Blood & Laboratory Values

Phenylalanine – Function & Diseases


Phenylalanine is a proteinogenic, essential amino acid with an aromatic six-membered ring that functions as a building block for many proteins and peptides.

In addition, phenylalanine plays an important role in nitrogen metabolism and can be converted in the liver into the proteinogenic amino acid tyrosine . 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 which – unlike most proteinogenic amino acids – is not only bioactive in the L-form, but also to a limited extent as an enantiomer in the R-form.

Although R-phenylalanine is biochemically largely inactive and only occurs during the artificial production of the amino acid, the role of D-phenylalanine in the control of certain neurotransmitters within the pain complex is discussed. A characteristic structural feature of phenylalanine is a simple aromatic six-membered ring (benzene ring) with a connected hydrocarbon chain. The chemical structural formula is C6H5-CH2-CH(NH2)-COOH, where the C6H5 group indicates the benzene ring. The amino acid is amphiphilic, which means that it is both fat and water soluble.

The chemical formula also shows that phenylalanine consists exclusively 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 sufficient amounts of phenylalanine from tyrosine, but is dependent on the supply from food. Phenylalanine is present in sufficient quantities in many animal and vegetable foods, so that with a normal, mixed diet – assuming normal absorption in the digestive tract – there is no need to worry about a lack of the amino acid.

Function, effect & tasks

The most important function and task of phenylalanine is to participate in the construction of many proteins and peptides. It is also involved in the synthesis of some hormones that play a central role in controlling metabolic processes. 

They are hormones such as adrenaline , norepinephrine , L-dopa, PEA, and melanin . In addition, L-phenylalanine serves as a basic substance from which z. B. the messenger substance dopamine, serotonin, tyramine and others are synthesized. L-phenylalanine also serves as a starting material for the essential amino acid tyrosine. For this purpose, phenylalanine is converted to tyrosine in the liver in two steps by hydroxylation and elimination of a water molecule. Phenylalanine hydroxylase is the enzyme that catalyzes the conversion to tyrosine.

An alternative supply of the equally essential amino acid tyrosine can – as with phenylalanine – come from food intake. Unlike all other amino acids, which only show a bioactive effect in their L-form, the D-enantiomer of phenylalanine at least seems to have an influence on pain perception. A mixture of L- and D-phenylalanine (racemic mixture) has been found to have an analgesic effect. The DL mixture probably blocks the breakdown of enkephalins – the body’s own opioids – so that the pain-relieving effect is prolonged and strengthened.

Formation, Occurrence, Properties & Optimal Values

The essential amino acid phenylalanine is ingested through food. It is not present in free form, but usually as part of a protein or polypeptide in a chemically bound form. In order to make the amino acid available for metabolism, the corresponding protein must first be broken down during digestion and then separated from the “fragments” by further enzymes in the further metabolism.

L-phenylalanine is synthesized via the so-called shikimic acid pathway. It is a complex biocatalytic chain reaction that autotrophic plants and bacteria have. The special feature of autotrophic organisms is their ability to form organic matter from only inorganic material. Free L-phenylalanine tastes bitter, while its D-enantiomer, which is only produced during industrial production, has a sweet taste. The amino acid is z. B. offered as a dietary supplement and is also part of the artificial sweetener aspartame. Many foods contain bioavailable L-phenylalanine 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. The phenylalanine requirement is highly dependent on the supply of tyrosine. If there is no tyrosine in the diet, the body needs 38 to 52 mg per kg of body mass. With a rich supply of tyrosine in food, the daily requirement drops to just 9 mg per kg of body mass. As a rule, foods containing phenylalanine also contain a corresponding amount of tyrosine.

The recommendation of the FAO/WHO from 1985 amounts to a combined daily requirement of L-phenylalanine and L-tyrosine of 14 mg per kg body mass. An adult with a body mass of 80 kg therefore needs 1,120 mg of the two substances daily.

Diseases & Disorders

Deficiency symptoms caused by a permanently low supply of phenylalanine and tyrosine in the diet are extremely rare, but can have serious consequences, especially in the neuronal area.

Apart from an impairment of the synthesis of many hormones and neurotransmitters, the deficiency can also manifest itself as a disturbance in the myelination of nerve fibers . The opposite of a deficiency, an overconcentration of phenylalanine (phenylketonuria), can occur as a result of a genetic metabolic disorder. The disease is inherited in an autosomal recessive manner and results in reduced production of the enzyme phenylalanine hydroxylase, which can convert phenylalanine to tyrosine.

The reduced enzyme activity leads to a sharp increase in the amino acid, so-called phenylketonuria, because the conversion to tyrosine is also the degradation pathway of phenylalanine. At the same time, there is a lack of tyrosine because the synthesis pathway is blocked. Another hereditary disease in this context is Hartnup syndrome. It is a metabolic disorder that disrupts phenylalanine transport across the cell membrane. This leads to serious problems in the CNS , on the skin and in the digestive tract.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.