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Amino acids are the building blocks for the formation of polypeptides and ultimately proteins. Therefore, they are the basic components of our body and are essential for physiological functions such as protein synthesis, tissue repair and nutrient absorption. Here, we carefully study the properties of amino acids, how they are used in the body, and where they come from.

There are 20 kinds of amino acids that make up proteins. They all have the same basic structure. The only difference lies in the R group or side chain they have. The simplest and smallest amino acid is glycine, and its R group is a hydrogen (H). They can be subdivided according to their characteristics, determined by the functional groups they possess. In a broad sense, they are divided by charge, hydrophobicity, and polarity. These properties affect the way they interact with surrounding amino acids in peptides and proteins, thereby affecting the 3D structure and properties of proteins.

This diagram shows the chemical structure of the 20 amino acids that make up proteins.

The following table shows the abbreviations and one-letter codes of the 20 amino acids in proteins. In addition, pyrrolysine, which is used for protein biosynthesis in some archaea and bacteria but does not exist in humans, and selenocysteine, a cysteine ​​analogue found only in certain lineages, are included in blue In color. Finally, abbreviations and stop codons for amino acid residues with multiple potential identities are shown in red to complete the alphabet of single-letter abbreviations.

Alanine was found in protein in 1875, accounting for 30% of silk residue. Its low reactivity contributes to the simple, slender structure of silk with almost no crosslinking, which gives the fiber strength, stretch resistance and flexibility. Only the l-stereoisomer is involved in protein biosynthesis.

In humans, arginine is produced when protein is digested. It can then be converted by the body into nitric oxide, a chemical known to relax blood vessels. Due to its vasodilatory effect, arginine has been proposed for the treatment of patients with chronic heart failure, high cholesterol, circulatory disorders and hypertension, although research on these aspects is still ongoing. Arginine can also be synthesized artificially, and arginine-related compounds can be used to treat people with liver insufficiency due to their role in promoting liver regeneration. Although arginine is necessary for growth, but not for body maintenance, research shows that arginine is essential to the wound healing process, especially in people with poor blood circulation.

In 1806, asparagine was purified from asparagus juice and became the first amino acid isolated from natural sources. However, it was not until 1932 that scientists were able to prove that asparagine was present in proteins. Only the l-stereoisomer is involved in the biosynthesis of mammalian proteins. Asparagine is important in removing toxic ammonia from the body.

Aspartic acid was discovered in protein in 1868 and is usually found in animal proteins, but only the l-stereoisomer is involved in protein biosynthesis. The water solubility of this amino acid allows it to exist near the active site of enzymes such as pepsin.

Cysteine ​​is particularly rich in protein in hair, hoof and skin keratin. It was isolated from urinary stones in 1810 and from horns in 1899. Subsequently, it was chemically synthesized, and the structure was resolved in 1903-4. The thiol group in the side chain of cysteine ​​is the key to its properties. It can form a disulfide bond (such as insulin) between the two peptide chains or form a loop in a single chain, thereby affecting the final protein structure . Two cysteine ​​molecules linked together by a disulfide bond constitute the amino acid cystine, which is sometimes listed separately in the list of common amino acids. Cysteine ​​is made from serine and methionine in the body and is only present in the l-stereoisomer of mammalian proteins.

People with a genetic disease of cystinuria cannot effectively reabsorb cystine into the blood. As a result, high levels of cystine accumulate in their urine, where they crystallize and form stones that block the kidneys and bladder.

Glutamine was first isolated from beet juice in 1883, from a protein in 1932, and then chemically synthesized the following year. Glutamine is the most abundant amino acid in our body and has many important functions. In humans, glutamine is synthesized from glutamate, and this conversion step is essential for regulating the level of toxic ammonia in the body and forming urea and purines.

Glutamic acid was isolated from wheat gluten in 1866 and chemically synthesized in 1890. Commonly found in animal proteins, only l-stereoisomers exist in mammalian proteins, and humans can synthesize it from the common intermediate α-ketoglutarate. The monosodium salt of L-glutamic acid, monosodium glutamate (MSG) is commonly used as a seasoning and flavor enhancer. The carboxyl side chain of glutamic acid can act as a donor and acceptor of ammonia that is toxic to the body, allowing ammonia to be safely transported to the liver, where it is converted to urea and excreted by the kidneys. Free glutamic acid can also be degraded into carbon dioxide and water or converted into sugars.

Glycine is the first amino acid isolated from protein, in this case gelatin, and is the only amino acid with no optical activity (no d or l stereoisomers). It is the simplest alpha-amino acid in structure, and it is very inactive when it is incorporated into a protein. Even so, glycine is also important in the biosynthesis of the amino acids serine, coenzyme glutathione, purine, and heme (an important part of hemoglobin).

Histidine was isolated in 1896, and its structure was confirmed by chemical synthesis in 1911. Histidine is the direct precursor of histamine and an important carbon source in purine synthesis. When bound to protein, the side chain of histidine can act as a proton acceptor and donor. When bound to enzymes (such as chymotrypsin) and enzymes involved in carbohydrate, protein, and nucleic acid metabolism, it will deliver important Characteristics. For infants, histidine is considered an essential amino acid. Adults may not consume food for a short time, but it is still considered essential.

Isoleucine was separated from beet molasses in 1904. The hydrophobicity of the side chain of isoleucine is important for determining the tertiary structure of the protein containing it. People with a rare genetic disease called maple syrup urine disease have defective enzymes in the common degradation pathways of isoleucine, leucine, and valine. If left untreated, metabolites will accumulate in the patient’s urine, creating a unique odor, which is why this odor is named.

Leucine was separated from cheese in 1819 and crystalline from muscle and wool in 1820. In 1891, it was synthesized in the laboratory. Only the l-stereoisomer appears in mammalian proteins and can be degraded by enzymes in the body into simpler compounds. Some DNA binding proteins contain regions where leucine is arranged in a structure called a leucine zipper.

Lysine was first isolated from milk protein casein in 1889, and its structure was 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 have very low lysine content, which leads to a lack of lysine in some people who rely heavily on lysine as a food, as well as vegetarians and low-fat dieters. Therefore, efforts have been made to develop lysine-rich corn strains.

Methionine was separated from milk protein casein in 1922, and its structure was solved by laboratory synthesis in 1928. Methionine is an important source of sulfur for many compounds in the body, including cysteine ​​and taurine. Related to its sulfur content, methionine helps prevent fat accumulation in the liver and helps detoxify metabolic waste and toxins. Methionine is the only essential amino acid not found in a large number of soybeans, so it is commercially produced and added to many soybean meal products.

Phenylalanine was first isolated from a natural source (lupin sprouts) in 1879 and then chemically synthesized in 1882. The human body is usually able to break down phenylalanine into tyrosine, but in individuals with hereditary phenylketonuria (PKU), this enzyme produces this conversion inactivity. If left untreated, phenylalanine will accumulate in the blood, leading to mental retardation in children. There are 10,000 children born with this disease, and adopting a low-phenylalanine diet early in life can alleviate this effect.

In 1900, proline was chemically synthesized. The following year, it was separated from the milk protein casein, and its structure showed the same. Humans can synthesize proline from glutamic acid, which only appears in the form of l-stereoisomer in mammalian proteins. When proline is incorporated into a protein, its unique structure will cause the peptide chain to bend or kink sharply, which makes a great contribution to the final structure of the protein. Proline and its derivative Hydroxyproline accounts for 21% of the amino acid residues of fibrin collagen, which is very important for connective tissue.

Serine was first isolated from silk protein in 1865, but its structure was not determined 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 generally important for the catalytic function of serine-containing enzymes (including chymotrypsin and trypsin). Nerve gas and some insecticides work by binding to the serine residue in the active site of acetylcholinesterase, completely inhibiting the enzyme. Esterase activity is essential for breaking down the neurotransmitter acetylcholine, otherwise it will form dangerously high levels, which can quickly lead to convulsions and death.

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

It was separated from casein (milk protein) in 1901, and the structure of tryptophan was established in 1907, but only l-stereoisomers appeared in mammalian proteins. In the human intestines, bacteria decompose dietary tryptophan, release skatole and indole and other compounds, making the stool emit an unpleasant aroma. Tryptophan is converted into vitamin B3 (also called niacin or niacin), but the conversion rate is not enough to keep us healthy. Therefore, we must also take vitamin B3, otherwise it will lead to a deficiency called pellagra.

Tyrosine was separated from the degradation of casein (a protein from cheese) in 1846, then synthesized in the laboratory, and its structure was determined in 1883. Only exists in the l-stereoisomer of mammalian protein, humans can synthesize tyrosine from phenylalanine. Tyrosine is an important precursor of the adrenal hormones adrenaline and norepinephrine. Thyroid hormones include thyroxine And hair and skin pigment melanin. In enzymes, tyrosine residues are usually associated with the active site, and changing the active site will change the specificity of the enzyme or completely eliminate the activity. Patients with severe genetic disease phenylketonuria (PKU) cannot convert phenylalanine to tyrosine, while patients with alkaliuria have defects in tyrosine metabolism and produce unique urine when exposed to air It will darken when it is medium.

The structure of valine was established in 1906 after it was first isolated from albumin in 1879. Only the l-stereoisomer appears in mammalian proteins. Valine can be degraded into simpler compounds in the body, but for people with a rare genetic disease called maple syrup urine disease, a defective enzyme can interrupt this process, and if not treated in time, it may be fatal.

All amino acids have a carboxyl group and an amino group. In the process of amino acid polymerization, the carboxyl group of one amino acid is connected to the amino group of the next amino acid through a peptide bond, and a water molecule is lost at the same time.

The amino acids that belong to the hydrophobic classification are alanine, valine, isoleucine, leucine, methionine, phenylalanine, tryptophan, and tyrosine. As their classification indicates, side chains are often repelled by water, so this affects the positioning of these amino acids in the tertiary structure of proteins.

Due to the hydrophilic nature of the side chain, polar amino acid residues usually appear on the outside of the protein after polymerization. The four amino acids are classified as polar but uncharged (asparagine, glutamine, serine, and threonine).

Aromatic amino acids (phenylalanine, tyrosine, and tryptophan) all belong to other categories, but they all have aromatic side chains. Therefore, they all absorb ultraviolet light to varying degrees, with tyrosine absorbing the most and phenylalanine absorbing the least.

To form a protein, amino acids polymerize and form peptide bonds, starting at the N-terminus and ending at the C-terminus.

-The messenger RNA (mRNA) copied from DNA provides instructions on which amino acid to incorporate in which position to synthesize a specific protein.

-In the ribosome, transfer RNA (tRNA) binds to one end of the mRNA and carries the required amino acid at the other end.

-Additional protein factors contribute to the initiation, extension and termination of protein synthesis.

-The genetic information needed to determine which position to incorporate which amino acid is encoded in the mRNA as a series of three bases or triplets, also known as the triplet code. The 64 possible triplets and their designated amino acids are called the genetic code or amino acid code.

-Many amino acids are encoded by more than one triplet code, such as arginine, arginine is added when encountering CGU, CGC, CGA or CGG. In most organisms, three (sometimes two) tripartite signal chains terminate.

The human body can synthesize 11 of the 20 amino acids, but we cannot synthesize the other 9 types. This may be due to gene loss or mutations over time in response to changing selection pressure, such as the abundance of specific foods containing specific amino acids. Therefore, these are called essential amino acids and must be obtained through our diet. Specific animal species can synthesize different amino acids, so their dietary requirements are also different. For example, humans can synthesize arginine, but dogs and cats cannot-they must be obtained through dietary intake. Unlike humans and dogs, cats cannot synthesize taurine. This is one of the reasons why commercial dog food is not suitable for cats. For humans, the nine amino acids that must be obtained through diet are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Foods containing all nine essential amino acids are called "complete protein" and include meat, seafood, eggs, dairy products, soybeans, quinoa, and buckwheat. Other protein sources, such as nuts, seeds, grains, and legumes, contain some but not all essential amino acids and are therefore called incomplete.

This table shows the recommended daily intake of nine essential amino acids per kilogram of body weight in the United States.

Recommended daily intake (mg/kg body weight)

Let's talk about supplements. A healthy and balanced diet can meet all the essential amino acid needs of your body. However, some advocate taking high-concentration supplements to improve mood, sleep, exercise performance, weight loss, and prevent muscle loss. Looking at many "health and wellbeing" pages, someone is taking full advantage of the benefits of amino acid supplements, but is there sufficient evidence to support this?

The essential amino acid tryptophan is necessary for the production of serotonin, a neurotransmitter that plays an important role in sleep, mood and behavior. Therefore, the effects of manipulation of tryptophan levels on sleep and mood have been studied in many studies. Although there is evidence that consumption of tryptophan levels can negatively affect sleep and mood, many studies have small sample sizes, lack of adequate control, or other problems. Therefore, although it is clearly a key component of the diet, and supplements may have beneficial effects, there is currently a lack of evidence supporting that tryptophan intake is higher than healthy diet intake, and further investigation is needed.

Although there are some studies that show that taking amino acid supplements can have a positive effect on the athletic performance of certain groups, the results between studies vary widely, and many studies show little or no benefit. A clinical trial is also studying the effect of taking amino acid food supplements on skin photoaging, but the results have not yet been announced.