In humans, phenylalanine is normally oxidized by the enzyme phenylalanine hydroxylase (PAH) to form the amino acid tyrosine; this is in fact the normal biosynthetic route to tyrosine for humans. From tyrosine, there are further connections to the biosynthesis of catecholamines, melanin, hormones, etc. Usually, dietary intake of Phe and Tyr, and the body's demand for Phe and Tyr, are fairly closely balanced. However, when there is too much phenylalanine in the body's pool of amino acids, it must be eliminated, either by excretion or by biochemical reaction.
There are two routes by which the excess Phe can be metabolized: oxidation to tyrosine (the normal and main route for degradation of Phe, and the normal route for biosynthesis of Tyr), and transamination to phenylpyruvate and subsequent further metabolism (a minor route, which comes to the fore when the main route is blocked).
If there is sufficient phenylalanine in the diet, then humans usually have no difficulty in synthesizing adequate amounts of tyrosine from the dietary phenylalanine. The reaction is complex:
Picture of Reaction of "Main Route" (Phe to Tyr)
This reaction is also the first reaction in the normal route of catabolism of phenylalanine.
The enzyme catalyzing the reaction is phenylalanine hydroxylase (PAH), a mixed-function mono-oxygenase that uses molecular oxygen. This enzyme also uses the cofactor tetrahydrobiopterin (BH4), which is oxidized in the course of the reaction to dihydrobiopterin (BH2). The cofactor must be regenerated by a separate system of enzymes for PAH action to continue.
Native PAH is a polymeric enzyme; it is not yet clear if the subunits are identical, but SDS-polyacrylamide electrophoresis shows two 50kDa subunit bands. However, there is only one genetic locus for PAH.
PAH is expressed only in the liver. An assay of its activity would thus require a liver biopsy, an invasive and stressful procedure.
Transaminases catalyze the transfer of -NH2 groups from the amino acids, onto alpha-ketoglutarate. Many different transaminases are known, and they are generally of broad specificity for amino acids (that is, one enzyme can accept as substrates two or more different amino acids). All have the same cofactor requirement - pyridoxal phosphate (vitamin B6).
Transamination of phenylalanine to phenylpyruvate is normally of negligible importance, so long as the main route is functioning. However, if the main route is blocked for some reason, then transamination of Phe becomes quite important. In fact, the production of the distinctive minor metabolite, phenylpyruvate, can be used to diagnose deficiencies in the main route of metabolism of phenylalanine.
"Minor Route" (Phe to Phenylpyruvate)
If there is inadequate PAH activity, with consequently little or no conversion of phenylalanine to tyrosine, then the catabolism of phenylalanine is blocked and serum levels of phenylalanine rise (hyperphenylalaninemia). Side reactions that under normal conditions are undetectable then start to produce metabolites of phenylalanine. These include phenylacetate, phenyllactate, phenylpyruvate, and phenylethylamine. These metabolites are excreted through the urine.
The disease is called phenylketonuria because one of the dominant metabolites contains a ketone group; this is phenylpyruvate, an alpha-keto acid.
A secondary cause of lack of PAH activity is a defect in the generation of adequate amounts of the cofactor tetrahydrobiopterin (BH4). Defects in biopterin metabolism account for 1% - 3% of all cases of hyperphenylalaninemia.
When the main route
of degradation of Phe is blocked, then serum concentrations of Phe and its metabolites
will rise. Three forms of hyperphenylalaninemia (HPA) can be distinguished,
based on serum concentrations of phenylalanine: benign, variant,
and classic.
| Compound
Concentration |
Normal | Benign HPA | Variant HPA | Classic
HPA / Classic PKU |
| Phenylalanine | approx. 1 mg/dL
(0.061 mM) |
4-10 mg/dL
(0.242-0.605 mM) |
10-20 mg/dL
(0.605-1.21 mM) | above 20 mg/dL
(above 1.21 mM) |
Under conditions of hyperphenylalaninemia a minor route of Phe metabolism becomes important. The minor route for degradation of Phe starts with the transamination of Phe to phenylpyruvate:
This transamination reaction is a standard one in the metabolism of amino acids. Note the role of alpha-ketoglutarate as the acceptor of the amino group from Phe, with consequent formation of glutamate.
The phenylpyruvate is further metabolized. Decarboxylation of phenylpyruvate gives phenylacetate, while a reduction reaction gives phenyllactate. The phenylacetate can be further conjugated with glutamine to give phenylacetyl glutamine. All of these metabolites can be detected in serum and urine by suitable clinical tests.
Defects in the regeneration of the cofactor tetrahydobiopterin (BH4) account for a small fraction of PKU cases. Such cases are sometimes identified as "malignant" PKU, because of the progressive deterioration in neurological function which cannot be alleviated by simple dietary restriction in phenylalanine intake. These cases may be distinguished from the classical form of PKU which is due to a defect in the enzyme phenylalanine hydroxylase (PAH). There are several possible causes of a defect in biopterin metabolism, and the consequences of such a defect can be profound, extending beyond phenylketonuria to defects in neurotransmission.
Regeneration of tetrahydrobiopterin (BH4)
In the course of
oxidizing phenylalanine to tyrosine, the enzyme PAH also oxidizes BH4
to BH2 (dihydrobiopterin). The necessary BH4
must then be regenerated by the action of a separate enzyme, dihydropteridine
reductase (DHPR).
Defects in these enzymes result in many of the same symptoms as are seen in "classical" PKU (due to defective PAH). However, these account for only a small fraction of all patients with hyperphenylalaninemia.
Deficiencies in biopterin metabolism can be distinguished from the classical PKU disease by the profile of various pterins in serum, urine, and cerebrospinal fluid.
High levels
of phenylalanine can cause inhibition of the decarboxylation of 5-hydroxytryptophan,
so that little or no serotonin is formed.
Phenylalanine can also inhibit the enzyme tyrosinase. Tyrosinase converts
tyrosine into 3,4-dihydroxyphenylalanine (DOPA) through an oxidation reaction.
DOPA can then be used for catecholamine biosynthesis, or in a separate set of reactions it goes on to form the pigment melanin. The blockage in melanin biosynthesis by the high levels of Phe is responsible for the fair complexions of patients with PKU.
Phe can also inhibit glutamate decarboxylase, resulting in lower GABA levels in the brain and thus interfering with neurologic function on a level apart from defective catecholamine metabolism.