L11-Urea cycle.pdf
- 1. 1 UREA CYCLE OBJECTIVES: *IMPORTANT *EXTRA - Understand the reactions for removal of α-amino group of amino acids and formation of ammonia. - Identify the importance of blood transport of ammonia to the liver in the form of glutamine/alanine. - Understand the importance of conversion of ammonia into urea by the liver through urea cycle - Identify urea as the major form for the disposal of amino groups derived from amino acids -Identify the causes (hereditary & acquired), clinical manifestations and management of hyperammonemia CORRECTIONS LINK
- 2. 2 OVERVIEW Hyperammonemia is a metabolic disturbance characterized by an excess of ammonia in the blood. It is a dangerous condition that may lead to encephalopathy and death. Ammonia is a substance that contains nitrogen. It is a product of the catabolism of protein. It is converted to the less toxic substance urea prior to excretion in urine by the kidneys. The metabolic pathways that synthesize urea involve reactions that start in the mitochondria and then move into the cytosol. The process is known as urea cycle, which comprises several enzymes acting in sequence. FIRST we will start with how ammonia is formed then we will see how ammonia will be transported to the liver to enter urea cycle then we will discuss the cycle. Enjoy..
- 3. INTRODUCTION 3 1.α-amino group to produce ammonia (NH3) by 2 ways: 2.Remaining1 carbon skeleton (α-keto acid) is used to produce energy A.Transamination to glutamate B.Oxidative deamination2 of glutamate 1: (the rest) after removal of α-amino,α-keto acid is formed 2: Deamination: removal of amino group in NH3 form So, excess amino acid should be degraded into : Unlike glucose (stored as glycogen) and fatty acids(stored as TAGs), amino acids are not stored by the body. FORMATION
- 4. A. TRANSAMINATION TO GLUTAMATE 4 ALT will transfer the amino group from Alanin to α-ketoglutarate, resulting in the formation of pyruvate and glutamate. Reversible action AST transfers amino group from Asparatate to α-ketoglutarate forming glutamate & oxaloacetate. Reversible action PLP: Pyridoxal phosphate, a co-enzyme that is derived from vitamin B6. & PLP The first step in the catabolism of most amino acids is the transfer of their α-amino group to α-ketoglutarate1, producing α-keto acid (derived from the original amino acid) and glutamate. α-ketoglutarate rules as a receptor for the amino groups. Thereby becoming GLUTAMATE. This transfer of amino groups from one carbon skeleton to another is catalyzed by a family of enzymes that we call Aminotransferases or Transaminases. & PLP FORMATION
- 5. B. OXIDATIVE DEAMINATION 5 NAD NADH Glutamate Dehydrogenase NH3 summary Amino groups of amino acids are funneled to glutamate BUT Why glutamate ? Because It is the only amino acid that undergoes rapid oxidative deamination (so easy to take the amine group from it) Glutamate α-ketoglutarate FORMATION So we funneled all the amino groups to glutamate and when We need to remove it, we do by glutamate dehydrogenase to α-ketoglutarate and NH3 (ammonia).
- 6. In the liver: 1. Glutamine is converted into glutamate by glutaminase. 2. Alanine will give its amino group to α-ketoglutarate to form glutamate by ALT. 3. Glutamate is converted into α-ketoglutarate and releasing NH3 by glutamate dehydrogenase. 4. NH3 will start Urea Cycle. NEXT: TRANSPORT OF NH3 FROM PERIPHERAL TISSUES INTO THE LIVER 6 From the muscle : 1.NH3 will be transferred into α-ketoglutarate to form Glutamate. 2.Glutamate will give its amino group to pyruvate to form ALANINE Ammonia is produced by all tissues and the main disposal is via formation of urea in liver Blood level of NH3 must be kept very low, Normal blood level5 – 50 μmol/L, otherwise, hyperammonemia and CNS toxicity will occur (NH3 is toxic to CNS) To solve this problem, NH3 is transported from peripheral tissues to the liver via formation of: Glutamine (most tissues), Alanine (muscle) muscle LIVER MOST TISSUE From “most” Peripheral Tissue: 1.NH3 is transferred to α-Ketoglutarate to form Glutamate 2. Glutamate by Glutamine Synthetase is converted to GLUTAMINE Transportation
- 7. UREA CYCLE 7 Urea is the major form for disposal of NH3 Urea cycle occurs in the liver, starts in the mitochondria. The urea cycle (also known as the Ornithine cycle) is a cycle of biochemical reactions occurring in many animals that produces urea ((NH2)2CO) from ammonia (NH3). So it helps the body to get red of the toxic Ammonia as the less toxic Urea. There are 5 enzymes in the cycle: 1. Carbamoyl phosphate synthetase I 2. Ornithine transcarbamoylase (OCT) 3. Argininosuccinate synthase 4. Argininosuccinate lyase 5. Arginase The doctor: the names of the enzymes imp. The cycle
- 8. 8 UREA CYCLE The first step is the formation of Carbamoyl phosphate , by the enzyme carbamoyl phosphate synthetase I , which is a rate limiting enzyme* The substrate of CPSI is (A nitrogen atom from free ammonia + A carbon atom from CO2 + 2ATP) CPSI has an absolute requirement for ( N-acetyl- glutamate ) which acts as an allosteric activator. N-acetyl-glutamate is synthesized by N-Acetyl- glutamate synthetase (NAGS) enzyme in presence of Arginine. NAGS deficiency is efficiently treated with Carbaglue (Carglumic acid), a CPS1 activator * rate-limiting step The slowest step in a metabolic pathway or series of chemical reactions, which determines the overall rate of the other reactions in the pathway. NAGS N-Acetylglutamate Glutamate ASS Later in the cycle another Nitrogen atom from aspartate joins the substrate. So one nitrogen of urea is from NH3 and the other nitrogen from aspartate The cycle
- 9. FATE OF UREA 9 • ~75% of the urea is then transported to the kidneys where it is excreted. • ~25% of urea diffuses from the blood into the intestine , and is cleaved into CO2 and ammonia by the bacterial urease, This ammonia is partly lost in the feces and partly reabsorbed to the circulation. In patients with renal failure, plasma urea is elevated, promoting greater transfer of urea from the blood to the intestine, so the bacterial urease in the intestine becomes clinically significant in this case ( Acquired hyperammonemia ) Normal blood level of ammonia: 5 – 50 µmol/L NH3 + CO2 Reabsored into blood Lost in feces
- 10. HYPERAMMONEMIA 10 Acquired Liver disease Acute: Viral hepatitis or hepatotoxic Chronic: Cirrhosis by hepatitis or alcoholism Renal failure Hereditary Genetic deficiencies of any of the 5 enzymes of urea cycle or the activator enzyme for CPSI: Ornithine transcarbamoylase deficency: X-linked recessive Most common of congenital hyperammonemia Marked decrease of citrulline1 and arginine2 Other enzymes deficiencies are Autosomal recessive Clinical presentation: Lethargy and somnolence Tremors Vomiting and cerebral edema Convulsions Coma and death 1.The end-product of this enzyme reaction 2.The end-product of the cycle
- 11. HYPERAMMONEMIA 11 Sodium phenyl butyrate (Buphenyl): Pro-drug that is converted to phenyl- acetate which condenses with glutamine forming phenylacetylglutamine that is excreted in urine Drug treatment A. Drugs that scavenge ammonia by creating an alternate pathway to excrete N2- precursors: 1. I.V. Sodium phenyl- acetate & sodium benzoate (Ammonul) 2. Oral sodium phenyl butyrate (Buphenyl) 3. I.V. Arginine: for all UCDs except UCD2 due to arginase deficiency (argininemia) B. Activators to CPSI (Carglumic acid “Carbaglue”): For hyperammonemia due to NAGS deficiency Management 1. Protein restriction1 2. Volume repletion to maintain renal function Use 10% dextrose in water but limit the use of normal saline 3. Ammonia removal by hemodialysis &/or drugs 4. Avoid drugs that increase protein catabolism (eg, glucocorticoids) or inhibit urea synthesis (eg, valproic acid), or have direct hepatotoxicity 1.restriction but don’t stop the intake, especially in growing children who need proteins for their development. 2. urea cycle disorders; remember: Arginine>> NAGS>> NAG>> carbamoyl phosphate synthetase I
- 12. MCQS 1- which one of the following is the major form of disposal of amino group? A. Ammonia B. Urea C. CPS1 D. CO2 2- Carbaglu is given to treat : A. Hyperammonemia secondary to renal failure. B. Hyperammonemia secondary to CPS1 deficiency. C. Hyperammonemia secondary to NAGS deficiency. D. Hyperammonemia secondary to OCT deficiency. 3- which of the following is the rate limiting enzyme of urea cycle ? A. Ornithine transcarbamoylase B. Carbamoyl phosphate synthetase C. Argininosuccinate synthase D. Argininosuccinate lyase 12 1-B 2-C 3-B 4-B 5-B 6-A 7-B 4-NH3 is transported from Muscles to Liver via Formation of: A. Glutamine B. Alanine C. Nitrogen D. Pyruvate 5-Alanine will give its amino group to α-ketoglutarate to form glutamate by ? A- AST B-ALT C- glutamate dehydrogenase. 6-Amino Group of Amino Acids are funneled to Glutamine by: A. Transamination B. Oxidative Deamination C. Decarboxylation D. Hydrolysis 7) The action of intestinal urease to form NH3 is clinically significant in: A. Liver Failure B. Renal Failure C. Hepatic Toxicity D. Renal Injury
- 13. SAQS 13 1. Interpret signs and symptoms of hyperammonemia. Ammonia is a toxic product and it can cross the Blood brain barrier, causing: Lethargy and somnolence, Tremors, Vomiting and cerebral edema, Convulsions, Coma and death 2. Briefly describe the transport of NH3 from peripheral tissue into the liver: 1. From the Muscles 1) NH3 is transferred to α-Ketoglutarate to form Glutamate 2) Glutamate will give its Amino Group to Pyruvate1 to form Alanine by ALT NH3 leaves Muscles in the form of ALANINE 2. Most peripheral tissues 1)NH3 is transferred to α-Ketoglutarate to form Glutamate 2)Glutamate is converted to Glutamine by Glutamine Synthetase NH3 leaves peripheral tissue in the form of GLUTAMINE* 3. What’s the normal blood level of Ammonia? 5 – 50 μmol/L 4. Name the enzymes invlolved in The Urea Cycle: 1. CarbamoylPhosphateSynthetaseI 2. OrnithineTranscarbamoylase(OCT) 3. ArgininosuccinateSynthase 4. ArgininosuccinateLyase 5. Arginase 5. Mention the conditions which can cause Hyperammonemia: Inherited: Ornithine transcarbamoylase deficiency and others Autosomal dominant Acquired: Liver disease like Viral Hepatitis, Hepatotoxic ex Alcohol, Cirrhosis Renal Failure
- 14. Amerah Bin Zuair Nuha AlHumaidhi Najla Aldraiweesh Mohammad Alotaibi 14 DONE BY: If You Have Any Questions Or Comments Please Inform Us: Biochemistry434@gmail.Com