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17.8 Amino Acid Catabolism

17.8 Amino Acid Catabolism. Amino acids from degraded proteins or from diet can be used for the biosynthesis of new proteins During starvation proteins are degraded to amino acids to support glucose formation First step is often removal of the a -amino group

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17.8 Amino Acid Catabolism

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  1. 17.8 Amino Acid Catabolism • Amino acids from degraded proteins or from diet can be used for the biosynthesis of new proteins • During starvation proteins are degraded to amino acids to support glucose formation • First step is often removal of the a-amino group • Carbon chains are altered for entry into central pathways of carbon metabolism

  2. 17.9 The Urea Cycle Converts Ammonia into Urea • Waste nitrogen must be removed (ammonia is toxic to plants and animals) • Terrestrial vertebrates synthesize urea (excreted by the kidneys) • Birds, reptiles synthesize uric acid

  3. Fig 17.25 • Synthesis of carbamoyl phosphate (removal of NH3) • Catalyzed by carbamoyl phosphate synthetase I (CPS I)

  4. Fig 17.25 (cont) (from previous slide)

  5. Carbamoyl phosphate synthetase is allosterically activated by N-acetylglutamate KEY REGULATION STEP in the pathway!

  6. Urea cycle (Fig 17.26 next two slides) Rxn 1 (mitochondria), Rxns 2,3,4 (cytosol) • Two transport proteins are required: Citrulline-ornithine exchanger Glutamate-aspartate exchanger • Overall reaction for urea synthesis is: • NH3 + HCO3- + Aspartate + 3 ATP • Urea + Fumarate + 2 ADP + 2 Pi + AMP + PPi A. The Reactions of the Urea Cycle

  7. B. Ancillary Reactions of the Urea Cycle • Supply of nitrogen for the urea cycle can be balanced by supply of NH3 and amino acids • Glutamate dehydrogenase and aspartate transaminase catalyze near equilibrium reactions • Flux through these enzymes depends upon relative amounts of ammonia and amino acids • Two cases (next slides): (a) NH3 in excess, (b) aspartate in excess

  8. Fig 17.27 (a) • Balancing the supply of nitrogen for the urea cycle

  9. Fig 17.27 (b)

  10. 17.10 Catabolism of the Carbon Chains of Amino Acids • After removal of amino groups, carbon chains of the 20 amino acids can be degraded • Degradation products: • Citric acid cycle intermediates • Pyruvate • Acetyl CoA or acetoacetate

  11. Glucogenic vs ketogenic amino acids • Glucogenic amino acids can supply gluconeogenesis pathway via pyruvate or citric acid cycle intermediates • Ketogenic amino acids can contribute to synthesis of fatty acids or ketone bodies • Some amino acids are both glucogenic and ketogenic

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