Dr. Saidunnisa, MD Professor of Biochemistry Aromatic amino acid (phenylalanine and tyrosine...

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Dr. Saidunnisa

Dr. Saidunnisa, MDProfessor of BiochemistryAromatic amino acid (phenylalanine and tyrosine )metabolism and its inborn errors.

Learning ObjectivesAt the end of the session the student shall be able to:Explain the major and minor pathways of phenylalanine and tyrosine metabolism.Interpret the biochemical aspects of disorders like PKU and Tyrosinemia and Alkaptunuria associated with them.Explain the melanin pathway with albinismExplain the neurotransmitters and thyroid hormone metabolic pathway Interpret parkinsonism, goiter ,hyper and hypothyroidism.OverviewPhenylalanine and tyrosine metabolism:Major pathwayMinor pathwayDisorders:Phenyl Ketonuria Tyrosinemia AlkaptonuriaAlbinismParkinsonism, Hyper and hypothyroidism.Phenylalanine and tyrosine MetabolismPhenylalanine is an essentialTyrosine is an non-essential amino acid.These are both glucogenic and ketogenic amino acids.The Predominant metabolism of phenylalanine occurs through tyrosine.These are involved in a variety of biologically important compounds:Epinephrine, nor-epinephrine, dopamine, (catecholamine)MelaninThyroid hormones

4Overview- Major and minor pathway

Major route of phenylalanine metabolism

6Conversion of phenylalanine to tyrosinePhenylalanine is hydroxylated at Para-position by phenylalanine hydroxylase to produce tyrosine.This is an irreversible reaction.It requires a specific coenzyme biopterin.

Mechanism of the reactionPhenylalanine hydroxylase is present in liver.involves incorporation of one atom of molecular oxygen into para position of phenylalanine while the other atom of oxygen is reduced to water (Monoxygenase).

Mechanism of the reactionThe active form of biopterin is H4-biopterin.In this reaction H4-biopterin is oxidized to H2-biopeterin.H4-biopterin is then regenerated by an NADPH-dependent dihydrobiopterin reductase.

Inborn Errors of MetabolismAn inherited enzyme deficiency caused by mutant genes, leading to the disruption of normal bodily metabolismAccumulation of a toxic substrate (compound acted upon by an enzyme in a chemical reaction)Impaired formation of a product normally produced by the deficient enzyme.What is a metabolic disease?Garrods hypothesis

product deficiencysubstrate excess toxic metaboliteADBC11Inborn errors of metabolismMore than 50 of these disorders have been described.Collectively they constitute a very significant portion of pediatric genetic diseases.Phenylketonuria is the most common disease.


In the United States, PKU is detected in 1/10,000 (Caucasian population) to 1/50,000 (African American population) newborns. Phenylketonuria (PKU), caused by a deficiency of phenylalanine hydroxylase.Biochemically, it is characterized by accumulation of phenylalanine .Hyperphenylalaninemia may also be caused by deficiencies in any of the several enzymes required to synthesize BH4, or in dihydropteridine (BH2) reductase,

Due to defect in phenylalanine hydroxylase, the conversion of phenylalanine to tyrosine is blocked resulting in high levels of phenylalanine in the body.Due to the deficiency phenylalanine is converted to phenylpyruvate and this is further converted to Phenyl lactate, and to phenyl acetate.

Minor pathwayCharacteristics of classic PKU:

Phenylalanine is present in elevated concentrations in tissues, plasma, and urine. Phenyllactate, phenylacetate, and phenylpyruvate, which are not normally produced in significant amounts in the presence of functional phenylalanine hydroxylase, are also elevated in PKU. [Note: The disease acquired its name from the presence of a phenylketone (now known to be phenylpyruvate) in the urine.

Signs and symptoms:Mental retardation, failure to walk or talk, seizures, hyperactivity, tremor, microcephaly, and failure to grow These metabolites give urine a characteristic musty (mousey) odor.

[Note: These clinical manifestations are now rarely seen as a result of neonatal screening programs.]

Biochemical basis for CNS symptomsAccumulation of phenylalanine in brain impairs the transport and metabolism of other aromatic amino acids (tryptophan, tyrosine).The synthesis of serotonin an excitatory neurotransmitter from tryptophan is insufficient.Learning checkCan PKU cause hypo pigmentation give the biochemical reasoning?

AlbinismDiagnosis Ferric chloride test: (Urine)qualitative test gives green colour due to presence of phenylpyruvic acid.Guthrie test: which is a bacterial (bacillus subtilis) bioassay: for phenylalanine.Normal levels: 1-2mg/dl.

Treatment Maintenance of plasma phenylalanine concentration within normal limits is a challenging task in treatment of PKU.This is done by strictly limiting dietary intake of phenylalanine and supplementing intake of tyrosine.Most experts believe that the diet should be adhered to throughout life but this can be very difficult, especially during adolescence and early adulthood.Some foods PKU patients should restrict

This diet essentially excludes milk and dairy products, meat, fish, chicken, eggs, beans and nuts which all contain very high levels of phenylalanine. Fruits, vegetables, breads and pastas also contain some phenylalanine and cannot be eaten freely. Soft drinks and foods containing the sweetener aspartame must also be avoided.

TyrosinemiaThis disorder is also known as Richner-Hanhart syndrome.Deficiency of tyrosine transaminase.Accumulation of tyrosine and its metabolites Phenyllactate, phenylacetate, and phenylpyruvate.Characterized by: dermatitis, eye lesions, mental retardation.


Alkaptonuria is a rare metabolic disease involving a deficiency in Homogentisic acid oxidase, resulting in the accumulation of Homogentisic acid.

Conversion of Homogentisate to Alkapton Homogentisate Poly phenol oxidase Benzoquinone acetate Polymerization Alkapton Binds to tissues: of connective tissue, bones and various organs (nose, ear etc) resulting in a condition known as Ochronosis.

Signs of Alkaptonuria


Dark staining of the diapers sometimes can indicate the disease in infants, but usually no symptoms are present until later in life.Change in colour of the urine on standing to brown or dark.

Alkaptonuria symptoms

The illness has three characteristic symptoms: homogentisic aciduria the patient's urine contains elevated levels of Homogentisic acid, Which is oxidized to a dark pigment on standing, large joint arthritis, and Black ochronotic pigmentation of cartilage and collagenous tissue.

Diagnosis Change in colour of the urine on standing to brown or dark.Urine gives positive with ammonical silver nitrate. This is due to strong reducing activity of homogentisate.

Treatment Diets low in phenylalanine and tyrosine: help in reducing the levels of Homogentisic acid, and decrease the amount of pigment deposited in body tissues. Although Alkaptonuria is not life-threatening, the associated arthritis may be severely crippling.

Melanin synthesis

Melanin is a pigment that occurs in several tissues, particularly the eye, hair, and skin. It is synthesized from tyrosine in the epidermis by pigment-forming cells called melanocytes. Its function is to protect underlying cells from the harmful effects of sunlight.


A defect in melanin production results in albinism, the most common form being due to defect in tyrosinase a copper-containing enzyme. Autosomal recessive

Sings and symptoms

Absence of melanin pigment (hypo pigmentation) from the skin, hair, and eyes. Affected individuals have vision defects and photophobia (sunlight hurts their eyes). They are at increased risk for skin cancer.

Oculocutaneous albinism having whit eyebrows and lashes.

Tyrosine-Derived Neurotransmitters

Tyrosine is converted to the catecholamine's. These are dopamine, nor epinephrine, and epinephrine These are neurotransmitters .The first step is conversion to DOPA, by tyrosine hydroxylase, which requires BH4 as cofactor.DOPA is converted to dopamine.

Tyrosine-Derived NeurotransmittersDopamine is converted to Norepinephrine and to epinephrine.. Within the substantia nigra and some other regions of the brain, synthesis proceeds only to dopamine. Within the adrenal medulla dopamine is converted to nor epinephrine and epinephrine.

Parkinsons disease Is commonly seen in elderly people.Due to degeneration of certain parts of the brain mainly substantia niagra there is decreased production of dopamine.

Signs and symptoms Four cardinal features:Remember acronym TRAPTremorRigidityAkinesiaPostural changes

Synthesis of thyroid hormonesThyroid hormones- thyroxine (T4 tetraiodothronine) and T3 (Triiodothyronine) are synthesized from tyrosine.Iodination of tyrosine ring occurs to produce mono, di, tri, tetra iodotyrosine in thyroglobulin molecule.Breakdown of thyroglobulin molecule releases the free hormones. Branched chain amino acids Metabolism

All the 3 AA are essential.Valine -GlycogenicLeucine-KetogenicIso leucine-Glycogenic and KetogenicThey serve an important role as alternate source of fuel for the brain during starvation.Overview of the pathway

PathwayThe first 3 common Metabolic reactions are:Transamination: removal of amino groups of all 3AA by PLP requiring transaminase and forming respective ketoacidsOxidative decarboxylation: removal of carboxyl group from keto acids by the enzyme alpha keto acid dehydrogenase (similar to PDH) are converted to respective acyl CoA esters.Dehydrogenation: oxidation of acyl CoA esters to succinyl CoA and acetyl CoA (similar to fatty acid oxidation).

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