Amino Acid Oxidation and the Urea Cycle

Transcription

1 Amino Acid Oxidation and the Urea Cycle

2 Amino Acids: Final class of biomolecules whose oxidation contributes significantly to the generation of energy Undergo oxidation in three metabolic circumstances in animals: During normal synthesis and degradation of proteins When a diet is rich in protein; amino acids are not stored During starvation or in diabetes mellitus

3 Overview of Amino Acid Catabolism: Dietary protein Intracellular protein Amino Acids Glucose TCA Cycle α-keto acid NH 4 Biosynthesis of amino acids nucleotides, biological amines Respiration Excretion

4 Metabolic Fates of Amino Groups Most amino acids are metabolized in the liver Some of the ammonia generated is used in biosynthesis; the excess is excreted Excess ammonia generated in extrahepatic tissues is transported to liver for conversion to the appropriate excreted form

5 Removal of the α-amino groups occurs in the cytosol by transamination reactions catalyzed by aminotransferases (transaminases): α-keto acid 2 amino acid 1 α-keto acid 1 amino acid 2 In liver α-keto acid 1 is usually α-kg; in muscle it is usually pyruvate

6 C=O α-kg H 3 N C H R amino acid NH C-H 3 C=O CH R 2 COO α-keto - glutamate acid C=O α-kg H 3 N C H CH3 3 alanine GPT alanine aminotransferase NH 3 C-H glutamate C=O CH 3 pyruvate

7 C=O H 3 N C H CH2 2 NH 3 C-H C=O α-kg aspartate glutamate OAA Aspartate aminotransferase or Glutamate-OAA transaminase (GOT)

8 Serum GPT (SGPT) and GOT (SGOT) are sensitive indicators for a number of disease conditions. During heart attacks, damaged heart cells leak aminotransferases. Damaged liver cells also leak aminotransferases. SGPT and SGOT levels are monitored in people exposed to industrial chemicals.

9 The effect of transamination is to collect amino groups from many amino acids and convert them into one, glutamate Glutamate channels amino groups into biosynthetic pathways or into reactions where nitrogenous waste products are formed

10 How are amino groups removed from glutamate and prepared for excretion? Glutamate is transported into the mitochondrial matrix where it undergoes oxidative deamination catalyzed by glutamate d hase: Glutamate glutamate d hase α-kg NH 4 NAD(P) NAD(P)H

11 Ammonia is extremely toxic to animal tissues; it is converted to glutamine for transport from extrahepatic tissues to the liver or kidneys. NH 3 C-H glutamine synthetase NH 3 C-H glutamate NH 4 H 2 N C=O glutamine

12 In the liver glutamine is converted back to glutamate by glutaminase: NH 3 C-H glutaminase NH 3 C-H H 2 N C=O glutamine H 2 O NH 4 glutamate UREA

13 Glutamine is the major transport form of ammonia; it is present in blood in much higher concentrations than other amino acids. Alanine also plays a role in transport of amino groups to the liver by the glucose-alanine cycle: Muscle glucose Liver AA glutamate pyr glutamate glutamate pyr alanine α Kg α Kg alanine

14 Habitat determines the Molecular Pathway for Nitrogen Excretion Aquatic organisms (bacteria, protozoa, fish) release ammonia to their aqueous enviroment (ammonotelic) Birds and reptiles convert amino nitrogen into uric acid; they cannot carry enough water for the excretion of nitrogen as urea (uricotelic) Terrestrial animals excrete amino nitrogen in the form of urea (ureotelic) Plants recycle virtually all amino groups; there is no general pathway for nitrogen excretion

15 The Urea Cycle Discovered by Hans Krebs and Kurt Hanseleit Occurs in the liver Takes place in two intracellular compartments; the cytosol and the mitochondrial matrix

16 - NH 4 HCO 3 NH 4 2 ATP carbamoyl phosphate synthetase I 2 ADP, Pi O O H 2 N-C-O-P-O - O - carbamoyl phosphate ornithine transcarbamoylase O H 2 N-C-NH-( ) 3 -CH- Citrulline Pi NH 3 NH 3 NH 3 -( ) 3 -CH- Ornithine Transported to cytosol

17 O NH 3 H 2 N-C-NH-( ) 3 -CH- Citrulline Aspartate ATP PPi arginosuccinate synthetase NH 2 NH 3 - OOC-CH -C-NH-( ) 3 -CH- 2 -CH-NH Arginosuccinate

18 NH 2 NH 3 - OOC-CH -C-NH-( ) 3 -CH- 2 -CH-NH Arginosuccinate arginosuccinate lyase - OOC-CH=CH- Fumarate NH 2 NH 3 NH -C-NH-( ) 3 -CH- 2 Arginine

19 NH 2 NH 3 NH -C-NH-( ) 3 -CH- 2 Arginine arginase NH 3 NH 3 -( ) 3 -CH- Ornithine O H 2 N-C-NH 2 Back to mitochondrion UREA

20 Overall equation for Urea synthesis: - NH 3 HCO 3 Aspartate 3 ATP urea fumarate 2ADP 2Pi AMP PPi

21 Regulation of the Urea Cycle: Carbamoyl Phosphate Synthetase I is allosterically activated by N-acetylglutamate. High levels of transamination during amino acid breakdown lead to elevated glutamate with concommitant increases in the concentration of N-acetylglutamate.

22 Breakdown of Individual Amino Acids Degradation of the carbon skeletons of the 20 common amino acids yields one of 7 intermediates: a-kg, succinyl CoA, pyruvate, fumarate, OAA, acetoacetate, acetyl CoA

23 α-kg, succinyl CoA, pyruvate, fumarate, OAA can all serve as precursors for glucose synthesis; hence amino acids giving rise to these intermediates are Glucogenic. Acetoacetate and acetyl CoA can serve as precursors for fatty acid or ketone synthesis; hence amino acids giving rise to these compounds are termed Ketogenic

24 Alanine, Serine, Glycine, Cysteine, Threonine, Tryptophan Pyruvate Isoleucine, Leucine, Threonine, Tryptophan Acetyl CoA Leucine, Lysine, Phenylalanine, Tyrosine Acetoacetate

25 Arginine, Glutamine, Glutamate, Proline, Histidine α Kg Isoleucine, Methionine, Valine Succinyl CoA Aspartate, Tyrosine, Phenylalanine Fumarate Aspartate, Asparagine OAA

26 H HS-CH C- 2 NH 2 Cysteine Alanine CH 3 H CH C- 2 NH NH 2 Tryptophan H C- NH 2 Threonine Glycine H H H 3 C-C-C- HO NH 2 H H C- NH 2 O CH 3 -C- Pyruvate H HOCH C- 2 NH 2 Serine

27 Alanine is converted to pyruvate by transamination. Asparagine is converted to Aspartate by Asparaginase: O H O H C-CH C- 2 C-CH C- 2 NH 2 NH -O 2 NH 2 Aspartate can be converted to OAA by transamination: Aspartate α-kg Glutamate OAA Aspartate degradation via the urea cycle yields fumarate.

28 Methionine, Valine, Isoleucine O CH 3 - -C-S-CoA Propionyl CoA O -OOC- - -C-S-CoA Succinyl CoA

29 Arginine, Glutamine, Histidine, Proline Glutamate Glutamate D hase α Kg

30 Leucine and Lysine are the only two purely ketogenic amino acids. HMG-CoA is an intermediate in leucine degradation. Initial steps in valine, leucine, isoleucine degradation are identical: transamination to the corresponding a-keto acid decarboxylation to the corresponding CoA derivative dehydrogenation to form a double bond Defect in the decarboxylation reaction results in Maple Syrup Urine Disease, which is fatal unless treated early in life.

31 Phenylalanine is converted to Tyrosine by Phenylalanine Hydroxylase; then the pathway proceeds with the breakdown of tyrosine to fumarate plus acetoacetate. Homogentisate is an intermediate in this pathway. Alkaptonuria results in the urinary excretion of excess homogentisate; air oxidation causes this compound to turn dark. This disease is not fatal, individuals tend to suffer arthritis in later life. -OOC-H 2 C OH OH homogentisate

32 Phenylketonuria results from absence of phenylalanine hydroxylase. Phe is converted to phenylpyruvate and excreted. Severe mental retardation occurs unless infants are immediately placed on a diet low in Phe. C=O COO- phenylpyruvate

33 Condition Human Genetic Disorders of Amino Acid Catabolism Incidence (per 100,000 births) Defective Enzyme Symptoms Alkaptonuria 0.4 Homogentisate Dioxygenase Dark pigment in urine; arthritis in late life Maple Syrup Urine 0.4 Branched chain α-keto Mental retardation; Disease acid dedydrogenase convulsions; early death Phenylketonuria 8 phenylalanine Neonatal vomiting; hydroxylase mental retardation Methylmalonic Acidemia (MMA) (<0.5) Methylmalonyl CoA Mutase Mental retardation; convulsions; early death

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