline. microorganisms and plants synthesize many uncommon amino acids. for example, some microbes make 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged derivative of alanine. both of these amino acids are found in peptidic lantibiotics such as alamethicin. however, in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is an intermediate in the production of the plant hormone ethylene. primordial synthesis the formation of amino acids and peptides are assumed to precede and perhaps induce the emergence of life on earth. amino acids can form from simple precursors under various conditions. surface-based chemical metabolism of amino acids and very small compounds may have led to the build-up of amino acids, coenzymes and phosphate-based small carbon molecules. amino acids and similar building blocks could have been elaborated into proto-peptides, with peptides being considered key players in the origin of life. in the famous urey-miller experiment, the passage of an electric arc through a mixture of methane, hydrogen, and ammonia produces a large number of amino acids. since then, scientists have discovered a range of ways and components by which the potentially prebiotic formation and chemical evolution of peptides may have occurred, such as condensing agents, the design of self-replicating peptides and a number of non-enzymatic mechanisms by which amino acids could have emerged and elaborated into peptides. several hypotheses invoke the strecker synthesis whereby hydrogen cyanide, simple aldehydes, ammonia, and water produce amino acids. according to a review, amino acids, and even peptides, "turn up fairly regularly in the various experimental broths that have been allowed to be cooked from simple chemicals. this is because nucleotides are far more difficult to synthesize chemically than amino acids." for a chronological order, it suggests that there must have been a 'protein world' or at least a 'polypeptide world', possibly later followed by the 'rna world' and the 'dna world'. codon–amino acids mappings may be the biological information system at the primordial origin of life on earth. while amino acids and consequently simple peptides must have formed under different experimentally probed geochemical scenarios, the transition from an abiotic world to the first life forms is to a large extent still unresolved. reactions amino acids undergo the reactions expected of the constituent functional groups. peptide bond formation the condensation of two amino acids to form a dipeptide. the two amino acid residues are linked through a 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 first 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. 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. as a result of this mechanism, all proteins made by ribosomes are synthesized starting at their n-terminus and moving toward 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. in the first step, gamma-glutamylcysteine synthetase condenses cysteine and glutamate 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