Urea synthesis
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The document summarizes the urea cycle, which converts toxic ammonia into urea in the liver. It describes the five steps of the cycle, which include the formation of carbamoyl phosphate in mitochondria and the other four reactions occurring in the cytosol. Deficiencies in enzymes in the cycle can cause various urea cycle disorders. Blood urea levels are monitored as high levels can indicate kidney dysfunction. Symptoms of urea cycle disorders include elevated ammonia, vomiting, and neurological issues.
Urea synthesis
- 1. Happiness is something comes into our lives through doors We don’t even remember leaving it open Dr. Dhiraj J. Trivedi
- 2. Urea - Introduction • Urea cycle reactions converts toxic NH3 into non- toxic and water soluble urea. • Urea is the principle end product of the protein or amino acid metabolism in humans. • 80-90% of urinary nitrogen excreted is in the form of urea nitrogen.
- 3. • Krebs and Henseleit were the first to elucidate the steps. Hence Krebs Henseleit cycle • Other name -- Urea cycle, Ornithine cycle. • Site of urea synthesis– Liver • First two reactions occur in the mitochondria • and later three reactions occur in cytosol Urea synthesis
- 4. Requirements of urea synthesis • Substrate: • CO2 --- from HCO3 • NH4 --- Deamination of amino acids • Aspartic acid --- Transamination of OAA • Energy: ATP 3molecules • Modulators: • Mg+2, N Acetyl Glutamate (NAG)
- 5. Steps in the urea cycle are Cytosol Step 3: Synthesis of Argininosuccinate Step 4: Synthesis of Arginine Step 5: Release of urea and Ornithine Mitochondria Step 1: Formation of carbamoyl phosphate Step 2: Formation of citrulline
- 6. CO2 NH3 Carbamoyl phosphate Carbamoyl phosphate synthetase - I N – Acetyl Glutamate 2ATP 2ADP + 2Pi Mg+2 From HCO3 From deamination Of amino acids + Step 1 : Synthesis of carbamoyl phosphate Step 1: Takes place in mitochondria
- 7. • In liver mitochondria • NAG allosterically activates CPS-I • Two high energy phosphate bonds are utilized for the synthesis of carbamoyl Phosphate
- 8. Isomers of carbamoyl phosphate synthetase
- 9. Carbamoyl phosphate Ornithine trans Carbamoylase Pi Ornithine Citrulline Step 2 : Synthesis of Citrulline Step 2 : Takes place in mitochondria Carrier proteins transfer Ornithine from cytosol to mitochondria matrix
- 10. Step 3: Takes place in Cytosol Step 3 : Synthesis of Arginino succinate Carrier proteins Transfer citrulline To cytosol for further reactions Arginino succinate Arginino succinate synthetase ATP AMP + PPi Mg+2 Citrulline Aspartate Oxaloacetate NH4 Malate TCA cycle
- 11. Arginino succinate Arginino succinase Arginine Fumarate Malate Step 4: Takes place in Cytosol Step 4 : Synthesis of Arginine Arginine - Essential amino acid synthesized in adult
- 12. • Arginino succinate is cleaved to arginine and fumarate • Enzyme is present in cytoplasm of liver and kidney tissues • Link between Urea cycle and TCA cycle • Fumarate can be converted to malate and then to oxaloacetate by the intervention of certain TCA cycle enzyme • Aspartate may be regenerated by transamination
- 13. Urea Arginase Arginine Ornithine Present only in liver Step 5: Takes place in Cytosol Step 5 : Release of Ornithine and Urea
- 14. • Release of urea and ornithine • The guanido group of arginine Hydrolytically cleaved by arginase • The urea diffuses into blood from where it is cleared by the kidneys • Ornithine will enter mitochondria and become substrate for reaction 2
- 15. Urea Arginase Arginine Ornithine Arginino succinate Arginino succinase Fumarate Malate Arginino succinate synthetase ATP AMP +PPi Mg+2 Citrulline Oxaloacetate NH4 TCA cycle Carbamoyl phosphate Ornithine trans Carbamoylase Pi CO2 NH3 Carbamoyl phosphate synthetase - I N – Acetyl Glutamate 2ATP 2ADP +2Pi Mg+2 Mitochondrial matrix
- 16. Regulation of urea cycle • 1. Substrate availability ( feed forward ) • 2. High protein diet and prolonged starvation increases urea synthesis • 3. Allosteric mechanism (by N acetyl glutamate) • 4. CPS-I, ornithine transcarbamoylase, arginino succinate synthetase and arginase all undergo feed back inhibition
- 17. Role of CPS – I in regulation of urea cycle • CPS-I is the rate limiting enzyme of the urea cycle • CPS-I is allosterically activated by N-acetyl glutamate (NAG), which lowers its Km for ATP • On binding with NAG, the enzyme changes its conformation to the active form • This will exposes specific –SH group and increases the ATP affinity of the enzyme
- 18. • This promotes the transfer of phosphate group from ATP to an enzyme bound carbamate intermediate, changing the carbamate to carbamoyl phosphate • Energetic – 3 molecules ATP • Formation of 1 molecule of urea requires 4 high energy bonds Role of CPS – I in regulation of urea cycle
- 19. Dependency of Urea cycle on TCA cycle • For ATP • CO2 produced in TCA cycle is used for urea synthesis • Aspartate supply can be replenished by converting fumarate produced in urea cycle to malate and oxaloacetate via TCA cycle.
- 20. Urea cycle disorders • Hyperammonemia Type I • Hyperammonemia Type II (Ornithinemia) • Citrullinemia • Arginino Succinic Aciduria • Hyper Argininemia
- 21. Hyper ammonemia I Hyper ammonemia II Citrullinemia Arginino succinic aciduria Hyper Argininemia Carbamoyl phosphate synthetase - Ornithine trans carbamoylase Arginino succinate synthetase Arginino succinase Arginase
- 22. Hyperammonemia Type I • This is due to the deficiency of carbamoyl phosphate synthetase-I in mitochondria • Features are high levels of NH3 in blood and urine
- 23. Hyperammonemia Type-II (Ornithinemia) • Due to the deficiency of ornithine transcarbamoylase • Results in elevated levels of NH3 in tissues
- 24. Citrullinemia • A rare disorder • Probably due to a mutational change in the catalytic site of arginino succinate synthetase • Blood level and urinary excretion of citrulline increases
- 25. Arginino Succinic Aciduria • Due to the deficiency of Arginino Succinase • Increased level of Arginino Succinate in blood and its increased excretion in urine
- 26. Hyper Argininemia • Due to the deficiency of Arginase • Increased level of Arginine in blood and increased urinary excretion
- 27. Significance of urea cycle 1. Major route of disposal of toxic Ammonia 2. Synthesis of semi-essential amino acid ARGININE 3. Urea cycle, Citric acid cycle and transamination reactions are linked
- 28. Blood urea • Normal level- 15 - 45 mg/dl • Uremia or azotemia is increased blood urea level due to renal failure • Normally urea excretion is about 20-30 gm per day • A high value of blood urea indicates significant reduction in the GFR and kidney disease • Causes for reduction in GFR--- Pre renal, Renal and Post-renal
- 29. Pre-renal causes- mainly reduction in GFR (reduction in plasma volume and fall in renal blood flow) –Diarrhea –Vomiting –Severe dehydration –Increased protein breakdown observed in cases of prolonged fevers, diabetic coma, thyrotoxicosis, after major surgery
- 30. Renal causes such as- • Acute glomerular nephritis • Chronic nephritis • Nephrosis • Polycystic kidney • Hydronephrosis
- 31. Post renal causes like • Urinary tract obstruction due to – Tumors, –stones in the bladder –Enlargement of prostate gland • All these may lead to increased reabsorption of urea from the renal tubules
- 32. Hyper ammonemia I Hyper ammonemia II Citrullinemia Arginino succinic aciduria Hyper Argininemia Carbamoyl phosphate synthetase - Ornithine trans carbamoylase Arginino succinate synthetase Arginino succinase Arginase
- 33. Common symptoms of urea cycle disorders includes • Elevated blood NH3 level • Aversion to protein intake • Tendency for vomiting • Mental retardation • Coma, convulsions and death • Lowering the dietary protein intake has been suggested as a way of treatment
- 34. 2 -2015