Lecture Notes on Metabolic Pathway of Phenylalanine, Tyrosine & Related Inborn Disorders

 Metabolic Pathway for Phenylalanine and Tyrosine 

(For MCQ Practice click here)

Conversion of Phenylalanine to Tyrosine 

- Phenylalanine hydroxylase (PAH) converts phenylalanine to tyrosine 
- PAH is encoded by the PAH gene located in Chromosome 12 and consist of 13 exons 
- Tetrameters with each monomer consisting of the catalytic site, regulator site, and subunit binding domain 
- Phenylalanine hydroxylase is tetrahydrobiopterin (BH4) requiring enzyme 
- Dihydrobioteridine reductase catalyzes the conversion of dihydrobiopteridine to tetrahydrobiopterin 
- Deficiency of enzyme caused phenylketonuria 

Figure- Overview of Phenylalanine and Tyrosine Metabolism 

Transamination of Tyrosine 
- Tyrosine aminotransferase catalyzes the conversion to tyrosine to p-hydroxyphenylpyruvate 
- Tyrosine aminotransferase is a Pyridoxal-5-phosphate requiring enzyme 
- In the process, alpha-ketoglutarate is converted into glutamate 
- Deficiency of the enzyme causes neonatal tyrosinemia (Type III) 

Hydroxylation of p-hydroxyphenylpyruvate to homogentisate 
- The reaction is catalyzed by an enzyme p-hydroxyphenylpyruvate hydroxylase (dioxygenase) 
- Requires Vit C (ascorbate) for its activity 
- Deficiency of the enzyme causes Tyrosinemia Type II 

Oxidation of Homogentisate to 4-maleylacetoacetate 
- Catalyzed by an enzyme homogentisate oxidase 
Deficiency of the enzyme causes Alkaptonuria 

Conversion of Maleylacetoacetase to 4-fumarylacetoacetate 
- Catalyzed of by an enzyme isomerase 
- Conversion of 4-fumarlyacetoacetate to acetoacetate and fumarate 
- Catalyzed of by an enzyme 4-fumarlyacetoacetate hydrolase 
- Deficiency of the enzyme causes Tyrosinemia Type-I 

Inborn Errors of Phenylalanine and Tyrosine Pathway

Phenylketonuria: PAH Deficiency 

Prevalence 
- Rare Genetic Disorder of Phenylalanine Metabolism 
- The most common form of amino acid disorders with US prevalence of 1 in 10,000 to 1 in 15,000 
Biochemical Basis
- Deficiency of enzyme that converts phenylalanine to tyrosine 
- Mutation of gene encoding phenylalanine hydroxylase (PAH) or dihydrobiopteridine reductase. 
- Mutations include missense (62%) and the remainder of insertion or deletions, nonsense, splicing defects, etc. 
- Increased Blood Phenylalanine (>1200 µM) & Urine Phenylketones 

Pathological Manifestation 
-In uncontrolled PKU, the Phenylalanine (Phe) level is >1200 µM in blood 
-Impairment caused by the accumulation of toxic by-products of Phe. 
-Clinical manifestation includes growth failure, microcephaly, seizures, and intellectual impairment. 
- Two possible mechanisms of clinical manifestation including neurological disorders. 
a) Decreased or absent PAH activity can lead to a deficiency of Tyr and its downstream products, including melanin, l-thyroxine, and the catecholamines neurotransmitters. 
b) Elevated Phe competes for large neutral amino acid Transporter blocking transport Tyr and Trp, thereby reducing serotonin and catecholamines 
-Decreased melanin synthesis leads to hyperpigmentation 

Diagnosis 
Screening test: 
Blood phenylalanine (normal- <120 µM) and tyrosine (high phenylalanine & Low tyrosine)- Chromatographic techniques 
Guthrie Test: B. substilis Inhibition test- use of bacteria to measure the presence of high Phe in a sample 
Confirmed by Mutation analysis of the PAH gene using southern blot, Restriction analysis, Sequencing analysis 

Treatment 
Diet
- A low Phe diet is used for treatment 
-Regimen intake of low protein diets and exclusion of high protein diets such as eggs, milk, cheese, meat, poultry, fish, dried beans, and legumes 
Monitoring Phe (Target <600 µM) 
Kuvan (BH4) in BH4 responsive individuals 
Palynziq- FDA approved enzyme substitution therapy that degrades and clear Phe from circulations. 

Type I Tyrosinemia: Fumaryl-Acetoacetate Hydrolase (FAH) Deficiency 
Prevalence: 
- 1 in 100,000 to 120,000 
Inheritance Pattern 
- Autosomal Recessive 
Biochemical Basis 
- Defective FAH gene (15q25.1), leading to accumulation of fumarylacetoacetate, succinylacetoacetate, and succinylacetone 

Pathological Manifestation 
- Accumulation of fumarylacetoacetate causes cellular damage and apoptosis in liver 
- Succinylacetone interferes hepatic enzyme including para hydroxyphenyl pyruvic acid hydroxylase and ALA-dehydratase 

Diagnosis (Newborn screening)- 
- Elevated blood tyrosine 
- Increased succinylacetone in blood and urine -Low delta-ALA dehydratase enzyme activity 
Molecular diagnosis 
-Sequence analysis of the FAH gene 

Treatment 
- Nitisinone (Orfadin), 22-(2-nitro-4-trifluoro-methylbenzyol)-1,3 cyclohexanedione (NTBC), which blocks para hydroxyphenyl pyruvic acid dioxygenase (p-HPPD), and prevents the accumulation of fumarylacetoacetate with dietary management 

Type II Tyrosinemia: Tyrosine Aminotransferase (TAT) Deficiency (Richner Hanhart syndrome)


Prevalence
Rare less than 1 in 1,000,000 

Inheritance Pattern 
- Autosomal Recessive 

Biochemical Basis 
- Defective TAT gene (16q22.2), leading to accumulation of tyrosine 
- Increased formation and excretion of p-hydroxyphenypyruvate, p-hydroxyphenyllactate, and p-hydroxyphenylacetate in urine 

Pathological Manifestation 
- Ophthalmic manifestation- recalcitrant pseudodendritic keratitis 

Diagnosis (Newborn screening)
- Elevated blood tyrosine (usually > 500 µM) 
- Presence of p-hydroxyphenypyruvate, p-hydroxyphenyllactate, and p-hydroxyphenylacetate in urine 
Molecular diagnosis 
-Sequence analysis of the TAT gene 

Treatment 
- Dietary management 

Tyrosinemia III (Neonatal ): Hydroxy-phenylalanine Pyruvate Hydroxylase Deficiency 

Prevalence: 
- Rarest less than 1 in 1,000,000 

Inheritance Pattern 
- Autosomal Recessive 

Biochemical Basis 
- Defective HPD gene (16q22.2), leading to accumulation of tyrosine 
- Increased formation and excretion of p-hydroxyphenypyruvate, p-hydroxyphenyllactate and p-hydroxyphenylacetate in urine 

Pathological Manifestation 
-Neurological manifestation with intellectual disability or ataxia 
- No liver involvement 

Diagnosis (Newborn screening)- 
- Elevated blood tyrosine (usually > 350 to 650 µM) 
- Presence of p-hydroxyphenypyruvate, p-hydroxyphenyllactate and p-hydroxyphenylacetate in urine 
Molecular diagnosis 
-Sequence analysis of the HPD gene 

Treatment 
- Dietary management 

Alkaptonuria: Homogentisate Oxidase Deficiency 

Prevalence: 
- In US, 1 in 250,000 to 1,000,000 

Inheritance Pattern 
- Autosomal Recessive 

Biochemical Basis 
- Defective HGD gene (3q13.33), leading to accumulation of homogentisate in blood and urine, 
- Urine to turn dark on standing 

Pathological Manifestation 

- Accumulation of HGA and its oxidation products (e.g., benzoquinone acetic acid) in connective tissue leads to ochronosis 
- Brown pigmentation of the sclera is observed 
- Ear cartilage pigmentation is seen in the concha and antihelix 
- Ochronotic arthritis 

Diagnosis (New born screening)
-Urinary HGA > 1 gm/day (Normal- 30mg/day) 
Molecular diagnosis 
-Sequence analysis of HGD gene 

Treatment 
- Dietary management 



(For MCQ Practice click here)

 Lecture Video on Phenylalanine and Tyrosine Metabolism



 Phenylketonuria: Biochemical Basis, Diagnosis, and Treatment


Alkaptonuria: Pathogenesis, Diagnosis, and Treatment




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