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