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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 -amino group
• Carbon chains are altered for entry into central pathways of carbon metabolism
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
Fig 17.25
• Synthesis of carbamoyl phosphate (removal of NH3)
• Catalyzed by carbamoyl phosphate synthetase I (CPS I)
Fig 17.25 (cont)
(from previous slide)
Carbamoyl phosphate synthetase is allosterically activated by N-
acetylglutamate
KEY REGULATION STEP in the pathway!
A. The Reactions of the Urea Cycle
• Urea cycle (Fig 17.26 next two slides) Rxn 1 (mitochondria), Rxns 2,3,4 (cytosol)
• Two transport proteins are required: Citrulline-ornithine exchangerGlutamate-aspartate exchanger
• Overall reaction for urea synthesis is:
NH3 + HCO3- + Aspartate + 3 ATP
Urea + Fumarate + 2 ADP + 2 Pi + AMP + PPi
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
Fig 17.27 (a)
• Balancing the supply of nitrogen for the urea cycle
Fig 17.27 (b)
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
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