Tyrosine Catabolism

• Tyrosine Catabolism involves breaking down the amino acid tyrosine into simpler, usable compounds through a series of biochemical reactions:

1. Tyrosine Aminotransferase (TAT):

   • Reaction: Tyrosine is converted into p-hydroxyphenylpyruvate.
   • Mechanism: The enzyme tyrosine aminotransferase (TAT) transfers an amino group from tyrosine to alpha-ketoglutarate, resulting in p-hydroxyphenylpyruvate and glutamate.

2. p-Hydroxyphenylpyruvate Dioxygenase (HPPD):

   • Reaction: p-Hydroxyphenylpyruvate is converted into homogentisate.
   • Mechanism: The enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD) catalyzes this conversion, requiring molecular oxygen.

3. Homogentisate 1,2-Dioxygenase (HGD):

   • Reaction: Homogentisate is converted into 4-maleylacetoacetate.
   • Mechanism: The enzyme homogentisate 1,2-dioxygenase (HGD) adds oxygen to homogentisate, producing an unstable intermediate that rearranges into 4-maleylacetoacetate.

4. Fumarylacetoacetate Hydrolase (FAH):

   • Reaction: 4-Maleylacetoacetate is converted into fumarate and acetoacetate.
   • Mechanism: The enzyme fumarylacetoacetate hydrolase (FAH) catalyzes this final step, where fumarate enters the citric acid cycle and acetoacetate is used in ketogenesis.

Metabolic Disorders of Tyrosine Catabolism

• Several inherited disorders result from defects in enzymes involved in tyrosine catabolism, affecting the body’s ability to process tyrosine properly:

1. Albinism of Tyrosine Catabolism:

   • Cause: Defects in the enzyme tyrosinase.
   • Symptoms: Reduced or absent melanin production, vision problems, sensitivity to sunlight, and increased skin cancer risk.
   • Management: Protective measures against sunlight.

2. Alkaptonuria:

   • Cause: Deficiency in homogentisate 1,2-dioxygenase (HGD).
   • Symptoms: Accumulation of homogentisic acid, dark urine upon air exposure, and ochronosis (bluish-black discoloration of connective tissue).
   • Treatment: Symptom management and possibly joint replacement surgery.

3. Tyrosinemia of Tyrosine Catabolism:

   • Type I:

     • Cause: Deficiency in fumarylacetoacetate hydrolase (FAH).
     • Symptoms: Liver and kidney damage.
     • Treatment: Specific diet, nitisinone medication, and potentially liver transplantation.

   • Type II (Richner-Hanhart Syndrome):

     • Cause: Deficiency in tyrosine aminotransferase (TAT).
     • Symptoms: Corneal ulcers and palmoplantar hyperkeratosis.
     • Treatment: Special diet.

   • Type III (Hawkinsinuria):

     • Cause: Deficiency in p-hydroxyphenylpyruvate dioxygenase (HPPD).
     • Symptoms: Developmental and intellectual disabilities.
     • Treatment: Dietary adjustments and supplementation.

• This structured format provides a clear overview of tyrosine catabolism and its associated metabolic disorders, outlining the processes and implications of each condition in an organized manner.

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