Amino acids

Amino acids, carbohydrates, and fat are among the central organic building blocks of all cells and tissues. All amino acids contain a basic amino group (-NH2), an acidic carboxyl group (-COOH), and the rest of the molecule grouped around a central carbon atom. This configuration allows two spatial structures, which are comparable to our left and right hands and are called L- and D-form most of the time.

Only 20 out of the several hundred known amino acids serve as building blocks for tissue and enzymes. Nearly all of them belong to the L-form, which makes this property negligible in most cases.

Amino acids are able to form chains by connecting their amino and carboxyl groups. When forming these so-called peptide bonds water is split off. The generated amino acid chains are referred to as peptides or proteins depending on their length.

Oligopeptides, i.e., chains made up of only a few amino acids, usually serve as signaling molecules within the body, such as hormones or neurotransmitters. Chains of several hundred amino acids are required for enzyme function. Enzymes are functional proteins (biocatalysts) that control most of the biological processes in the body. Structural proteins found in muscles, bones, connective tissue, cartilage, hair, and others usually consist of even longer chains of several thousand amino acids.

Their properties are determined by the type, sequence and number of amino acids, and additional molecules such as sugar molecules, metal complexes, and others. Due to their enormous variability, peptides and proteins participate in many other parts of the body, e.g., blood coagulation, antibody production for the defense against infections, and toxins such as snake venoms. In addition, proteins serve as an energy reserve for sugar metabolism during periods of starvation.

Proteins are present in every body cell and make up more than half of its dry weight. Eight out of the amino acids used in proteins are so-called essential amino acids (E); i.e., the body cannot produce them by itself. This is why they have to be ingested with food. Therefore, not only the total amount, but also the composition of the proteins has to be considered to provide a healthy diet with all necessary amino acids in a reasonable quantity.

The German Nutrition Society (DGE) recommends a daily intake of approx. 0.8 grams protein per kilogram body weight for adults (babies and toddlers need slightly more). Carnivorous animals, such as dogs, need a slightly higher amount of protein, i.e., some 2 grams per kilogram body weight.

Amino acids not only serve as protein building blocks, but also have a number of other important properties which support joint function, for example, as shown in numerous studies. Some amino acids have comparable functions and can be interconverted in the body.

Figure: Amino acid molecule picked from collagen triple helix

Last update: February 2018

Properties of the 20 Amino Acids Found in Proteins

Listed below is a short overview of the 20 amino acids and their properties in alphabetical order:

Explanation of abbreviations: Alanine (Ala; A; NE; 9.0%) means (Ala= int. abbreviation; A=1-letter code used for amino acid sequence; E=essential/NE=non-essential/SE=semi-essential; % = average proportion in proteins)

Alanine (Ala; A; NE; 9.0%) mainly occurs in α-helical proteins together with other amino acids such as leucine and glutamic acid. They are called helix-forming amino acids as they facilitate the formation of secondary structures. After cleavage of its amino group, alanine can flow directly into sugar metabolism and be used for energy production or the body’s own glucose production (gluconeogenesis). At the same time, alanine raises blood sugar levels by promoting the excretion of glucagon (counter-hormone of insulin), thus providing the body with sufficient energy when needed.

Arginine (Arg; R; SE; 4.7%) shows the highest nitrogen mass fraction of all amino acids. Arginine belongs to the group of basic amino acids together with lysine and histidine. Arginine is involved in numerous biological functions as it is an important source of high-energy nitrogen-phosphate compounds. It is an intermediate product of the urea cycle, the metabolic pathway for the degradation of nitrogen compounds into urea. Arginine also functions as a precursor of nitrogen monoxide (NO), one of the smallest messenger molecules in the human body. NO is involved in the regulation of smooth muscle tone (e.g., in vasodilatation or blood pressure regulation) and blood clotting. Arginine thereby lowers the risk for cardiovascular diseases.

Asparagine (Asn; N; NE; 4.4%) serves as a precursor of aspartic acid, a closely related amino acid, which acts as an intermediate in the transfer of amino groups to urea in the urea cycle (see also glutamine). This is why asparagine/aspartic acid are considered diuretic and blood cleansing.

Aspartic acid (Asp; D; NE; 5.5%) is also called aspartate. Together with glutamine, it acts as a neurotransmitter (a signaling molecule) in over 50% of all synapses (nerve connections). Aspartic acid is involved in the immune system and the coding of genetic information in DNA and RNA. It is also needed in sugar metabolism, when carbohydrates are converted to energy, or sugar to glycogen for energy storage.

Cysteine (Cys; C; NE; 2.8%) is a sulfur-containing amino acid with special properties due to its thiol (-SH) group. Two of those thiol groups can form stable disulfide bridges (-SS-) providing special stability to the spatial protein structure. Thiol groups directly participate in enzyme reactions or form sulfur-iron complexes acting as cofactors. In addition, cysteine serves as starting material for the biosynthesis of many other compounds such as glutathione (a tripeptide from glutamic acid, cysteine and glycine), and as a crucial antioxidant protecting cells from harmful oxidation processes.

Glutamine (Gln; Q; NE; 3.9%) is one of the main free amino acids in blood plasma, and occurs in very high concentrations in muscle cells. Among other functions, it is responsible for the storage of water in cells and promotes the formation of proteins and glycogen during physical stress. In addition, glutamine provides amino groups (-NH2) for the nitrogen metabolism and is involved in the detoxification of the body. This means that nitrogen-containing degradation products of the protein metabolism are converted into urea in the liver and finally excreted by the kidneys.

Glutamic acid (Glu; E; NE; 6.2%) is needed to fix cytotoxic ammonia released during protein and amino acid degradation, a process that finally leads to the formation of glutamine. As the so-called citrate cycle directly links glutamic acid to sugar metabolism, glutamic acid also participates in the energy production of the cell. In addition, glutamic acid serves as an important neurotransmitter at the synapses (signal transmission system at nerve endings) of the central nervous system

Glycine (Gly; G; NE; 7.5%) is the smallest and most simple amino acid. Due to its composition, no distinction between L- or D-form is possible. Glycine is found in almost all proteins and is the starting molecule for many different metabolic pathways. Glycine is part of certain nucleotides and is used for the biosynthesis of genetic material, for the synthesis of hem (a hemoglobin component important for oxygen binding in the blood), creatine (energy storage in the muscle), or glutathione (central antioxidant of the cells), and last but not least as an important signal transmitter in the central nervous system. Notably, glycine is particularly abundant in collagen, where it makes up more than one third of all amino acids. Its small size allows collagen to be twisted into a triple helical structure.

Histidine (His; H; SE; 2.1%) has special biochemical properties important for the binding of metal ions such as ion. Histidine is important for the blood pigment hemoglobin and the muscle pigment myoglobin, which are both involved in oxygen transport. Histidine also plays a role in the electron transport chain in mitochondria (the cell’s power plants) and in plant chlorophyll involved in photosynthesis. Apart from this, our body uses histidine to produce the tissue hormone histamine, which plays a central role in the immune system to defend the body against foreign antigens, promoting inflammatory reactions and other defense mechanisms.

Isoleucine (Ila; I; E; 4.6%) serves as a building block in protein synthesis. If the body has to fall back on its own energy reserves, isoleucine is used for energy production in muscle cells. This is one of the reasons why it is an integral ingredient of amino acid infusion solutions and chemically defined diets in medicine.

Leucine (Leu; L; E; 7.5%) plays a key role in the development and maintenance of muscle tissue. It supports protein biosynthesis in muscles and the liver, inhibits the breakdown of muscle protein, and promotes healing processes as well. Leucine (and isoleucine) stimulate the release of insulin from the pancreas, and are therefore involved in blood sugar regulation and other crucial metabolic pathways. Leucine, just like isoleucine, can serve as an energy reserve, if necessary.

Lysine (Lys; K; E; 7.0%) is quite often used in a modified version in the body. For example, hydroxylysine is an important co-factor in the glycosylation of collagen, and fosters the formation of this crucial constituent of connective tissue. Hydroxylysine is, therefore, indispensable for the stability of collagen and connective tissue. Its hydroxy groups are cross-linked via sugar molecules leading to stable glycoproteins which are typical for collagen. Besides this, lysine is used as an accelerator in pain-relieving drugs, especially in combination with ibuprofen.

Methionine (Met; M; E; 1.7%) is another sulfur-containing amino acid. As protein synthesis always starts with methionine, this amino acid forms the first amino acid at the N-terminus (NH2-terminus) of each newly formed protein, at least for a moment. Methionine has a urinary acidifying effect and is therefore used in urinary tract diseases and to prevent kidney stones. In addition, methionine dissolves fat and prevents excessive fat accumulation in the liver. It also promotes detoxification by increasing heavy metal excretion. In addition, free methionine is required for the utilization of selenium, a trace element in the body.

Phenylalanine (Phe; F; E; 3.5%) is involved in the synthesis of the messenger molecules adrenaline, noradrenaline, dopamine, and melanin, and serves as a starting material for many other signaling molecules. On some food packages you will find the statement ‘contains a source of phenylalanine’. This is important information especially for people with phenylketonuria as it refers to the presence of the sweetener aspartame which releases phenylalanine during digestion.

Proline (Pro; P; NE; 4.6%) has a particular structure which promotes special folding of a protein chain. This makes proline and its derivative hydroxyproline indispensable for the spatial formation of collagen containing structures. Proline also protects against the degradation of collagen by blocking digestive enzymes (collagenases). This is an important issue in inflammation.

Serine (Ser; S; NE; 7.1%) plays a special role in the activation or inactivation of enzymes by (de-)phosphorylation of molecules, e.g., in nerve cells. In the form of phosphatidylserine, it is part of the basic structure of many membranes. Serine deficiency has an important impact on the brain leading to reduced concentration and inattention.

Threonine (Tre; T; E; 6.0%) is used in amino acid infusion solutions and chemically defined diets in medicine. It is also added to compound feed, as many grains used for livestock feeding are short of this amino acid. Threonine is found in many antibodies as well as in the collagen of connective tissue.

Tryptophan (Trp; W; E; 1.1%) is regarded as mood-lifting when converted into serotonin. It is also part of many other hormones, such as the sleep hormone melatonin, and forms the precursors (provitamin) of vitamin B3 and NAD, an important cofactor for redox reactions in cell metabolism.

Tyrosine (Tyr; Y; NE; 3.5%) is mainly used as starting material for some hormones, biogenic amines and neurotransmitters involved in signaling within the body. As an example, the formation of adrenaline and noradrenaline in the adrenal glands requires tyrosine and phenylalanine.

Valine (Val; V; E; 6.9%) is a very common amino acid which also serves as an energy reserve, e.g., in the muscle. Valine also serves as a precursor of other amino acids, such as glutamate, that act as the body’s own messenger substance.

Overview of Amino Acids Found in Bioactive Collagen Peptides

Amino acid	Weight share %	mol %
Alanine	8,1	10,1
Arginine	8	5,1
Aspartic acid	5,6	4,7
Glutamic acid	9,7	7,3
Glycine	21,8	32,1
Histidine	1,1	0,8
Hydroxylysine	1,6	1,1
Hydroxyproline	11,7	9,9
Isoleucine	1,4	1,2
Leucine	2,9	2,4
Lysine	3,7	2,8
Methionine	0,7	0,5
Phenylalanine	2,1	1,4
Proline	13,1	12,6
Serine	3,2	3,3
Threonine	1,8	1,7
Tyrosine	1	0,6
Valine	2,5	2,4