- How are amino acids classified?
- The 20 protein amino acids
- Glycine (Gly, G)
- Alanine (Ala, A)
- Proline (Pro, P)
- Valine (Val, V)
- Leucine (Leu, L)
- Isoleucine (Ile, I)
- Methionine (Met, M)
- Phenylalanine (Phe, F)
- Tyrosine (Tyr, Y)
- Tryptophan (Trp, W)
- Serine (Ser, S)
- Threonine (Thr, T)
- Cysteine (Cys, C)
- Asparagine (Asn, N)
- Glutamine (Gln, G)
- Lysine (Lys, K)
- Histidine (His, H)
- Arginine (Arg, R)
- Aspartic acid (Asp, D)
- Glutamic acid (Glu, E)
- References
The amino acids are organic compounds that form proteins which are amino acid chains. As their name implies, these contain both basic groups (amino, NH2) and acid groups (carboxyl, COOH).
These subunits are key to the formation of hundreds of thousands of different proteins in organisms as different as a bacterium and an elephant or a fungus and a tree.
Ven diagram for amino acids (Source: Amino_Acids_Venn_Diagram.png: Klaus Hoffmeierderivative work: solde9 / Public domain, via Wikimedia Commons)
More than 200 different amino acids have been described, but scholars of the subject have determined that the proteins of all living beings (simple or complex) are always made up of the same 20, which are joined together to form characteristic linear sequences.
Since all amino acids share the same main “backbone”, it is their side chains that differentiate them; therefore, these molecules can be thought of as the "alphabet" in which the language of the structure of proteins is "written".
The common backbone for all 20 amino acids consists of a carboxyl group (COOH) and an amino group (NH2) linked through a carbon atom, known as α-carbon (the 20 common amino acids are α-amino acids).
The 20 amino acids and their structures
The α-carbon is also joined by a hydrogen atom (H) and a side chain. This side chain, also known as the R group, varies in size, structure, electrical charge and solubility according to each amino acid in question.
How are amino acids classified?
The 20 most common amino acids, that is, protein amino acids, can be divided into two groups: essential and non-essential. The latter are synthesized by the human body, but the former must be acquired with food and are necessary for the functioning of cells.
The essential amino acids for humans and other animals are 9:
- histidine (H, His)
- isoleucine (I, Ile)
- leucine (L, Leu)
- lysine (K, Lys)
- methionine (M, Met)
- phenylalanine (F, Phe)
- threonine (T, Thr)
- tryptophan (W, Trp) and
- valine (V, Val)
The non-essential amino acids are 11:
- Alanine (A, Ala)
- arginine (R, Arg)
- asparagine (N, Asn)
- aspartic acid (D, Asp)
- cysteine (C, Cys)
- glutamic acid (E, Glu)
- glutamine (Q, Gln)
- glycine (G, Gly)
- proline (P, Pro)
- serine (S, Ser) and
- tyrosine (Y, Tyr)
In addition to this classification, the 20 protein amino acids (of which they form proteins) can be separated according to the characteristics of their R groups in:
- Non-polar or aliphatic amino acids: glycine, alanine, proline, valine, leucine, isoleucine and methionine.
- Amino acids with aromatic R groups: phenylalanine, tyrosine and tryptophan.
- Uncharged polar amino acids: serine, threonine, cysteine, asparagine and glutamine.
- Positively charged polar amino acids: lysine, histidine and arginine.
- Negatively charged polar amino acids: aspartic acid and glutamic acid.
The 20 protein amino acids
Here is a brief description of the main characteristics and functions of each of these important compounds:
This is the amino acid with the simplest structure, since its R group consists of a hydrogen atom (H), so it is also small in size. It was isolated for the first time in 1820 from gelatin, but it is also very abundant in the protein that makes up silk: fibroin.
Glycine (Source: Borb, via Wikimedia Commons)
It is not an essential amino acid for mammals, as it can be synthesized by the cells of these animals from other amino acids such as serine and threonine.
It participates directly in some "channels" in cell membranes that control the passage of calcium ions from one side to the other. It also has to do with the synthesis of purines, porphyrins and some inhibitory neurotransmitters in the central nervous system.
This amino acid, also known as 2-aminopropanoic acid, has a relatively simple structure, since its R group consists of a methyl group (-CH3), so its size is also quite small.
Alanina (Source: Borb, via Wikimedia Commons)
It is part of many proteins and, since it can be synthesized by body cells, it is not considered essential, but it is metabolically important. It is very abundant in silk fibroin, from where it was first isolated in 1879.
Alanine can be synthesized from pyruvate, a compound produced by a metabolic pathway known as glycolysis, which consists of the breakdown of glucose to obtain energy in the form of ATP.
It participates in the glucose-alanine cycle, which takes place between the liver and other tissues of animals, and which is a catabolic pathway that depends on protein for the formation of carbohydrates and for energy production.
It is also part of transamination reactions, in gluconeogenesis and in the inhibition of the glycolytic enzyme pyruvate kinase, as well as in hepatic autophagy.
Proline (pyrrolidine-2-carboxylic acid) is an amino acid that has a particular structure, since its R group consists of a pyrrolidine ring, formed by five carbon atoms linked together, including the α carbon atom.
Proline (Source: The original uploader was Paginazero at Italian Wikipedia. / Public domain, via Wikimedia Commons)
In many proteins, the rigid structure of this amino acid is very useful for introducing "twists" or "folds." Such is the case with collagen fibers in most vertebrate animals, which are made up of many proline and glycine residues.
In plants it has been shown that it participates in the maintenance of cellular homeostasis, including redox balance and energy states. It can act as a signaling molecule and modulate different mitochondrial functions, influence cell proliferation or death, etc.
This is another amino acid with an aliphatic R group, which is composed of three carbon atoms (CH3-CH-CH3). Its IUPAC name is 2-3-amino-3-butanoic acid, although it can also be found in the literature as α-aminovalerian acid.
Valine (Source: Borb, via Wikimedia Commons)
Valine was purified for the first time in 1856 from an aqueous extract of the pancreas of a human being, but its name was coined in 1906 due to its structural similarity to valeric acid extracted from some plants.
It is an essential amino acid, as it cannot be synthesized by the body, although it does not appear to perform many other functions in addition to being part of the structure of many globular proteins.
From its degradation other amino acids such as glutamine and alanine, for example, can be synthesized.
Leucine is another essential amino acid and is part of the group of branched chain amino acids, along with valine and isoleucine. The R group that characterizes this compound is an isobutyl group (CH2-CH-CH3-CH3), so it is very hydrophobic (repels water).
Leucine (Source: Borb, via Wikimedia Commons)
It was discovered in 1819 as part of the proteins of animal muscle fibers and sheep's wool.
It is very abundant in proteins such as hemoglobin and participates directly in the regulation of protein turnover and synthesis, as it is an active amino acid from the point of view of intracellular signaling and genetic expression. In many cases it is an enhancer of the flavor of some foods.
Also a branched-chain amino acid, isoleucine was discovered in 1904 from fibrin, a protein that is involved in blood clotting.
Isoleucine (Source: Taekyubabo, via Wikimedia Commons)
Like leucine, it is an essential amino acid, whose side chain consists of a branched chain of 4 carbon atoms (CH3-CH-CH2-CH3).
It is extremely common in cellular proteins, and can represent more than 10% of their weight. It also works in the synthesis of glutamine and alanine, as well as in the balance of branched chain amino acids.
Methionine, also called γ-methylthiol-α-aminobutyric acid, is an amino acid discovered during the first decade of the 20th century, isolated from casein, a protein present in cow's milk.
Methionine (Source: Borb, via Wikimedia Commons)
It is an essential amino acid, it is hydrophobic, since its R group consists of an aliphatic chain with a sulfur atom (-CH2-CH2-S-CH3).
It is essential for the synthesis of many proteins, including hormones, proteins from the skin, hair and nails of animals. It is marketed in the form of tablets that work as natural relaxants, useful for sleeping and, in addition, to maintain the good condition of the hair and nails.
Phenylalanine or β-phenyl-α-aminopropionic acid, is an aromatic amino acid whose R group is a benzene ring. It was discovered in 1879 in a plant of the Fabaceae family and today it is known to be part of many natural resins such as polystyrene.
Phenylalanine (Source: Borb, via Wikimedia Commons)
As a hydrophobic amino acid, phenylalanine is present in virtually all hydrophobic domains of proteins. In many plants, this amino acid is essential for the synthesis of secondary metabolites known as phenylpropanoids and flavonoids.
In animals, phenylalanine is also found in highly important peptides such as vasopressin, melanotropin, and enkephalin, all essential for neuronal function.
Tyrosine (β-parahydroxyphenyl-α-aminopropionic acid) is another aromatic amino acid, whose R group is an aromatic ring associated with a hydroxyl group (-OH), which is why it is capable of interacting with different elements. It was discovered in 1846 and is generally obtained from phenylalanine.
Tyrosine (Source: NEUROtiker / Public domain, via Wikimedia Commons)
It is not an essential amino acid, but it can be if its biosynthetic pathways fail. It has many functions in the human body, among which its participation as a substrate for the synthesis of neurotransmitters and hormones such as adrenaline and thyroid hormone stand out.
It is essential for the synthesis of melanin, a molecule that gives us protection against the sun's ultraviolet rays. It also contributes to the production of endorphins (endogenous pain relievers) and antioxidants such as vitamin E.
It acts directly in the phosphorylation of proteins, as well as in the addition of nitrogen and sulfur groups.
This amino acid, also known as 2-amino-3-indolylpropionic acid, is part of the group of essential amino acids and is also an aromatic amino acid, since its R group consists of an indole group.
Tryptophan (Source: The original uploader was Paginazero at Italian Wikipedia. / Public domain, via Wikimedia Commons)
Its main functions in animals have to do, in addition to protein synthesis, with the synthesis of serotonin, a neurotransmitter, and melatonin, an antioxidant that also functions in the sleep and wake cycles.
This amino acid is also used by cells as a precursor for the formation of the cofactor NAD, which participates in multiple oxidation-reduction enzymatic reactions.
In plants, tryptophan is one of the main precursors for the synthesis of the plant hormone auxin, which participates in the regulation of growth, development and other physiological functions in these organisms.
Serine, or 2-amino-3-hydroxypropanoic acid, is a nonessential amino acid that can be produced from glycine. Its R group is an alcohol of formula -CH2OH, so it is a polar amino acid without charge.
Serina (Source: Borb, via Wikimedia Commons)
It is functionally important in many essential proteins and is required for the metabolism of fats, fatty acids, and cell membranes. Participates in muscle growth and the health of the immune system of mammals.
Their functions are also related to the synthesis of cysteine, purines and pyrimidines (nitrogenous bases), ceramide, and phosphatidylserine (a membrane phospholipid). In bacteria it participates in the synthesis of tryptophan and in ruminants in gluconeogenesis.
It is part of the active site of enzymes with hydrolytic activity known as serine proteases and also participates in the phosphorylation of other proteins.
Threonine or threo-Ls-α-amino-β-butyric acid is another essential amino acid that is part of a large number of cellular proteins in animals and plants. It was one of the last amino acids discovered (1936) and has many important functions in cells, including:
- It is a binding site for the carbohydrate chains of glycoproteins
- It is a recognition site for protein kinases with specific functions
- It is part of important proteins such as those that form tooth enamel, elastin and collagen, as well as others of the nervous system
- Pharmacologically it is used as a food supplement, anxiolytic and antidepressant
Threonine (Source: Borb, via Wikimedia Commons)
The R group of threonine, like that of serine, contains an -OH group, so it is an alcohol with the structure -CH-OH-CH3.
This non-essential amino acid was discovered in 1810 as the main constituent of the protein found in the horns of different animals.
Cysteine (Source: The original uploader was Paginazero at Italian Wikipedia. / Public domain, via Wikimedia Commons)
Its R group consists of a thiol or sulfhydryl group (-CH2-SH), so it is essential for the formation of intra- and intermolecular disulfide bridges in proteins where it is found, which is very important for the establishment of the structure. three-dimensional of these.
This amino acid is also involved in the synthesis of glutathione, methionine, lipoic acid, thiamine, coenzyme A, and many other biologically important molecules. In addition, it is part of keratins, structural proteins that are very abundant in animals.
Asparagine is a non-essential amino acid, belonging to the group of uncharged polar amino acids. This was the first amino acid discovered (1806), isolated from asparagus juice.
Asparagine (Source: Borb, via Wikimedia Commons)
It is characterized by a group R is a carboxamide (-CH2-CO-NH2), so it can easily form hydrogen bonds.
It is active in cell metabolism and body physiology of animals. It works in the regulation of gene expression and the immune system, in addition to participating in the nervous system and in the detoxification of ammonia.
The R group of glutamine is described by some authors as an amide of the side chain of glutamic acid (-CH2-CH2-CO-NH2). This is not an essential amino acid, as there are routes for its biosynthesis in animal cells.
Glutamine (Source: Borb, via Wikimedia Commons)
It participates directly in the turnover of proteins and in cell signaling, in gene expression and in the immune system of mammals. It is considered as a "fuel" for proliferating cells and has functions as an inhibitor of cell death.
Glutamine is also involved in the synthesis of purines, pyrimidines, ornithine, citrulline, arginine, proline, and asparagine.
Lysine or ε-aminocaproic acid is an essential amino acid for humans and other animals. It was discovered in 1889 as part of casein, gelatin, egg albumin, and other animal proteins.
Lysine (Source: Borb, via Wikimedia Commons)
In its R group, lysine has a positively charged amino group (-CH2-CH2-CH2-CH2-NH3 +) characterized by its hydrophobicity.
It not only acts as a micronutrient for cells in the body, but it is also a metabolite for different types of organisms. It is essential for muscle growth and remodeling and also appears to have antiviral activity, in protein methylation, and in other modifications.
It is a "semi-essential" amino acid, as there are routes for its synthesis in adult human beings, but they do not always meet the body's requirements.
Histidine (Source: NEUROtiker / Public domain, via Wikimedia Commons)
It is a polar amino acid that has an R group known as imidazolium, which has a cyclic structure with two nitrogen atoms whose characteristics allow it to participate in different enzymatic reactions where proton transfers occur.
Histidine participates in the methylation of proteins, is part of the structure of hemoglobin (the protein that carries oxygen in the blood of animals), is in some antioxidant dipeptides and is a precursor of other important molecules such as histamine.
This positively charged amino acid was first isolated in 1895 from the antler proteins of some animals. It is not an essential amino acid, but it is very important for the synthesis of urea, one of the ways in which nitrogen is excreted in animals.
Arginine (Source: Borb, via Wikimedia Commons)
Its R group is -CH2-CH2-CH2-NH-C-NH-NH2 and acts as an antioxidant, regulator of hormonal secretion, ammonium detoxifier, regulator of gene expression, nitrogen reservoir, in protein methylation, etc..
Aspartic acid has one R group with a second carboxyl group (-CH2-COOH) and is part of the group of negatively charged amino acids.
Aspartic acid (Source: The original uploader was Paginazero at Italian Wikipedia. / Public domain, via Wikimedia Commons)
Its main functions have to do with the synthesis of purines, pyrimidines, asparagine and arginine. It participates in transamination reactions, in the urea cycle and in the synthesis of inositol.
It also belongs to the group of negatively charged amino acids, with an R group with the structure -CH2-CH2-COOH, very similar to that of aspartic acid. It was discovered in 1866 from hydrolyzed wheat gluten and is known to be part of many common proteins in many living things.
Glutamic acid (Source: Borb, via Wikimedia Commons)
This nonessential amino acid has many important functions in animal cells, especially in the synthesis of glutamine and arginine, two other protein amino acids.
Furthermore, it is an important mediator of the transmission of excitatory signals in the central nervous system of vertebrate animals, so its presence in certain proteins is crucial for brain function, for cognitive development, memory and learning.
References
- Fonnum, F. (1984). Glutamate: a neurotransmitter in the mammalian brain. Journal of Neurochemistry, 18 (1), 27–33.
- Nelson, DL, Lehninger, AL, & Cox, MM (2008). Lehninger principles of biochemistry. Macmillan.
- Szabados, L., & Savoure, A. (2010). Proline: a multifunctional amino acid. Trends in plant science, 15 (2), 89-97.
- Wu, G. (2009). Amino acids: metabolism, functions, and nutrition. Amino acids, 37 (1), 1-17.
- Wu, G. (2013). Amino acids: biochemistry and nutrition. CRC Press.