Urea cycle

it is about how ammonia is detoxified to urea and its biomedical significance. This PPT can be used by students of MBBS, MD, BDS and general Biochemistry students

1. Prof. Dr. V. P. Acharya MD, PhD UREA CYCLE

2. Pathway for final disposal of ammonia Synonym: Krebs- Henseleit cycle Organ: Liver Organellar location: Mitochondria& Cytoplasm

3. Sir Hans Krebs (1932) 1 st metabolic cycle to be discovered N. P. prize for TCA cycle

4. UREA CYCLE Urea- CO (NH 2 ) 2 C- from HCO 3 - N- one from ammonium ion - one from Aspartate 5 steps 2 in mitochondria 3 in cytoplasm

5. 1. Formation of Carbamoyl Phosphate Rate –limiting step of urea synthesis Takes place in mitochondria CPS-I enzyme N-acetyl glutamate Irreversible & allosterically regulated N-Acetyl glutamate

6. Difference between CPS-I and CPS-II Characteristics CPS-I CPS-II Cellular location Mitochondria Cytosol Pathway involved Urea cycle Pyrimidine synthesis Source of nitrogen Ammonia Glutamine Allosteric activator N-acetylglutamate (NAG) Nil

7. 2. Formation of Citrulline Also mitochondrial Ornithine transcarbamoylase Present in milk; but not in tissue proteins or blood

8. 3. Formation of Argininosuccinate 2 nd atom of nitrogen of urea Argininosuccinate synthase ATP hydrolyses to AMP PPi is an inhibitor to this step

9. 4. Formation of Arginine Argininosuccinate lyase Fumarate inhibits the enzyme But Fumarate is funneled to TCA cycle

10. 5. Formation of Urea Arginase - hydrolytic enzyme Ornithine & Urea formed Ornithine- enters mitochondria

11. Summary of Urea Cycle

12. Bioenergetics 4 ATPs consumed 2 ATPs in 1 st step Formation of Argininosuccinate requires 2 high energy bond ATP →AMP+ PPi

13. Regulation of Urea cycle Feed-forward reaction- Availability of ammonia Induction of urea cycle enzymes- High protein diet - Prolonged starvation 3 . Allosteric regulation- CPS-I is allosterically activated by NAG

14. 4. Compartmentalization- Prevents inhibition of Argininosuccinate lyase by Fumarate

15. Krebs Bicycle or Tricycle???

16. Significance of Urea cycle Toxic Ammonia → Less-toxic Urea Bicarbonate also consumed Forms Arginine Integrates with TCA cycle Ornithine → Proline Ornithine → Precursor of polyamines

17. Disorders of Urea Cycle UCDs - 6 enzyme deficiencies , 2 transporter defects All present with hyperammonaemia 1 defect is X-linked (OTC) – most common – the rest are autosomal recessive Age of onset varies from 2 days to adulthood Severity varies from lethal hyperammonaemic encephalopathy to asymptomatic 1 ST 2 enz defect- more severe symptoms as Ammonia accumulates

18. Diseases Enzyme deficit Features Hyperammonemia I CPS-I Mental retardation, ↑↑NH 3 Hyperammonemia II OTC ↑NH 3 , ↑Gln, orotic aciduria Hyperornithinemia Defective Ornithine transporter ↑NH 3 & Ornithine Citrullinaemia Argininosuccinate synthetase ↑NH 3 & Citrulline; citrullinuria Argininosuccinic aciduria Argininosuccinate lyase Argininosuccinate in blood & urine. Trichorrhexis nodosa Hyperargininemia Arginase ↑Arginine in blood & CSF; Cys & Lys in urine

19. Symptoms of Ammonia intoxication CNS affected Convulsions Flapping tremor Slurred speech Blurred vision Nausea, vomiting Lethargy Coma, death

20. Causes of toxic effect of NH 3 ↓ATP due to diversion of excess of α - ketoglutarate from TCA cycle intermediates to form Glu & Gln in brain - ↓ glucose in brain (major fuel of brain) ↑formation of GABA from Glutamate- ↓neural transmission ↑Glutamine- osmotic effect- brain edema

21. Uraemia Normal blood level : 15-45 mg/dl 75% of the NPN > 90% excreted through kidney, rest via GI tract & skin Concentration affected by - dietary protein, - rate of protein catabolism - muscle wasting as in starvation - Digestion of blood proteins after GI hemorrhage

22. Types Causes Pre-renal uremia High Protein diet Any causes of ↑ catabolism Any cause of impaired renal perfusion Renal Uremia Any cause that impedes GFR Post-renal uremia Obstruction to urine outflow Reduced plasma urea concentration Low protein diet Severe liver disease Water retention

23. Clinical significance Measuring GFR- Urea clearance test Together with creatinine estimation to know renal status

24. DO NOT PRETEND – BE DO NOT THINK- ACT DO NOT DREAM- REALISE

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