Week 6 Urea Cycle Case Study

Question 1

Describe the biochemical process by which the body catabolizes amino acids and eliminates nitrogen.

Answer 1

• HCO₃^- + NH₄⁺ + 2 ATP → Carbamoyl phosphate
  
  • via CPS I
• Carbamoyl phosphate + Ornithine → Citrulline
  
  • via Ornithine transcarbamoylase (OTC)
• Citrulline leaves mitochondria
• Citrulline + Aspartate + ATP → Argininosuccinate
  
  • via Argininosuccinate synthetase
• Argininosuccinate - Fumarate → Arginine
  
  • via Argininosuccinate lyase
• Arginine + H₂O → Urea + Ornithine
  
  • via Arginase
• Urea is excreted in the urine
• Ornithine re-enters the mitochondria

Question 2

Why does ammonia build up in inherited disorders of the urea cycle?

Answer 2

• The urea cycle gets rid of nitrogen (in the form of ammonia) by converting it to urea and excreting it into the urine.
• If the urea cycle does not function properly, ammonia builds up

Question 3

What are the systemic effects associated with hyperammonemia?

Answer 3

• Nausea
• Vomiting
• FTT
• Liver damage (fatty liver)
• cerebral edema
• headache
• confusion
• seizure

Question 4

What are the sources of nitrogen for the urea cycle?

Answer 4

• Free NH₄⁺
• Glutamine
• Alanine
• Aspartate

Question 5

What are the clinical presentation and laboratory differences in CPS I deficiency?

Answer 5

• Clinical presentation:
  
  • Poor appetite, Tiredness, Vomiting, Trouble regulating body temperature, Trouble breathing, Seizures (also called epilepsy), Uncontrolled body movements, Delayed growth, FTT
• Laboratory Results:
  
  • Hyperammonemia
  • Normal urinary orotic acid
  • Liver biopsy showing decreased activity of CPS I

Question 6

What are the clinical presentation and laboratory differences in OTC deficiency?

Answer 6

• Clinical presentation
  
  • elevated urinary orotic acid
  • elevated glutamine levels
  • Lower levels of Citrullin, argininosuccinic acid, and arginine levels

Question 7

What are appropriate treatments for individuals with OTC deficiency?

Answer 7

• Neonatal
  
  • Liver transplant is necessary for survival
• Late onset
  
  • low protein diet
  • Infusion of citrulline (allows the urea cycle to bypass the OTC/Ornithine step and allows urea cycle to produce urea)
  • Liver transplant is optimal

Question 8

In the case study, Ornithine had a higher than normal Km value. What does this suggest about the OTC function?

Answer 8

• Higher Km means that more ornithine was needed to fill half of the OTC binding sites
  
  • more substrated needed
  • suggests OTC has lower affinity for ornithine
  • possible binding trouble or defect in active site

Question 10

In the case study, OTC function was optimal at a higher than normal pH. What does this suggest about the Ornithine transcarbamoylase enzyme?

Answer 10

• Higher optimum pH
  
  • change in AA structure alters relationship with environment

Question 11

What are the different modes of inheritance of OTC and CPS I?

Answer 11

• X-linked recessive
  
  • males more likely to be affected
• Point mutation (G → A)
  
  • Arginine (+) → Glutamine (neutral)
• Heterozygous female can still be affected, due to varied X-inactivation
  
  • especially after increased protein intake
• Normal father + Heterozygous Mother =
  
  • 25% chance of having affected male
  • 25% chance of having heterozygous female

Question 12

Describe western blot.