AMINO ACID METABOLISM

• Amino acids (AAs) are precursors for proteins.

• Precursors for many other biological N-containing compounds.

• Energy metabolites: When degraded, amino acids produce glucose, carbohydrates and ketone bodies.

• Excess dietary AAs are neither stored nor excreted. Rather, they are converted to common metabolic intermediates.

Fate of Amino Group

1. Ureotelic: urea for excretion most terrestrial vertebrates

2. Uricotelic: uric acid for excretion birds, reptiles

3. Ammonotelic: NH₄⁺ for excretion aquatic animals

Fate of Carbon Skeletons

Converted into 7 common metabolites:

• pyruvate; • acetyl-CoA; • acetoacetate; • a-ketoglutarate;

• succinyl-CoA; • fumarate; • oxaloacetate

FATE OF AMINO GROUP

I. DEAMINATION

A. Transamination by Aminotransferase (or Transaminase)

• Funnel a-amino groups from a variety of AAs to glutamate by reacting with a-ketoglutarate.

amino acid + a-ketoglutarate ⇌ a-keto acid + glutamate

· Does not result in any net deamination.

B. Oxidative Deamination

1. Glutamate Dehydrogenase (in mitochondria)

· See p.692

· Glu + NAD⁺ (or NADP⁺) + H₂O ⇌ NH₄⁺ + a-ketoglutarate + NAD(P)H +H⁺

· An enzyme unusual (but not the only one as stated in the Textbook) in being able to use NAD⁺ and NADP⁺.

· Plays a central role in AA metabolism. In most organisms glutamate is the only AA which has such an oxidative deamination enzyme.

· Glutamate DH is allosterically regulated. It is inhibited by GTP and ATP, and activated by GDP and ADP.

· The NH₄⁺ so obtained can feed into urea cycle.

2. L-Amino Acid Oxidase

· Requires FAD as a cofactor.

· D-Amino acid oxidase also exists in mammalian tissues. Real physiological function unknown.

C. Direct Deamination of Serine and Histidine

1. Serine Dehydratase

· Fig. 20-15.

· PLP-dependent

· serine + H₂O ® pyruvate + NH₄⁺

2. Histidine Ammonia Lyase

· Fig. 20-17, Reaction 8.

· histidine ® urocanate + NH₄⁺

UREA CYCLE

· 1932 by Hans Krebs and Kurt Henseleit as the first metabolic cycle elucidated. See Fig. 20-9.

· Overall Reaction:

· NH₃ + HCO₃^– + aspartate + 3 ATP + H₂O ® urea + fumarate + 2 ADP + 2 Pi + AMP + PPi

· Requires 5 enzymes: 2 from mitochondria and 3 from cytosol.

1. Carbamoyl phosphate synthetase (Mitochondrial)

· Eukaryotes have two forms of CPS, the mitochondrial CPS I uses ammonia as the N donor for urea synthesis. The cytosolic CPS II uses glutamine as its N donor for pyrimidine biosynthesis.

· 2 ATP + HCO₃^– + NH₃ ® carbamoyl phosphate + 2 ADP + Pi

2. Ornithine transcarbamoylase (Mitochondrial)

· carbamoyl phosphate + ornithine ® citrulline

Antiport: (cytosolic ornithine ® mitochondria) coupled to (mitochondrial citrulline ® cytosol).

3. Argininosuccinate synthetase (Cytosolic)

· citrulline + aspartate + ATP ® argininosuccinate + AMP + PPi

4. Argininosuccinase (Cytosolic)

· argininosuccinate ® fumarate + arginine

· The skeleton of Asp is recovered in fumarate. Up to this point, the reactions are the same for all organisms that are capable of synthesizing arginine.

5. Arginase (Cytosolic)

· Only the ureotelic animals have large amounts of the arginase.

· arginine + H₂O ® urea + ornithine

REGULATION OF UREA CYCLE

1. Mitochondrial carbamoyl phosphate synthetase I (CPS I)

· CPS I catalyzes the first committed step of the urea cycle.

· CPS I is also an allosteric enzyme sensitive to activation by N-acetylglutamate which is derived from glutamate and acetyl-CoA.

· Increased rate of AA degradation requires higher rate of urea synthesis.

·  AA degradation ® ↑glutamate concentration → ↑synthesis of N-acetylglutamate ® ↑CPS I activity ® ↑urea cycle efficiency

2. All other urea cycle enzymes are controlled by the concentrations of their substrates.

· Deficiency in an E ® ↑(substrate) ® ↑rate of the deficient E.