UREA CYCLE

Biochemistry
Metabolism of Proteins
Nitrogen excretion and urea cycle

1. METABOLISM OF PROTEINS AND AMINO ACIDS Prepared by : Rabia Khan Baber Course Title : Biochemistry Topic : Nitrogen Excretion, Ammonia and Urea Cycle

2. AIMS AND OBJECTIVES OF PPT Nitrogen excretion from an aa What is ammonia and its role Urea cycle introduction Important enzymes of urea cycle Reactions of urea cycle Regulation of urea cycle Related diseases of urea cycle

3. NITROGEN EXCRETION FROM AN AA During fasting, muscle protein is cleaved to amino acids, some of which are partially oxidized to produce energy. Portions of these amino acids are converted to alanine and glutamine, which, along with other amino acids are released into the blood. Glutamine is oxidized by various tissues, including the gut and kidney, which convert some of its carbons and nitrogen to alanine. Alanine and other amino acids travel to the liver, where the carbons are converted to glucose and ketone bodies and the nitrogen is converted to urea, which is excreted by the kidneys. Several enzymes are important in the process of interconverting amino acids and in removing nitrogen so that the carbon skeletons can be utilized. These include transaminases, glutamate dehydrogenase and deaminases. Because reactions catalyzed by transaminases and glutamate dehydrogenase are reversible, they can supply amino groups for the synthesis of non-essential amino acids.

4. Transamination is the major process for removing nitrogen from amino acids. transfer of an amino group from one amino acid to another α-keto acid by Transaminase (aminotransferase). The nitrogen from one amino acid thus appears in another amino acid. Because transamination reactions are reversible they can be used to remove nitrogen from amino acids or to transfer nitrogen to α-keto acids to form amino acids. They participate in both amino acid degradation and amino acid synthesis.

5. AMMONIA Ammonia (NH 3 ) is a metabolite that results predominantly from protein and amino acid degradation. Ammonia is an extremely toxic base and its accumulation in the body would quickly be fatal so it is converted to urea, which is nontoxic, very soluble, and readily excreted by the kidneys through urine.

6. ROLE AND SIGNIFICANCE OF AMMONIA Ammonia does not have a physiologic function. However, it is important clinically because it is highly toxic to the nervous system. Because ammonia is being formed constantly from the deamination of amino acids derived from proteins, it is important that mechanisms exist to provide for the timely and efficient disposal of this molecule. The liver is critical for ammonia catabolism because it is the only tissue in which all elements of the urea cycle, providing for the conversion of ammonia to urea. Ammonia is also consumed in the synthesis of nonessential amino acids, and in various facets of intermediary metabolism.

7. Ammonia in the circulation originates in a number of different sites. A diagram showing the major contributors to ammonia levels AMMONIA IN THE CIRCULATION

8. UREA CYCLE (KREBS–HENSELEIT CYCLE) The urea cycle is the metabolic pathway that transforms nitrogen to urea for excretion from the body . Liver cells play a critical role in disposing of nitrogenous waste by forming urea hrough the action of the urea cycle . Nitrogenous excretory products are then removed from the body through in the urine. The urea excreted each day by a healthy adult (about 30 g) accounts for about 90% of the nitrogenous excretory products. The cycle occurs mainly in the liver.

9. Location of Urea Cycle: Cytosol and mitochondria of hepatocytes . Substrates: NH 3 (as derived from oxidative deamination of glutamate); CO 2 ; aspartate; three ATP . Products: Urea ; fumarate; H 2 O . Purpose of the Urea Cycle: The urea cycle allows for the excretion of NH 4 + by transforming ammonia into urea, which is then excreted by the kidneys.

10. IMPORTANT ENZYMES IN UREA CYCLE Carbamoyl phosphate synthetase I : Converts ammonium and bicarbonate into carbamoyl phosphate. This is the rate-limiting step in the urea cycle. This reaction requires two ATP and occurs in the mitochondria. Ornithine transcarbamoylase : Combines ornithine and carbamoyl phosphate to form citrulline . Located in mitochondria. Argininosuccinate synthetase : Condenses citrulline with aspartate to form arginosuccinate . This reaction occurs in the cytosol and requires one ATP. Argininosuccinate lyase : Splits argininosuccinate into arginine and fumarate. Occurs in the cytosol. Arginase: Cleaves arginine into one molecule of urea and ornithine in the cytosol. The ornithine is then transported back into the mitochondria for entry back into the cycle.

11. REACTIONS OF THE UREA CYCLE

12. Step #1; Synthesis of Carbomyl phosphate Carbamoyl phosphate is synthesized in the first reaction This is the rate-limiting step in the urea cycle. This reaction requires two ATP and occurs in the mitochondria . Enzyme: carbamoyl phosphate synthetase I, which is located in mitochondria and is activated by N- acetylglutamate . NH3 + CO2 + 2ATP → carbamoyl phosphate + 2ADP + Pi Step#2:Synthesis of Citruline Ornithine reacts with carbamoyl phosphate to form citrulline . Inorganic phosphate is released. Enzyme: ornithine transcarbamoylase , which is found in mitochondria. The product, citrulline , is transported to the cytosol in exchange for cytoplasmic ornithine. Carbamoyl phosphate + ornithine → citrulline + Pi

13. Step#3;Synthesis of Argininosuccinate The third step is catalyzed by an enzyme called argininosuccinate synthetase , which uses citrulline and ATP to form a citrullyl -AMP intermediate, which reacts with an amino group from aspartate to produce argininosuccinate Enzyme : Argininosuccinate synthetase Citrulline + ATP + aspartate → argininosuccinate + AMP + PPi Step#4;Cleavage of Argininosuccinate Argininosuccinate is cleaved to form arginine and fumarate. Enzyme: argininosuccinate lyase . This reaction occurs in the cytosol. Argininosuccinate → arginine + fumarate

14. Step#5; Cleavage of Arginine to Ornithine and Urea Arginine is cleaved to form urea and regenerate ornithine. Enzyme: arginase, which is located primarily in the liver and is inhibited by ornithine. Urea passes into the blood and is excreted by the kidneys. Arginine → urea + ornithine Fate of Ornithine; Ornithine is transported back into the mitochondrion (in exchange for citrulline ) where it can be used for another round of the cycle. When the cell requires additional ornithine, it is synthesized from glucose via glutamate.

15. Regulation of Urea Cycle: Carbamoyl phosphate synthetase I catalyzes the rate-limiting step of the cycle and is stimulated by N - acetylglutamate . Although the liver normally has a great capacity for urea synthesis, the enzymes of the urea cycle are induced if a high-protein diet is consumed for 4 days or more . Related Diseases of Urea Cycle: Hyperammonemia occurs when there is a deficiency in one of more of the urea cycle enzymes, causing insufficient removal of NH 4 + . Ammonia intoxication leads to CNS deterioration in the form of mental retardation, seizure, coma, and death.

16. OVER ALL ENERGETICS OF THE CYCLE

17. SIGNIFICANCE OF THE UREA CYCLE The main purpose of the urea cycle is to eliminate toxic ammonia from the body. About 10 to 20 g of ammonia is removed from the body of a healthy adult every day. A dysfunctional urea cycle would mean excess amount of ammonia in the body, which can lead to hyperammonemia and related diseases. The deficiency of one or more of the key enzymes catalyzing various reactions in the urea cycle can cause disorders related to the cycle. Defects in the urea cycle can cause vomiting, coma and convulsions in new born babies. This is often misdiagnosed as septicemia and treated with antibiotics in vain. Even 1mm of excess ammonia can cause severe and irreversible damages.

18. UREA CYCLE

19. REFERENCES Text book of medical biochemistry, MN Chatterjee Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins. Lehninger , A. L., Nelson, D. L., & Cox, M. M. (2000).  Lehninger principles of biochemistry. New York: Worth Publishers.