When Tyrosine becomes an essential amino acid

In patients with PKU, Tyrosine becomes essential, since it is formed from Phenylalanine in the reaction that is impaired in Phenylketonuria.

Additional information:

Most of the textbooks classify amino acids from the nutritional point of view, in two groups: essential or not essential.  Essential amino acids are considered those amino acids that can not be synthesized by an organism and so should be consumed in the diet; non essential amino acids are those amino acids that can be synthesized.  This classification is not related to the importance of the amino acids, but with the fact of them being required in the diet or not.

According to this classification, the essential amino acids are:

Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine

(Mnemonics: PVT TIM HALL:

Phenylalanine

Valine

Threonine

Tryptophan

Isoleucine

Methionine

Histidine

Arginine

Leucine

Lysine)

Non essential amino acids are:

Alanine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Proline, Serine, Tyrosine

Like most facts in biology, this “black and white” classification is not 100 % accurate. Actually, some amino acids are conditionally essential or partially essential, since some “essential” amino acids, like arginine,  can be synthesized by the body. Arginine is synthesized in the urea cycle, for example, but it is considered essential since the quantity of Arginine that is synthesized is not enough during the growing process.

 Tyrosine is an amino acid that is synthesized in the body from Phenylalanine, that is an essential amino acid. This reaction is catalyzed by the enzyme Phenylalanine Hydroxylase, that use as cofactor reduced tetrahydobiopterine.

If Phenylalanine is deficient in the diet, then the body requires tyrosine in the diet.

In Phenylketonuria there is an excess of Phenylalanine, since the body can not metabolize it, but Phenylketonuria is a consequence of a deficit of Phenylalanine Hydroxylase (Classic Phenylketonuria) or a deficit of Tetrahydrobiopterin Reductase. In both cases, the organism is not able to synthesize Tyrosine from Phenylalanine, so even when there is an accumulation of Phe in these patients, it can not be used to synthesize Tyrosine.

In fact, some of the signs and symptoms of Phenylketonuria, like mental retardation and other neurological symptoms, have been related to the unavailability of tyrosine for the synthesis of the neurotransmitters that derive from tyrosine.

The lack of pigmentation of PKU patients, has been related also to the lack of tyrosine, since Tyrosine is a precursor of melanine also.

It is obvious that if Tyrosine is formed in normal persons from phenylalanine through the reaction cited above, in case that this reaction can not be produced, like in PKU, it is necessary to supplement the patient with Tyrosine, since the patient can not synthesize it, so Tyrosine becomes an essential amino acid for these patients