1. 1 Dr P Kumar Nitrogen Metabolism 27 th March 2012 Amino Acids: Disposal of Nitrogen & Carbon Skeletons Skeletons Protein Metabolism-I

2. 2 Learning Objectives Know the: I. Fate of Amino acid Nitrogen  A. Protein turnover and amino acid catabolism.  B. General reactions of amino acid metabolism.  C. Removal of amino acid nitrogen as ammonia.  D. Role of glutamate in the metabolism of amino acid nitrogen.  E. Role of alanine and glutamine in transporting amino acid nitrogen to the liver.  F. Role of vitamin B 6 (Pyridoxal phosphate (PLP) as a coenzyme for transamination reactions.  G. Importance of Vit B 6 supplementation for TB patients on long term treatment on Isoniazid.  H. Reactions involving ALT & AST & Significance as a “Diagnostic Markers”. Lecture outline …

3. 3 An Overview of Body’s AA Pool-Sources & Utilization

4. 4 Dietary Proteins Pepsin Partiallly Digested Proteins Neutralise pH to 7 Gastric Mucosa Unfold / Partial Denaturation of Proteins Amino acids Enzymes Absorption @ Intestinal Epithelium Acidic pH~2-2.5 (Food & Stomach) Digestion of Proteins Hepatic Portal Circulation New Proteins Amino acids Dipeptides Proteolysis (Stomach & Intestine) Amino acids (AAs) Proteases Proteogenesis Receptors Peptide Hormones Carrier / Transport Proteins HCl Pancreatic Proteases Sodium Bicarbonate Bile salt Deodenum / Intestine Trypsin Chymotrypsin Peptidase

5. 5 Digestion of Dietary Proteins by the Proteolytic Enzymes of the Gastrointestinal Tract

6. 6 Dietary Protein Turnover These building blocks are generated by the digestion of proteins in the intestine and the degradation of proteins within the cell.

7. 7 H 2 N-C-NH 2 Urea O Carbon Nitrogen Other N-Containing Compounds Dietary Proteins Summary of Amino Acid Metabolism Amino Acids in Blood Amino Acids CO 2 + H 2 O Energy ATP Proteins Digestion (Stomach Intestine) Membrane  -Aminobutyrate DopamineNorepinephrineEpinephrineSerotonin Melanin, Heme CatecholaminesCreatine-P Purines, & Pyrimidines.

8. 8 Cellar Protein Turnover  Food intake and nutritional status of animals affects protein turnover.  Supply of amino acids must meet demand. The rate of amino acid oxidation is sensitive to a surplus or deficit or to hormonal factors that regulate the amino acid pool.  Increased synthesis and decreased degradation = positive nitrogen balance.  Mediated through hormones.  Insulin ( ↑ positive nitrogen balance).  Glucagon ( ↑ negative nitrogen balance).  Epinephrine ( ↑ negative nitrogen balance).  Glucocorticoids ( ↑ negative nitrogen balance).  Growth hormone ( ↑ positive nitrogen balance).  Protein synthesis and degradation are under independent controls.

9. 9 Cellular Protein Degradation  Degradation occurs primarily by proteases.  Primarily coordinated by 2 systems.  Lysosomal enzymes (Primarily degrade extracellular proteins).  Proteosomal (non-lysosomal).  Occurs in the cytosol.  Intracellular protein.  Protein brought into cells by endocytosis.  Varies depending on the cell type and physiological status.

10. 10 Cellular Protein Degradation Many cellular proteins are constantly degraded & resynthesized. To facilitate this recycling, a complex system for the controlled turnover of proteins has evolved. Damaged or unneeded proteins are marked for destruction by the covalent attachment of chains of a small protein, Ubiquitin. Polyubiquitinated proteins are subsequently degraded by a large, ATP-dependent complex called the “Proteasome”.

11. 11 http://www.youtube.com/watch?v=4DMqnfrzpKg The Ubiquitin-Proteasome Degradation Pathway of Proteins

12. 12 Protein Ubiquitination Multiple Ubiquitins can be polymerized to each other. Enzyme 1 Enzyme 2 Enzyme 3 Enzyme-Ub Complex Ubiquitin

13. 13 26S Proteosome Ubiquitinated Proteins are Degraded by the 26S Proteosome

14. 14 Half-Life of the Amino Acids What Determines Whether A Protein Will Become Ubiquinated? Amino terminal residues determines the half-lives of proteins.

15. 15 Amino Acid Breakdown: No storage form of Amino Acids, therefore excess need to be converted to other forms to be used as energy or stored as glycogen / fat. C H2NH2NH2NH2NCH O OH R Disposal of Nitrogen Atom (Urea) Recycling of the Carbon Skeleton

16. 16 Overview of Amino Acid Catabolism: Interorgan Relationships Intestine –Dietary amino acids absorbed. –Utilizes glutamine and asparagine as energy sources. Releases CO 2, ammonium, alanine, citrulline as endproducts.Releases CO 2, ammonium, alanine, citrulline as endproducts. Utilizes glutamine during fasting for energy.Utilizes glutamine during fasting for energy. –Dietary amino acids and catabolites released to portal blood.

17. 17 Overview of Amino Acid Catabolism: Interorgan Relationships Liver –Synthesis of liver and plasma proteins. –Catabolism of amino acids. Gluconeogenesis.Gluconeogenesis. Ketogenesis.Ketogenesis. Branched chain amino acids not catabolized.Branched chain amino acids not catabolized. Urea synthesis.Urea synthesis. –Amino acids released into general circulation. Enriched (% of total AA) in BCAA (2-3X).Enriched (% of total AA) in BCAA (2-3X).

18. 18 Overview of Amino Acid Catabolism: Interorgan Relationships Skeletal Muscle –Muscle protein synthesis. –Catabolism of BCAA. Amino groups transported away as alanine and glutamine (50% of AA released).Amino groups transported away as alanine and glutamine (50% of AA released). –Alanine to liver for gluconeogenesis. –Glutamine to kidneys. Kidney –Glutamine metabolized to a-KG + NH 4 a-KG for gluconeogenesisa-KG for gluconeogenesis NH 4 excreted or used for urea cycle (arginine synthesis).NH 4 excreted or used for urea cycle (arginine synthesis). –Important buffer preventing acidosis. –[NH 4 + ] : [NH 3 ] = 100 : 1.

19. 19 Vitamin-Coenzymes in Amino Acid Metabolism Vitamin-Coenzymes in Amino Acid Metabolism –Enzymes that bind amino acids use PLP as coenzyme for binding Transaminases.Transaminases. Amino acid decarboxylases.Amino acid decarboxylases. Amino acid deaminases.Amino acid deaminases. Vitamin B 6 : Pyridoxal Phosphate (PLP). Isoniazid is an antibiotic used to prevent & treat TB. To prevent development of resistant TB bacteria, people with TB are treated with long courses of combination drug therapy, most commonly isoniazid, rifampin, and pyrazinamide. Isoniazid can interfere with the activity of vit B 6. Vitamin B 6 supplementation is recommended, especially in people with poor nutritional status, to prevent development of isoniazid-induced peripheral neuritis (inflamed nerves).

20. 20 The general reactions of amino acids are:  Deamination.  Transamination.  Trans-deamination.  Methylation.  Decarboxylation.

21. Deamination The removal of amino group as ammonia from an amino acid. (a). Oxidative Deamination. (b). Non oxidative Deamination. Amino acid Imino acid Keto acids. (a) Oxidative deamination: The liberation of free ammonia from the amino group of amino acids coupled with oxidation to form ketoacids is known as oxidative deamination. NH 3 Amino acid oxidase FAD FADH 2

22. Non oxidative deamination: - is by special enzymes; - no oxidation. Serine Pyruvate [keto acids] +NH 3 +H 2 0 Cysteine Pyruvate + NH 3 +H 2 S Serine dehydratase Desulphydrase

23. 23 Disposal of Amino Acids Nitrogen: Key Reactions 1.Transamination Reactions. - Alanine transaminase & Aspartate transaminase. - Alanine transaminase & Aspartate transaminase. 2. Deamination Reactions. –Glutamate Dehydrogenase (GDH). –Hydrolytic Deamination. Glutaminase.Glutaminase. 3. Glutamine Synthesis.

24. 24 Have a Break

25. 25 Disposal of Amino Groups: Aminotransferase Reactions Using α–KG as a Amino Group Acceptor Often the first step of amino acid degradation.Often the first step of amino acid degradation. Transfer of amino group from many amino acids to limited number of keto acid acceptors.Transfer of amino group from many amino acids to limited number of keto acid acceptors. –Pyruvate Alanine. –Oxaloacetate Aspartate. –α-KG Glutamate. (Alanine) (Pyruvate)

26. 26 Disposal of Amino Groups: Transamination Reactions Transamination reactions tend to channel amino groups on to glutamate. –Glutamate’s central role in amino acid N metabolism.

27. 27

28. 28 Disposal of Amino Groups: Transamination Reactions Transaminase reactions are reversible –ALT = SGPT (Serum Glutamate Pyruvate Transaminase). ALA important in muscle where ~25% of AA-N is transported out on ALA.ALA important in muscle where ~25% of AA-N is transported out on ALA. In liver, reverse reaction moves AA-N back on Glutamate.In liver, reverse reaction moves AA-N back on Glutamate. –AST = SGOT (Serum Glutamate Oxaloacetate Transaminase) Aspartate important in liver since half of urea-N is from Aspartate.Aspartate important in liver since half of urea-N is from Aspartate.

29. 29 Disposal of Amino Groups: Deamination Reactions Glutamate Dehydrogenase. –oxidative deamination. –Important in liver where it releases ammonia for urea synthesis. Hydrolytic Deamination. Glutaminase & Asparaginase. Conversion to a-KG by Glutamate Dehydrogenase to release NH 3.

30. 30 Disposal of Amino Groups: Glutamine Synthetase Important plasma transport form of nitrogen from muscle.Important plasma transport form of nitrogen from muscle. Detoxification of ammonia.Detoxification of ammonia. –Brain. –Liver. Removes ammonia intestinal tract:Removes ammonia intestinal tract: –Bacterial deamination of amino acids –Glutamine utilization in intestinal cells. During amino acid breakdown, the α-amino Nitrogen gets incorporated as the α-amino group in Alanine or the amide group in Glutamine. Alanine and Glutamine are then released to the circulation.

31. 31 Aminotransferase The N is then transferred from Glutamate to Pyruvate, producing Alanine. Pyruvate a-ketoglutarate Alanine and Glutamine released by peripheral tissues are taken up by the Liver. The Nitrogen on Alanine is transferred to α-KG to produce Glutamate. Vitamin B 6 (PLP)

32. 32 Aminotransferase  -ketoglutarate Glutamate PLP

33. 33 Nitrogen flow in Liver Alanine Glutamate Aspartate NH 3 Glutamine Glutamine is hydrolyzed by Glutaminase to release NH 3

34. 34 Transport of Ammonia from Peripheral Tissues to the Liver

35. 35 α-Ketoglutarate Aminotransferase PLP Flow of Nitrogen: In tissues (e.g Muscle), most amino acids transfer their α-amino group to Glutamate. + Glutamate has two fates important for disposal of waste N. 1. Conversion to α-KG by Glutamate Dehydrogenase to release NH 3. 2. As N donor in the transamination of Oxaloacetate to Aspartate

36. 36 Gluamate As N Donor In The Transamination Of Oxaloacetate To Aspartate O Glutamate α-Ketoglutarate NH 2 Oxaloacetate Aspartate Aminotransferase Vit B 6 O O HO-C-CH 2 -C-C-OH OO HO-C-CH 2 -CH-C-OH Glutamate has two fates important for disposal of waste N. 2. Conversion to a-ketoglutarate by Glutamate Dehydrogenase to release NH 3. Dehydrogenase to release NH 3. 1. As N donor in the transamination of oxaloacetate to Aspartate.

37. 37 Movement of Amino Acid Nitrogen: Post-absorptive And Fasting States From extra-hepatic tissues (muscle) to liver.From extra-hepatic tissues (muscle) to liver. –Site of gluconeogenesis and ketogenesis. –Site of urea synthesis. All amino acids present in plasma but enriched (~50%) in Alanine & Glutamine.All amino acids present in plasma but enriched (~50%) in Alanine & Glutamine.

38. 38 Production of Alanine & Glutamine in Extrahepatic Tissues Transamination of AA to form GLU.Transamination of AA to form GLU. –AA + α-KG α-Keto acid + Glutamate. Formation of ALA.Formation of ALA. –Glutamate + Pyruvate α-KG + Alanine. Formation of Glutamine.Formation of Glutamine. –Glutamate α-KG + NH 3. –NH 3 + Glutamate Glutamine.

39. 39 Metabolism of Ammonia

40. 40 Involvement of Peripheral Tissues And Liver in the Ammonia Metabolism α-KG = α-Ketoglutarate

41. 41 Aminotransferase Reaction Using α –KG as the Amino-Group Acceptor Fate of Amino Acid Nitrogen Transamination Reactions α PLP

42. 42 α-Ketoglutarate Glutamate Dehydrogenase Glutamine Synthetase Molecualr Interconversions in Handling of Ammonia NH 4 + ATP Alanine Glutamate Pyruvate Glutamine AST ADP+Pi Aspartate Oxaloacetate NADH+H + NAD NADPH+H + NADPH + ALT NH 4 + H2OH2O The major enzyme responsible for the interconversion of Glutamate into α- KG is Glutamate Dehydrogenase. ALT= Alanine aminotransferase AST= Aspartate aminotransferase.

43. 43 Fate of Amino Acid Nitrogen  Transamination is the major process for removing nitrogen from AAs.  The nitrogen is transferred as an amino group from the original AA to α –KG, forming glutamate, whereas the original AA is converted to its corresponding α –ketoacid.  For e.g. Aspartate can be trasaminated to form its corresponding α -KG, OAA.  In the process, the –NH 3 + group is transferred to α -KG, which is converted to its corresponding AA, Glutamate.  All AAs except lysine and threonine undergo transamination rxns. A. Transamination reactions

44. 44 Fate of Amino Acid Nitrogen  All AAs except lysine and threonine undergo transamination  The enzymes catalyzing these reactions are known as transaminases or aminotransferases.  For most of these reactions, α -KG and Glutamate serve as one of the α –Ketoacid-amino acid pairs.  PLP is a necessary cofactor for these reactions.  Because these reactions are readily reversible, they can be used to remove nitrogen from AAs or transfer nitrogen to α –Ketoacids to form AAs.  Thus, they are involved both in AA degradation and in AA synthesis. A. Transamination reactions…

45. 45 Evaluation of Abnormal Liver Function Tests Reactions Involving ALT & AST & Significance as a Diagnostic Markers. A LT Reference range = 5-30 U/L. Increased in liver diseases. AST Reference range = 10-30 U/L. Increased in MI.  Markers of hepatocellular damage.  Cholestasis.  Evaluation of liver synthetic function. Liver Function Tests (LFT ’ s)

46. 46 Markers of Hepatocellular Damage (Transaminases)  AST - liver, heart skeletal muscle, kidneys, brain, RBCs.  In liver 20% activity is cytosolic & 80% mitochondrial.  Clearance performed by sinusoidal cells, half-life 17hrs.  ALT – more specific to liver, v.low concentrations in kidney & skeletal muscles.  In liver totally cytosolic.  Half-life 47 hrs.

47. 47  Viral – Hepatitis A, B, C, E, CMV, EBV.  ALT levels usually peak before jaundice appears.  Jaundice occurs in 70% Hep-A, 35% acute Hep-B, 25% Hep-C. Check for exposure.  Check Hep A IgM, Hep B core IgM and HepBsAg, Hep C IgG or Hep C RNA. CME= Cytomegalovirus ; EBV=. CME= Cytomegalovirus ; EBV=Epstein-Barr virus.  Viral, Ischaemic, Toxins.  Early phase of acute obstruction.  Autoimmune: raised IgG & autoantibodies (anti-SM, Liver/Kidney Microsomal antibodies (LKM),systemic lupus antibody (SLA).  Autoimmune: raised IgG & autoantibodies (anti-SM, Liver/Kidney Microsomal antibodies (LKM), systemic lupus antibody (SLA).  Acute hepatitis – transaminase > 10x ULN.  Cholestatic & Mild rise in ALT. LDH raised 80% & ↑ ALP. Patterns of Liver Enzyme Alteration

48. 48 Acute Hepatitis (ALT>10xULN)  Toxins - paracetamol (up to 50% of all cases of Acute Liver Failure).  Ecstasy (2 nd most common cause in the young <35).  Any drug. Steroids and azathioprine.  Herbal remedies.  Alcohol – almost never, AST <7xULN in 98%.  AST/ALT ratio > 1 in 92%, >2 in 70% cases.  Progressive liver failure.

49. 49 Common Causes of Abnormal LFTs In The UK  Transient mild abnormalities which are simply impossible to explain.  Drugs – e.g. Statins.  Alcohol excess.  Hepatitis C.  Non-Alcoholic Fatty Liver Disease (NAFLD).

50. 50 Investigation of Abnormal LFTs Principles  2.5% of population have raised LFTs.  Normal LFTs do not exclude liver disease.  Interpret LFTs in clinical context.  Take a careful history for risk factors, drugs (inc OTCs), alcohol, comorbidity, autoimmunity.  Physical examination for liver disease.  If mild abnormalities and no risk factors or suggestion of serious liver disease, repeat LFTs after an interval (with lifestyle modification).

51. 51 Investigation of Abnormal LFTs ALT/AST 2-5x normal (100-200 U/L)  History & Examination.  Discontinue hepatotoxic drugs.  Continue statins but monitor LFTs monthly.  Lifestyle modification (lose wt, reduce alcohol, diabetic control).  Repeat LFTs at 1 month and 6 months.

52. 52 Investigation of Abnormal LFTs - Raised ALT / AST If still abnormal at 6 months:  Consider referral to secondary care  Hepatitis serology (B, C).  Iron studies – transferrin saturation + ferritin.  Autoantibodies & immunoglobulins.  Consider caeruloplasmin.  Alpha-1- antitrypsin.  Coeliac serology.  Thyroid function tests, lipids/glucose.  Consider liver biopsy espicially if ALT > 100 U/L).

53. Transmethylation: is the transfer of methyl groups from Methionine to acceptor molecules. The enzyme involved is “methyl transferase”. First methionine has to become activated. The activated form is called S-adenosyl methionine [SAM] Methionine + ATP S- adenosyl methionine Donates methyl group to Methylated product Methyl acceptor 1. Guanido acetic acid Creatine. 2. Norepinephrine Epinephrine. 3. Serine Choline. Methyl transferase

54. DECARBOXYLATION: Removal of carbon dioxide from the amino acid to form the corresponding amines. [Biogenic amines] Aminoacid Decarboxylase B 6 PO 4 CO 2 Histidine Histidine decarboxylase Histamine 5 OH Tryptophan A AA decarboxylase Serotonin Glutamate GABA Amine ( Biogenic amines). Glutamate decarboxylase CO 2

55. Sources Disposal Dietary protein Amino acid catabolism Glutamine synthesis Purine nucleotide metabolism Glutamate synthesis Pyrimidine nucleotide metabolism Ammonia from the intestine UREA synthesis Monoamine oxidase reactions NH 3

57. 57 1.In the transamination reaction shown below, which of the following are the products, X, Y?. A. Alanine, α-Ketoglutarate. B. Glutamate, α-Ketoglutarate. C. Aspartate, α-Ketoglutarate. √ D. Pyruvate, Aspartate. E. Pyruvate, Alanine. A.Correct Answer =C. Transminase reactions always have an amino acid and an α-keto acid as substrates. The products of the reactions are also an amino acid (corresponding to the α-keto substrate) and an α-keto acid (corresponding to the amino acid subatrate). Three amino acid α-keto acid pairs commonly encounter in metabolism are: Alanine/Pyruvate. Aspartate/Oxaloacetate. Aspartate/Oxaloacetate. Glutamate/ α-Ketoglutarate. Glutamate/ α-Ketoglutarate.Oxaloacetate Glutamate X Y

58. 58 6. Which of the following is the common nitrogen acceptor for all reactions involving transaminase?. A. α-ketoglutarate.√ B. α-ketobutyrate. C. Pyruvate. D. Oxaloacetate. E. Acetoacetate. Answer: A. All transaminases have α-ketoglutarate as the common nitrogen acceptor. Oxaloacetate (OAA) is a common nitrogen donor. Serine is deaminated to pyruvate, and threonine is deaminated to α-ketobutyrate. Other amino acids are converted to acetoacetate.

59. 59 16. During a medical rotation, a medical student volunteered for a respiratory physiology exam that determines basal metabolic rate and the respiratory quotient. She followed the protocol for a resting Individual in the post-absorptive state. Which of the following amino acids would be found in the highest concentration in serum?. A.Alanine and glutamine.√ B.Orginine and ornithine. C.Glutamate and aspartate. D.Branched chain amino acid. E.Hydrophobic amino acids. Answer: A. Because the postabsorptive state (after a meal), is being referred to, we are referring to amino acid synthesis. essential amino acids that are not synthesized are not being referred to, therefore, choices B, D, and E cannot be correct. Regarding Choice C (glutamate and aspartate), although non-essential, these amino acids are the key elements involved in transamination, mediating transfer of nitrogen groups from one amino acid to another amino acid, so they are never in high concentrations themselves, but in flux. high concentrations themselves, but in flux.

60. 60 The proteins which marks proteins for degradation is _______. A.Chaperonin B.Ubiquitin.√ C.Proteasomin. D.Apoptosin. E.Calmodulin. Ans: B Ans: B.

61. 61 Ubiquitin is a 76-amino acid cytosolic peptide and is responsible for: A.degradation of many cytosolic proteins only. B.degradation of many nuclear proteins only. C.both of them.√ D.none of them. E.Degradation of nucleic acids. Ans: C Ans: C.

62. 62 Ubiquitinated proteins are recognized by the ________, which unfolds and transports the proteins to its proteolytic core. A.Nucleosome. B.Polysome. C.Proteasome cap.√ D.Ribozyme. E.None of them. Ans: C Ans: C.

63. What part of the 26S proteosome complex contains the protease function?. A. 26S proteosome core. B. 19S cap. C. E3 subunit. D. E2 subunit. E. All of the above. Ans: A. ?

64. 64

65. Which of the following enzymes function in the biological assimilation of ammonia?. A. glutamate dehydrogenase. B. glutamine synthetase. C. glutamate synthase. D. all of the above. Ans: D. ?

66. Which of the following protein modifications target a protein for degradation?. A. Phosphorylation. B. Ubiquitination. C. Zymogen activation. D. Subunit aggregation. E. All of the above. Ans: B. ?

67. Specific amino acids associated with the ______ of a protein determine its half life. A. C-terminal. B. N-terminal. C. All of the above. Ans: B. (N-terminal / amino terminal). ?

68. Ubiquitin is covalently attached to a protein through an amide linkage to which specific amino acid?. A. Aspartate. B. Glutamate. C. Lysine. D. Arginine. E. Histidine. Ans: C. ?

69. What part of the 26S proteosome complex recognizes an ubiquitinated protein?. A. 20S proteosome core. B. 19S cap. C. E3 subunit. D. E2 subunit. E. All of the above. Ans: B. ?

70. In a patient with cystic fibrosis, the mutant cytosolic transmembrane conductance regulator (CFTR) protein folds incorrectly. The patients cells modify this abnormal proteins by attaching ubiquitin molecules to it. What is the fate of this modified CFTR protein?. A.It performs its normal function, as the ubiquitin largely corrects for the effect of the mutation. B. It is secreted from the cell. C. It is placed into storage vesicles. D. It is degraded by the proteasome. E. It is repaired by cellular enzymes. Ans: D. Ubiquitination usually marks old, damaged, or misfolded proteins for destruction by proteasome. There is no known cellular mechanism for repair of damaged proteins. ?

71. A young boy has cystic fibrosis, which is due to a mutation resulting in misfolded protein. The protein is degraded very quickly inside the cell. This protein degradation is most probably undertaken by: A. lipoic acid. B. acetyl CoA. C. ubiquitin. E. linoleoic acid. E. linolenic acid. Ans: C. ?

72. After proteins are synthesized, their lifespan is regulated by proteolytic degradation. Some proteins are degraded by lysosomal enzymes. The process of _________ allows subcellular material, including organelles, to be enclosed by a membrane and subjected to lysosomal action. A. autophagy. B. phylogeny. C. phagocytosis. D. pinocytosis. E. exocytosis. Ans: C. ?

73. What part of the 26S proteosome complex contains the protease function?. A. 26S proteosome core. B. 19S cap. C. E3 subunit. D. E2 subunit. E. All of the above. Ans: A. ?

74. What is the fate of excess amino acids in animals and humans?. What is the fate of excess amino acids in animals and humans?. A. they are recycled for the synthesis of new proteins. B. they are stored for future use. C. they are catabolized to free ammonia and carbon skeletons. D. a and b. E. a and c. Ans: C. ?

75. Fish dispose of toxic ammonia by ___________. A. converting it to uric acid, which is subsequently secreted. B. converting it to urea, which is subsequently secreted. C. releasing it directly to the environment. D. all of the above. E. none of the above. Ans: C. ?

76. The enzyme of the urea cycle are localized within __________. A. the mitochondria.. B. the cytosol. C. the chloroplast. D. a and b. E. all of the above. Ans: D. ?

77. Which amino acid participates as a pathway intermediate in the urea cycle?. A. Lysine. B. Arginine. C. Glutamine. D. Histidine. E. Tyrosine. Ans: B. ?

78. In humans, the urea cycle functions primarily in the ___________. A. muscle tissue. B. heart. C. brain. D. liver. E. adipose tissue. Ans: D. ?

79. 79 Proteins destined for __________ is tagged with the molecules of ubiquitin. A.Degradation.√ B.Ribosylation. C.Deamination. D.Tranamination. E.Sulphuration. Ans: A Ans: A.

80. 80 Transamination reactions are catalyzed by a class of Enzymes, all of which require _______ as a coenzyme. A.pyridoxal phosphate.√ B.biotin. C.ascorbic acid. D.lipoic acid. E.Thiamine pyroposphate. Ans: A Ans: A.

81. The Reaction Catalyzed by Glutamate Dehydrogenase which reversibly converts glutamate to α-ketoglutarate require the cofactor, A.ATP B.NAD C.NAD(P)+ /NAD(P)H. D.Biotin E.Pyridoxal phosphate. Ans: C. The cofactors require by the glutamate dehydrogenase is NAD(P)+ /NAD(P)H.

82. 11. A 55-year-old man suffers from cirrhosis of the liver. Toxins such as ammonia are not properly metabolized by the liver and can now damage structures such as the brain. Which of the following amino acids covalently binds ammonia and transports and stores It in a nontoxic form?. A.Aspartate. B.Glutamate. C.Serine. D.Cysteine. E.Histidine. Answer: B. Glutamate and ammonia from glutamine, as catalyzed by glutamine synthetase at the cost of 1 ATP. Cysteine is not directly involved with ammonia. Aspartate and ammonia both donate a nitogen to form urea (along with ammonia). Histidine and serine are deaminated, forming ammonia. Central nervous system dysfunction due to high ammonia-level hepatic encephalopathy results in a sequelae of symptoms such as asterixis, confusion and coma.

83. 6. Which of the following is the common nitrogen acceptor for all reactions involving transaminase?. A. α-ketoglutarate. B. α-ketobutyrate. C. Pyruvate. D. Oxaloacetate. E. Acetoacetate. Answer: A. All transaminases have α-ketoglutarate as the common nitrogen acceptor. Oxaloacetate (OAA) is a common nitrogen donor. Serine is deaminated to pyruvate, and threonine is deaminated to α-ketobutyrate. Other amino acids are converted to acetoacetate.