Catabolism of phenylalanine and tyrosine and their metabolic disorders by BNP.pdf

Decarboxylation is the reaction by which CO2 is removed from the COOH group of an amino acid as a result an amine is formed. The reaction is catalyzed by the enzyme decarboxylase, which requires pyridoxal-P (B6-PO4) as coenzyme. Tissues like liver, kidney, brain possess the enzyme decarboxylase and also by microorganisms of intestinal tract. The enzyme removes CO2 from COOH and converts the amino acid to corresponding amine.
Phenylalanine is an essential, aromatic amino acid. The need for phenylalanine becomes minimal, if adequate tyrosine is supplied in the food. This is called the sparing action of tyrosine on phenylalanine.
Tyrosine is an aromatic amino acid. It is synthesized from phenylalanine, and so is a non-essential amino acid. The need for phenylalanine becomes minimal, if adequate tyrosine is supplied in the food. This is called the sparing action of tyrosine on the phenylalanine.
Seven amino acids produce acetyl CoA or acetoacetate and therefore are categorized as ketogenic. Of these, isoleucine, threonine, and the aromatic amino acids (phenylalanine, tyrosine, and tryptophan) are converted to compounds that produce both glucose and acetyl CoA or acetoacetate. Leucine and lysine do not produce glucose; they produce acetyl CoA and acetoacetate.
The name phenylketonuria is coined due to the fact that the metabolite phenylpyruvate is a keto acid (C6H5CH2−CO−COO−) excreted in urine in high amounts.
Phenylalanine cannot be converted to tyrosine. So, phenylalanine accumulates. Phenylalanine level in blood is elevated.

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DECARBOXYLATION REACTION AND BIOGENIC
AMINES
Decarboxylation

Decarboxylation is the reaction by which CO2 is removed from the COOH group of an amino
acid as a result an amine is formed. The reaction is catalysed by the enzyme decarboxylase,
which requires pyridoxal-P (B6-PO4) as coenzyme. Tissues like liver, kidney, brain possess the
enzyme decarboxylase and also by microorganisms of intestinal tract. The enzyme removes
CO2 from COOH and converts the amino acid to corresponding amine. This is mostly a process
confined to putrefaction in intestines and produces amines. Biogenic amines formed from
various amino acids and their biologic importances are listed in Table 27.1.

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Catabolism of phenylalanine and tyrosine and their metabolic
disorders

PHENYLALANINE (PHE) (F)

Phenylalanine is an essential, aromatic amino acid. The need for phenylalanine becomes
minimal, if adequate tyrosine is supplied in the food. This is called the sparing action of
tyrosine on phenylalanine. It is partly glucogenic and partly ketogenic. Phenylalanine was
isolated by Schulze in 1879 and Tyrosine by Liebig in 1846. In 1913, Abderhalden showed the
essentiality of phenylalanine.

Step 1: Phenylalanine to Tyrosine
The reaction involves addition of a hydroxyl group to the aromatic ring, by phenylalanine
hydroxylase (step 1, Fig.18.1). It needs NADPH, NADH and tetrahydrobiopterine as co-
enzymes. As this is an irreversible reaction, tyrosine cannot replenish phenylalanine. Hence,
phenylalanine is essential in food. It is a mixed function oxidase (mono-oxygenase). One
molecule of O2 is needed in this reaction; out of which one atom is incorporated in the OH
group and the other is reduced to water. This reaction also needs the electron carrier
tetrahydrobiopterin, which is regenerated by the reduction of dihydrobiopterin by a reductase
using NADPH (step 1A, Fig. 18.1).


Fig. 18.1: Phenylalanine catabolism; 1 = Phenylalanine hydroxylase; 1-A = NADPH dependent reductase

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TYROSINE (TYR) (T)

Tyrosine is an aromatic amino acid. It is synthesized from phenylalanine, and so is a non-
essential amino acid. The need for phenylalanine becomes minimal, if adequate tyrosine is
supplied in the food. This is called the sparing action of tyrosine on the phenylalanine.
Tyrosine is partly glucogenic and partly ketogenic.

Catabolism of Tyrosine (and Phenylalanine) Step 2: Transamination

Degradative pathway of phenylalanine and tyrosine are the same, since phenylalanine is
converted to tyrosine and then metabolized. Tyrosine is transaminated to give
parahydroxyphenylpyruvic acid by tyrosine transaminase (step 2, Fig.18.2). It is pyridoxal
phosphate dependent. It is induced by glucocorticoids.

Step 3: Production of Homogentisic acid

The next step (No. 3, Fig.18.2) is catalyzed by para-hydroxy-phenylpyruvate hydroxylase. It is
a di-oxygenase, which means that both oxygen atoms are incorporated into the product. It is a
copper containing enzyme. Interestingly, the reaction involves shifting of the side chain from
para-position to meta-position. A new OH group is freshly added to the para position to give
rise to 2,5-dihydroxyphenyl acetic acid or homogentisic acid. Ascorbic acid is helpful in this
reaction.

Step 4: Cleavage of Aromatic Ring

Homogentisic acid oxidase opens the ring (step 4, Fig.18.2). It is also a di-oxygenase with an
iron atom at the active site. The product is 4- maleyl acetoacetate.

Step 5: Isomerization

It then undergoes cis to trans isomerization to form fumaryl acetoacetate by an isomerase (step
5, Fig.18.2). The isomerase requires glutathione (GSH) as a cofactor.

Step 6: Hydrwolysis

Fumaryl acetoacetate is then hydrolyzed to fumarate and acetoacetate by a hydrolase (step 6,
Fig.18.2). This results in the production of a glucogenic product (fumarate) and a ketone body
(acetoacetate). Hence, phenylalanine and tyrosine are partly glucogenic and partly ketogenic.

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Fig. 18.2: Catabolism of phenylalanine and tyrosine

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Seven Amino Acids Are Degraded to Acetyl-CoA
Seven amino acids produce acetyl CoA or acetoacetate and therefore are
categorized as ketogenic. Of these, isoleucine, threonine, and the aromatic amino
acids (phenylalanine, tyrosine, and tryptophan) are converted to compounds that
produce both glucose and acetyl CoA or acetoacetate (Fig. 39.16). Leucine and
lysine do not produce glucose; they produce acetyl CoA and acetoacetate.

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Portions of the carbon skeletons of seven amino acids— tryptophan, lysine,
phenylalanine, tyrosine, leucine, isoleucine, and threonine—yield acetyl-CoA
and/or acetoacetyl-CoA, the latter being converted to acetyl-CoA (Fig.18–21).

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Some of the final steps in the degradative pathways for leucine, lysine, and
tryptophan resemble steps in the oxidation of fatty acids. The degradative pathways

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of two of these seven amino acids deserve special mention. Tryptophan break-down
is the most complex of all the pathways of amino acid catabolism in animal tissues;
portions of tryptophan (four of its carbons) yield acetyl-CoA via acetoacetyl- CoA.
Some of the intermediates in tryptophan catabolism are precursors for the synthesis
of other biomolecules (Fig.18–22),

Including nicotinate, a precursor of NAD and NADP in animals; serotonin, a
neurotransmitter in vertebrates; and indoleacetate, a growth factor in plants.

Disorders of tyrosine (phenylalanine) metabolism-
Several enzyme defects in phenylalanine/ tyrosine degradation leading to metabolic
disorders are known. In Fig.15.19, the deficient enzymes and the respective inborn
errors are depicted and they are discussed here under

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PHENYLKETONURIA (PKU)

1. Deficiency of phenylalanine hydroxylase (Fig.18.1) is the cause for this disease. The
genetic mutation may be such that either the enzyme is not synthesized, or a non-
functional enzyme is synthesized.
2. It is a recessive condition. Frequency of PKU was considered to be 1 in 10,000 births; but
recent introduction of better diagnostic facilities showed that the incidence is as high as 1
in 1,500 births (WHO, 2003). Incidence of PKU in India is lesser than western countries;
only 1 in 25,000 births.
3. There are 5 types of PKU described. Type I is the classical one, described below. It is due
to phenylalanine hydroxylase deficiency. Types II and III are due to deficiency of
dihydrobiopterin reductase. Type IV and V are due to the deficiency of the enzyme
synthesizing biopterin. Since tetrahydrobioptrerin is the co-enzyme required for serotonin
and dopamine, the decreased level of these neurotransmitters may also result in the
neurological symptoms. Phenylalanine hydroxylase gene is located in chromosome
no.12; and dihyrobiopterin reductase gene in chromosome no.4.

Biochemical Abnormalities
The name phenylketonuria is coined due to the fact that the metabolite phenylpyruvate is
a keto acid (C6H5CH2−CO−COO−) excreted in urine in high amounts.
 Phenylalanine cannot be converted to tyrosine. So, phenylalanine accumulates.
Phenylalanine level in blood is elevated.
 So alternate minor pathways are opened (Fig.15.23). Phenylketone (phenylpyruvate),
phenyllactate and phenylacetate are excreted in urine.

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Clinical Manifestations

A. The classical PKU child is mentally retarded with an IQ (Intelligence quotient) of
50. About 20% inmates of lunatic asylum may have PKU.
B. Agitation, hyperactivity, tremors and convulsions are often manifested. This may
be because phenylalanine interferes with neurotransmitter synthesis such as
serotonin.
C. Defect in myelin formation is observed in pku patients. (myelin sheath allow
electrical impulse to transmit quickly and efficientlyalong the nerve cell. The
sheath is made of up protein and amino acid)
D. The child often has hypopigmentation,that causes light skin colour, fair hair, blue
eyes etc. explained by the inhibition of tyrosinase which leads to inhibition of
melanin formation.
E. Phenyllactic acid in sweat may lead to mousy body odor (mouse body smell).

Laboratory Diagnosis
A. Blood phenylalanine: Normal level is 1 mg/dL. In PKU, the level is >20 mg/dL. This
may be demonstrated by chromatography. Tandem mass spectroscopy is the most
reliable test; but is costly.
B. Guthrie test is a rapid screening test, which is a bacterial (Bacillus subtilis) bioassay for
phenylalanine. The test is usually performed after the baby is fed with breast milk for a
couple of days by testing elevated levels of phenylalanine.
C. Ferric chloride test: Urine of the patient contains phenylketones about 500–3000
mg/day. This could be detected by adding a drop of ferric chloride to the urine. A
transient blue-green color is a positive test.But this is a less reliable test.
D. DNA probes are now available to diagnose the defects in phenylalanine hydroxylase and
dihydrobiopterin reductase.

Treatment

A. Early detection is very important. About 5 units of IQ are lost for each 10-week delay in
starting the treatment.
B. The treatment is to provide a diet containing low phenylalanine (10–20 mg/kg body
weight per day). Food based on tapioca (cassava) will have low phenylalanine content.
C. This special diet is to be continued during the first decade of life; after which the child
can have a normal diet. Lifelong compliance of special diet is advised, though not
mandatory.
D. In some seriously affected PKU patients, treatment includes administration of 5-
hydroxytryptophan and dopa to restore the synthesis of serotonin and catecholamines.
PKU patients with tetrahydrobiopterin deficiency require tetrahydrobiopterin
supplementation.

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E. Female child, on growing to adulthood may become pregnant. Then again special diet is
to be given, because the increased phenylalanine level will affect the brain development
of the fetus.


ALKAPTONURIA (Black urine disease)

The term alkaptonuria arises from the Arabic word alkapton for ‘alkali’ and Greek word ‘to
suck up oxygen greedily in alkali’. This is based on the observation that the urine becomes
black on standing when it becomes alkaline. Sir Archibald Garrod in 1902 reported that patients
complain that their underwears are getting blackened. Garrod concluded that the disease is
inherited and it is due to the deficiency of the enzyme required for further metabolism of
homogentisic acid. Alkaptonuria and albinism are two inborn errors included in Garrod’s tetrad;
the other two being pentosuria and cystinuria. Garrod introduced the term “inborn errors of
metabolism” in 1908. The condition had been vividly described by Zactus Luxtanus in 1649.
Egyptian mummies dating back 2000 BC had pigmented cartilages due to alkaptonuria.

Biochemical Defect

1. Alkaptonuria is an autosomal recessive condition with an incidence of 1 in
250,000 births.
2. The metabolic defect is the deficiency of homogentisate oxidase (step 3, Fig. 18.2
and item 3, Fig.18.7). This results in excretion of homogentisic acid in urine.
3. It is compatible with fairly normal life. The only abnormality is the blackening of
urine on standing. The homogentisic acid is oxidized by polyphenol oxidase to
benzoquinone acetate. It is then polymerized to black colored alkaptone bodies.
4. By the 3rd or 4th decade of life, patient may develop ochronosis (deposition of
alkaptone bodies in intervertebral discs, cartilages of nose, pinna of ear)
(Fig.18.6B). Black pigments are deposited over the connective tissues including
joint cavities to produce arthritis.
5. No specific treatment is required. But minimal protein intake with phenylalanine
less than 500 mg/day is recommended.

Diagnosis of Alkaptonuria

1. Urine becomes black on standing when it becomes alkaline. Blackening is
accelerated on exposure to sunlight and oxygen. The urine when kept in a test tube
will start to blacken from the top layer.
2. Ferric chloride test will be positive for urine.
3. Benedict’s test is strongly positive. Therefore, alkaptonuria comes under the
differential diagnosis of reducing substances in urine

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ALBINISM

1. The Greek word, albino means white. Albinism is an autosomal recessive disease with an
incidence of 1 in 20,000 population (Fig.18.7).
2. Tyrosinase is completely absent, leading to defective synthesis of melanin.
3. The ocular fundus is hypopigmented and iris may be gray or red. There will be associated
photophobia, nystagmus and decreased visual acuity.
4. The skin has low pigmentation, and so skin is sensitive to UV rays. The skin may show
presence of nevi and melanomas. Hair is also white.
5. Manifestations are less severe in tyrosinase positive type, where the abnormality is in the
uptake of tyrosine by melanocytes.
6. Albinism may be produced by the following causes:

i) Melanocyte deficiency secondary to a failure of melanoblasts to colonize the skin.
ii) Failure of melanocytes to form melanosomes
iii) Due to tyrosinase deficiency, melanin is not produced in the melanosomes
iv) Failure of melanosomes to form melanin owing to substrate deficiency
v) Failure of melanosomes to store melanin or to transport melanin to keratinocytes
vi) Excessive destruction of functional melanosomes.


Fig. 18.7: Summary of tyrosine metabolism

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HYPERTYROSINEMIAS

Hepatorenal Tyrosinemia (Tyrosinemia Type I)

1) It is also called as tyrosinosis. It is an autosomal recessive condition with an incidence of 1.5
per 1,000 births. It is due to a deficiency of enzyme fumaryl acetoacetate hydrolase (step
no. 5 in Fig.18.2 and item 3, Fig.18.7).
2) Symptoms manifest by the first 6 months of life and death occurs rapidly. Cabbage like odor
and hypoglycemia and eventual liver failure are seen. There may be mild mental retardation.
3) Urine contains tyrosine, para-hydroxyphenylpyruvic acid (p-HPPA) and
hydroxyphenyllactic acid; and serum shows tyrosine.
4) Tyrosine and phenylalanine restricted diet is advised.

Oculocutaneous Tyrosinemia (Tyrosinemia Type II)

It is due to deficiency of tyrosine amino transferase (tyrosine transaminase) (step no. 2,
Fig.18.2). Mental retardation, keratosis of palmar surface, painful corneal lesions and
photophobia are seen. There is increased excretion of tyrosine and tyramine in urine. A diet low
in protein is advised.

Neonatal Tyrosinemia (Tyrosinemia Type III)

This is due to the absence of the enzyme para-hydroxyphenylpyruvate hydroxylase (step no.3,
Fig.18.2). This deficiency may cause transient hypertyrosinemia in the new born; this will
respond to administration of ascorbic acid and dietary protein restriction.
A variant of the above disease is called Hereditary p-HPPA Oxidase Deficiency. This
is a more aggressive condition. It is characterized by neurological abnormalities and excretion
of tyrosine, para-hydroxy phenylpyruvic acid (p-HPPA), hydroxyphenyllactic acid and hydroxy
phenylacetic acid in urine.

Maple syrup urine disease (MSUD)

The normal metabolism of the branched chain amino acids Leucine, Isoleucine, and valine
involves loss of the α-amino acid by transamination followed by oxidative decarboxylation of
the respective keto acids. The decarboxylation step is catalysed by branched chain α keto acid
decarboxylase. In approximately 1 in 300,000 live births in the general US population are
affected by this enzyme defect leading to ketoaciduria. When untreated this condition may lead
to both physical and metal retardation of the newborn and a distinct maple syrup odor of the
urine.

This defect can be partially managed with a low protein or modified diet. In some instances,
supplementation with high doses of thiamine pyrophosphate is recommended.

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