Essential Amino Acids: Chart, Abbreviations and Structure

Discovered in protein in 1875, alanine makes up 30% of the residues in silk. Its low reactivity contributes to the simple, elongated structure of silk with few cross-links which gives the fibers strength, stretch resistance and flexibility. Only the l-stereoisomer participates in the biosynthesis of proteins.

Arg amino acid

In humans, arginine is produced when proteins are digested. It can then be converted into nitric oxide by the human body, a chemical known to relax blood vessels.

Due to its vasodilatory effects, arginine has been put forward for the treatment of people with chronic heart failure, high cholesterol, compromised circulation and high blood pressure, although research on these fronts is still ongoing. Arginine can also be produced synthetically, and arginine-related compounds can be used in treating people with liver dysfunction due to its role in promoting liver regeneration. Although arginine is necessary for growth but not body maintenance, research has indicated that arginine is crucial to the wound-healing process, particularly in those with poor circulation.

Asn amino acid

In 1806, asparagine was purified from asparagus juice, making it the first amino acid to be isolated from a natural source. However, it wasn’t until 1932 that scientists were able to prove that asparagine occurred in proteins. Only the l-stereoisomer participates in the biosynthesis of mammalian proteins. Asparagine is important in the removal of toxic ammonia from the body.

Asp amino acid

Discovered in proteins in 1868, aspartic acid is commonly found in animal proteins, however only the l-stereoisomer participates in the biosynthesis of proteins. The water solubility of this amino acid lends itself to the presence near the active sites of enzymes such as pepsin.

Cys amino acid

Cysteine is particularly abundant in the proteins of hair, hooves, and the keratin of the skin, having been isolated from a urinary calculus in 1810 and from horn in 1899. Subsequently, it was chemically synthesized, and the structure solved in 1903–4.

The sulfur-containing thiol group in cysteine’s side chain is key to its properties, enabling the formation of disulfide bridges between two peptide chains (as with insulin) or loop formation within a single chain, impacting the final protein structure. Two cysteine molecules linked together by a disulfide linkage make up the amino acid cystine, which is sometimes listed separately in common amino acid listings. Cysteine is made in the body from serine and methionine and only present in the l-stereoisomer in mammalian proteins.

People with the genetic condition cystinuria are unable to effectively reabsorb cystine into their bloodstream. Consequently, high levels of cystine build up in their urine where it crystallizes and forms stones that block the kidneys and bladder.

Gln amino acid

Glutamine was first isolated from beet juice in 1883, isolated from a protein in 1932 and subsequently synthesized chemically the following year. Glutamine is the most abundant amino acid in our bodies and performs several important functions. In humans, glutamine is synthesized from glutamic acid and this conversion step is vitally important in regulating the level of toxic ammonia in the body, forming urea and purines.

Glu amino acid

Glutamic acid was isolated from wheat gluten in 1866 and chemically synthesized in 1890. Commonly found in animal proteins, only the l-stereoisomer occurs in mammalian proteins, which humans are able to synthesize from the common intermediate α-ketoglutaric acid. The monosodium salt of l-glutamic acid, monosodium glutamate (MSG) is commonly used as a condiment and flavor enhancer. The carboxyl side chain of glutamic acid is able to act as a donor and acceptor of ammonia, which is toxic to the body, enabling the safe transport of ammonia to the liver where it is converted to urea and excreted by the kidneys. Free glutamic acid can also be degraded to carbon dioxide and water or transformed into sugars.

Gly amino acid

Glycine was the first amino acid to be isolated from a protein, in this case gelatin, and is the only one that is not optically active (no d- or l-stereoisomers). Structurally the simplest of the α-amino acids, it is very unreactive when incorporated into proteins. Even so, glycine is important in the biosynthesis of the amino acid serine, the coenzyme glutathione, purines and heme, a vital part of hemoglobin.

His amino acid

Histidine was isolated in 1896 and its structure was confirmed by chemical synthesis in 1911. Histidine is the direct precursor of histamine and is also an important source of carbon in purine synthesis. When incorporated into proteins, histidine’s side chain can act as a proton acceptor and donor, conveying important properties when combined into enzymes such as chymotrypsin and those involved in the metabolism of carbohydrates, proteins, and nucleic acids.

For infants, histidine is considered an essential amino acid, adults are able to go for short periods without dietary intake but is still considered essential.

Ile amino acid

Isoleucine was isolated from beet sugar molasses in 1904. The hydrophobic nature of isoleucine’s side chain is important in determining the tertiary structure of proteins in which it is included.

Those suffering from a rare inherited disorder called maple syrup urine disease, have a faulty enzyme in the degradation pathway common to isoleucine, leucine, and valine. Without treatment, metabolites build up in patient’s urine contributing the distinctive odor that gives the condition its name.

Leu amino acid

Leucine was isolated from cheese in 1819 and from muscle and wool in its crystalline state in 1820. In 1891, it was synthesized in the laboratory.

Only the l-stereoisomer appears in mammalian protein and can be degraded into simpler compounds by the enzymes of the body. Some DNA binding proteins contain regions in which leucines are arranged in configurations called leucine zippers.

Lys amino acid

Lysine was first isolated from the milk protein casein in 1889, and its structure elucidated in 1902. Lysine is important in the binding of enzymes to coenzymes and plays an important role in the way histones function.

Many cereal crops are very low in lysine which has led to deficiencies in some populations that rely heavily on these for food as well as in vegetarians and low-fat dieters. Consequently, efforts have been made to develop corn strains rich in lysine.

Met amino acid

Methionine was isolated from the milk protein casein in 1922, and its structure was solved by laboratory synthesis in 1928. Methionine is an important source of sulfur for numerous compounds in the body, including cysteine and taurine. Linked to its sulfur content, methionine helps to prevent fat accumulation in the liver, and helps to detoxify metabolic wastes and toxins.

Methionine is the only essential amino acid that is not present in significant amounts of soybeans and is therefore produced commercially and added to many soy meal products.

Phe amino acid

Phenylalanine was first isolated from a natural source (lupine sprouts) in 1879 and subsequently synthesized chemically in 1882. The human body is ordinarily able to break down phenylalanine into tyrosine, however in individuals with the inherited condition phenylketonuria (PKU), the enzyme that performs this conversion lacks activity. If left untreated, phenylalanine builds in the blood causing retarded mental development in children. On in 10,000 children are born with the condition, adopting a diet low in phenylalanine early in life can ease the effects.

Pro amino acid

In 1900, proline was chemically synthesized. The following year it was then isolated from the milk protein casein and its structure shown to be the same. Humans can synthesize proline from glutamic acid, appearing only as the l-stereoisomer in mammalian proteins. When proline is incorporated into proteins, its peculiar structure leads to sharp bends, or kinks, in the peptide chain, contributing greatly to the protein’s final structure. Proline and its derivate hydroxyproline, account for 21 % of the amino-acid residues of the fibrous protein collagen, essential to connective tissue.

Ser amino acid

Serine was first isolated from silk protein in 1865, but its structure was not established until 1902. Humans can synthesize serine from other metabolites, including glycine, although only the l-stereoisomer appears in mammalian proteins. Serine is important for the biosynthesis of many metabolites and is often important to the catalytic function of enzymes in which it is incorporated, including chymotrypsin and trypsin.

Nerve gases and some insecticides act by combining with a serine residue in the active site of acetylcholine esterase, inhibiting the enzyme completely. Esterase activity is essential to breakdown the neurotransmitter acetylcholine otherwise dangerously high levels build up, rapidly leading to convulsions and death.

Thr amino acid

Threonine was isolated from fibrin in 1935 and synthesized in the same year. Only the l-stereoisomer appears in mammalian proteins where it is relatively unreactive. Although important in many reactions in bacteria, its metabolic role in higher animals, including humans, remains unclear.

Trp amino acid

Isolated from casein (milk protein) in 1901, tryptophan’s structure was established in 1907, but only the l-stereoisomer appears in mammalian proteins. In the human gut, bacteria break down dietary tryptophan, releasing compounds like skatole and indole which give feces their unpleasant aroma. Tryptophan is converted to vitamin B3 (also called nicotinic acid or niacin), but not at a sufficient rate to keep us healthy. Consequently we must also ingest vitamin B3, failure to do so leading to a deficiency called pellagra.

Tyr amino acid

In 1846 tyrosine was isolated from the degradation of the casein (a protein from cheese), following which it was synthesized in the laboratory and its structure determined in 1883. Only present in the l-stereoisomer in mammalian proteins, humans can synthesize tyrosine from phenylalanine. Tyrosine is an important precursor to the adrenal hormones epinephrine and norepinephrine, thyroid hormones including thyroxine and the hair and skin pigment melanin. In enzymes, tyrosine residues are often associated with active sites, alteration of which can change enzyme specificity or wipe out activity entirely.

Suffers of the serious genetic condition phenylketonuria (PKU) are unable to convert phenylalanine to tyrosine, whilst patients with alkaptonuria have a defective tyrosine metabolism, producing distinctive urine which darkens when exposed to the air.

Val amino acid

The structure of valine was established in 1906, after first being isolated from albumin in 1879. Only the l-stereoisomer appears in mammalian protein. Valine can be degraded into simpler compounds in the body, but in people with a rare genetic condition called maple syrup urine disease, a faulty enzyme interrupts this process and can prove fatal if untreated.