Phenylalanine is a proteinogenic essential amino acid with an aromatic six-membered ring that functions as a building block of many proteins and peptides. In addition, phenylalanine plays an important role in nitrogen metabolism and can be converted in the liver to tyrosine, another proteinogenic amino acid. Phenylalanine and tyrosine play important roles in the synthesis of insulin, melanin, thyroxine, and neurotransmitters dopamine, serotonin, and tyramine.
What is phenylalanine?
Phenylalanine is an essential alpha-amino acid that, unlike most proteinogenic amino acids, is bioactive not only in the L-form but also to a limited extent as an enantiomer in the R-form. Although the R-phenylalanine is biochemically largely inactive and occurs exclusively in the artificial production of the amino acid, it is discussed what role D-phenyalanine plays in the control of certain neurotransmitters within the pain complex. As a characteristic structural feature, phenylalanine has a simple aromatic six-membered ring (benzene ring) with an attached hydrocarbon chain. The chemical structural formula is C6H5-CH2-CH(NH2)-COOH, with the C6H5 group indicating the benzene ring. The amino acid is amphiphilic, which means that it is soluble in both fat and water. From the chemical formula, it can also be seen that phenylalanine is composed entirely of carbon, hydrogen, nitrogen and oxygen, substances that are ubiquitous. Rare metals, minerals or trace elements are not part of the amino acid. Nevertheless, the human metabolism cannot synthesize phenylalanine from tyrosine to a sufficient degree, but is dependent on its supply from food. Phenylalanine is present in sufficient quantities in many animal as well as plant foods, so there is no need to fear a deficiency of the amino acid in a normal, mixed diet – assuming a normal absorption capacity in the digestive tract.
Function, effect and tasks
The most important function and task of phenylalanine lies in its participation in the construction of many proteins and peptides. Likewise, it is involved in the synthesis of some hormones that have a central position in the control of metabolic processes. They are hormones such as adrenaline, noradrenaline, L-dopa, PEA and melanin. In addition, L-phenylalanine serves as a basic substance from which, for example, the neurotransmitter dopamine, serotonin, tyramine and others are synthesized. L-phenylalanine also serves as a starting material for the essential amino acid tyrosine. Phenylalanine is converted to tyrosine in the liver in two steps by hydroxylation and by splitting off a water molecule. Phenylalanine hydroxylase is the enzyme that catalyzes the conversion to tyrosine. An alternative supply of the amino acid tyrosine, which is also essential, can be obtained – as with phenylalanine – through food intake. Unlike all other amino acids, which show bioactive effects exclusively in their L-form, the D-enantiomer of phenylalanine seems to have at least an influence on the sensation of pain. A mixture of L- and D-phenylalanine (racemic mixture) was found to exhibit analgesic activity. It is likely that the DL mixture blocks the breakdown of enkephalins – the body’s own opioids – so that the analgesic effect is prolonged and enhanced.
Formation, occurrence, properties, and optimal levels
The essential amino acid phenylalanine is absorbed through food. It is not present freely, but usually as part of a protein or polypetide in a chemically bound form. To make the amino acid available for metabolism, the corresponding protein must first be broken down in the course of digestion and then separated from the “fragments” in further metabolism by means of additional enzymes. L-phenylalanine is synthesized via the so-called shikimic acid pathway. This involves a complex biocatalytic chain reaction that autotrophic plants and bacteria possess. The special characteristic of autotrophic organisms is their ability to form organic matter from exclusively inorganic material. Free L-phenylalanine tastes bitter, while its D-enantiomer, which is formed exclusively during industrial production, has a sweetish taste. The amino acid is offered, for example, as a dietary supplement and is also part of the artificial sweetener aspartame.Bioavailable L-phenylalanine is contained in many foods in bound form. Its content is particularly high in dried peas and soybeans, in walnuts and pumpkin seeds, and in various types of fish and meat. Phenylalanine requirements are strongly dependent on the supply of tyrosine. If tyrosine is not available in the diet, the body requires 38 to 52 mg per kg of body mass. If tyrosine is abundantly available in the diet, the daily requirement drops to only 9 mg per kg of body mass. As a rule, foods containing phenylalanine also contain a corresponding amount of tyrosine. The 1985 FAO/WHO recommendation is a combined requirement of L-phenylalanine and L-tyrosine of 14 mg per kg body mass daily. Thus, an adult with 80 kg body mass requires 1,120 mg of the two substances daily.
Diseases and disorders
Deficiency symptoms in the case of a permanently insufficient supply of phenylalanine and tyrosine in the diet, are extremely rare, but can have serious consequences, especially in the neuronal field. Apart from an impairment of the synthesis of many hormones and neurotransmitters, the deficiency can also be manifested by a disturbance in the myelination of nerve fibers. The opposite of a deficiency, an overconcentration of phenylalanine (phenylketonuria), can occur due to a genetic metabolic disorder. The disease is inherited in an autosomal recessive manner and results in decreased production of the enzyme phenylalanine hydroxylase, which can convert phenylalanine to tyrosine. The reduced enzyme activity leads to a strong increase of the amino acid, the so-called phenylketonuria, because the conversion to tyrosine is at the same time the degradation pathway of phenylalanine. At the same time, a deficiency of tyrosine occurs because the synthesis pathway is blocked. Another hereditary disease in this context is Hartnup syndrome. This is a metabolic disorder that disrupts phenyalanine transport across the cell membrane. This causes serious problems in the CNS, skin, and digestive tract.
