NON-ESSENTIAL AMINO ACIDS
Non-essential amino acids.
There are 22 amino acids naturally incorporated into polypeptides and are called proteinogenic or natural amino acids. Of these, 20 are encoded by the universal genetic code and are referred to as standard amino acids. The remaining 2, selenocysteine and pyrrolysine, are incorporated into proteins by unique synthetic mechanisms. Pyrrolysine, "the 22nd amino acid", is not used by humans, which leaves 21 amino acids used by humans. Of these 21 amino acids, 12 are considered nonessential in the human diet.
Nonessential amino acids are those amino acids that can be made by the body from either essential amino acids or normal breakdown of proteins or other biochemical mechanism. These nonessential amino acids are: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, tyrosine, and selenocysteine.
Six of the 12 nonessential amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine. Five amino acids are dispensable in humans, meaning they can be synthesized in the body in sufficient amounts. These five are alanine, aspartic acid, asparagine, glutamic acid and serine.
Alanine is a small non-essential amino acid (nonessential means it can be made in the body) in humans and one of the 20 amino acids encoded by the human genetic code. Alanine can be synthesized by the body from pyruvate and branched chain amino acids such as valine, leucine, and isoleucine.
Arginine is a nonessential amino acid that is used in the biosynthesis of proteins. It is physiologically active in the L-form and is one of the 20 genetically encoded amino acids. Arginine is referred to as nonessential because it can be synthesized by the body. Although nonessential in humans, arginine is classified as a semi essential or conditionally essential amino acid, depending on the developmental stage and health status of the individual.
Aspartic acid, also known as aspartate, is an amino acid that is used in the biosynthesis of proteins. There are two forms or enantiomers of aspartic acid. The name "aspartic acid" can refer to either enantiomer or a mixture of two. Of these two forms, only one, "L-aspartic acid", is directly incorporated into proteins. The biological roles of its counterpart, "D-aspartic acid" are more limited.
Cysteine is a non-essential sulfur-containing amino acid in humans, related to cystine. Cysteine has the same structure as serine, but with one of its oxygen atoms replaced by sulfur; replacing it with selenium gives selenocysteine.
Glutamic acid is an amino acid with formula C5H9O4N and naturally occurring in the L-form. Glutamic acid is used by almost all living beings in the biosynthesis of proteins, being specified in DNA by the codons GAA or GAG.
Glutamine is a non-essential and conditionally essential amino acid in humans, meaning the body can usually synthesize sufficient amounts of it, but in some instances of stress, the body's demand for glutamine increases, and glutamine must be obtained from the diet. In human blood, glutamine is the most abundant free amino acid.
Glycine is the simplest possible amino acid. It is a colorless, sweet-tasting crystalline solid. It is the only achiral proteinogenic amino acid. It can fit into hydrophilic or hydrophobic environments, due to its minimal side chain of only one hydrogen atom.
Proline is a cyclic, amino acid synthesized from glutamic acid and other amino acids. Proline is a constituent of many proteins, it is found in high concentrations in collagen, proline constitutes almost a third of the residues. Collagen is the main supportive protein of skin, tendons, bones, and connective tissue and promotes their health and heeling.
L-serine is a non-essential amino acid occurring in natural form as the L-isomer. It is synthesized from glycine or threonine. It is involved in the biosynthesis of purines; pyrimidines; and other amino acids.
L-Tyrosine is the levorotatory isomer of the aromatic amino acid tyrosine. L-tyrosine is a naturally occurring tyrosine and is synthesized in vivo from L-phenylalanine. It is considered a non-essential amino acid; however, in patients with phenylketonuria who lack phenylalanine hydroxylase and cannot convert phenylalanine into tyrosine, it is considered an essential nutrient.
Asparagine is a derivative of aspartic acid and plays an important role in the biosynthesis of glycoproteins and other proteins.
Selenocysteine has a structure similar to that of cysteine, but with an atom of selenium taking the place of the usual sulfur, forming a selenol group which is deprotonated at physiological pH.
Important metabolic intermediates.
When taken up into the human body from the diet or synthesized within the body, the 12 nonessential amino acids either are used to synthesize proteins and other biomolecules or are oxidized to urea and carbon dioxide as a source of energy.
The nonessential amino acids that are glucogenic; Alanine, Arginine, Asparagine, Aspartic, Cysteine, Glutamic, Glutamine, Glycine, Proline, and Serine can also be converted into glucose, through gluconeogenesis.
In addition, cysteine, taurine (a precursor of cysteine), tyrosine, and arginine are considered semi essential amino acids in children (though taurine is not technically an amino acid), because the metabolic pathways that synthesize these amino acids are not fully developed.
In humans, non-protein amino acids also have important roles as metabolic intermediates, such as in the biosynthesis of the neurotransmitter gamma-amino-butyric acid (GABA). Many amino acids are used to synthesize other molecules, for example:
Tyrosine is a precursor of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine and various trace amines.
Glycine is a precursor of porphyrins such as heme (an iron-containing compound of the porphyrin class which forms the non-protein part of hemoglobin in blood and some other biological molecules.).
Arginine is a precursor of nitric oxide. Nitric oxide (NO) contributes to vessel homeostasis by inhibiting vascular smooth muscle contraction and growth, platelet aggregation, and leukocyte adhesion to the endothelium.
Aspartate, glycine, and glutamine are precursors of nucleotides. It should also be noted that, not all of the functions of amino acids are known.
Sources: High protein foods e.g. fish
Nonessential amino acids can be synthesized in the body from either essential amino acids or normal breakdown of proteins or other biochemical mechanism. This makes them to be attainable from any high protein foods like meat, eggs, milk, fish, and also from some vegetables like beans, sea weed etc. They can also be gotten from sources like grains and supplements like whey or soy protein.
Six of the 12 nonessential amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine.
The amino acid arginine plays a variety of roles in many different cell types. In addition to serving as substrate for protein synthesis, arginine is a precursor to nitric oxide (NO), polyamines, proline, glutamate, creatine, and agmatine. Consequently, deficiencies of arginine have the potential to disrupt many cellular and organ functions.
Infants are unable to effectively synthesize arginine, making it nutritionally essential for infants. Adults, however, are able to synthesize arginine in the urea cycle but in certain cases like illness, they may not be able to produce enough.
Cysteine: Low levels of cysteine may reduce your ability to prevent free radical damage, may result in impaired function of the immune system, apathy, loss of pigmentation in hair, edema, lethargy, liver damage, muscle loss, skin lesions, weakness, fat loss, and slowed growth in children.
Glutamine: Glutamine deficiency can cause damage to the intestinal tract, lead to muscle wasting, poor wound healing or slow recovery after workouts.
Glycine: Glycine helps form glutathione, a valuable antioxidant that's used to prevent cellular damage and various signs of aging. It is also used to form creatine and as a glucogenic amino acid, to generate energy among other functions. Deficiency would impair these functions.
Proline: Proline deficiencies are extremely rare, people at risk of having low proline levels are those typically on a protein restricted diet. However in the case of deficiency, it can lead to skin lesions, recurrent infections, joint pains, abnormalities of collagenous tissues etc.
Tyrosine: Deficiency may include; depression, edema, fatigue, lethargy, loss of pigmentation, low serum levels of related proteins (e.g. coenzyme q10), mood disorders, skin lesions, and thyroid problems.
The other five nonessential amino acids are dispensable in humans, meaning they can be synthesized in sufficient amounts in the body. These five are alanine, aspartic acid, asparagine, glutamic acid and serine.
High amino acid intake can cause kidney damage.
Alanine: High doses of alanine can cause flushing and tingling.
Arginine: Oral supplementation with L-arginine at doses up to 15 grams daily are generally well tolerated. The most common adverse reactions of higher doses from 15 to 30 grams daily are nausea, abdominal cramps and diarrhea. Some may experience these symptoms at lower doses.
Aspartic Acid: There are no known drug or supplement interactions reported with L-aspartic acid usage. According to researchers over consumption of aspartic acid might cause acne, headaches, diarrhea, mood swings, and depression. But there’s not enough evidence to imply that aspartic acid is the culprit.
Cysteine: There is a concern that cysteine might cause bronchospasm in people with asthma if inhaled or taken by mouth or through a tube in the windpipe. There is also concern that cysteine might slow blood clotting which can cause problems in people with bleeding disorders.
Glutamic Acid: Glutamic acid is in a class of chemicals known as excitotoxins, high levels of which can cause damage to areas of the brain unprotected by the blood–brain barrier and that a variety of chronic diseases can arise out of this neurotoxicity.
Glutamine: In the case of cancer, glutamine has been shown to fuel the growth of tumors. Cancer cells tend to use high amounts of glutamine to fuel their growth and in the process starving the body of this important amino acid.
Glycine: In large amounts, glycine can cause nausea, vomiting and upset stomach. It can also cause drowsiness since it’s a good sleeping aid especially if you are suffering from insomnia.
Proline: Hyperprolinemia, also referred to as prolinemia or prolinuria, is a condition which occurs when the amino acid proline is not broken down properly by the enzymes proline oxidase or pyrroline-5-carboxylate dehydrogenase, causing a buildup of proline in the body. Some individuals with type I Hyperprolinemia exhibit seizures, intellectual disability, or other neurological problems. Those with type II often involves seizures, convulsions, and intellectual disability.
Tyrosine: Apart from headaches, there are not that many issues that come up with L-tyrosine. If you are suffering from a thyroid condition, that might be a concern as tyrosine exacerbates this problem. There is also evidence that the supplement can affect the absorption of certain drugs, such as levodopa or MAOIs, which can be a problem if you are taking medication for something else.
Selenocysteine: No free pool of selenocysteine exists in the cell. Its high reactivity would cause damage to cells. Instead, cells store selenium in the less reactive selenide form (H2Se).
There are no serious concerns reported about serine and asparagine. They are one of the least likely to be supplemented individually and thus there source is most likely to be from whole proteins which are safer as supplements, or food.
The amounts of nonessential amino acids required are depend on the age and health of the individual, so it is hard to make general statements about the dietary requirements of these amino acids.