Lecture 24

Transaminases and Glutamate Dehydrogenase

• The tests typically measure two transaminases: sGOT (serum glutamate-oxaloacetate transaminase, also called aspartate aminotransferase, AST) and sGPT (serum glutamate-pyruvate transaminase, also called alanine aminotransferase, ALT).
• In the liver, glutamate is transported from the cytosol to mitochondria and is deaminated by oxidative deamination within the mitochondrial matrix, forming α-ketoglutarate.
• Glutamate dehydrogenase (GDH) can use either NAD+ or NADP+ as an electron acceptor.
• Ammonia is processed into urea for excretion.
• The pathway for ammonia excretion is a transdeamination = transamination (transaminases) + oxidative deamination (GDH).

Glutamate Dehydrogenase and Carbon-Nitrogen Metabolism

• Glutamate dehydrogenase operates at an important intersection in carbon and nitrogen metabolism.
• α-ketoglutarate made by the GDH reaction can be oxidized as fuel (in the citric acid cycle) or serve as a glucose precursor in gluconeogenesis (through oxaloacetate).
• Glutamate dehydrogenase is allosterically:
  • Activated by ADP (at low glucose or energy levels, increasing amino acid degradation and using carbon skeletons for energy)
  • Inhibited by GTP (at high levels of α-ketoglutarate)

Ammonia Metabolism in Muscle

• In the muscle, ammonia collected in glutamate is donated to pyruvate to make alanine.
• Vigorously working muscles rely on glycolysis for energy, producing pyruvate.
• To avoid lactic acid build-up, pyruvate can be converted to alanine (by taking up the amino group from glutamate) for transport into the liver.
• Once in the liver, alanine releases its amino group, leading to pyruvate, which is used to produce glucose.

Amino Acid Oxidation and Urea Production

• Amino acid oxidation occurs when:
  • Amino acids released during protein turnover are not needed for new protein synthesis
  • Ingested amino acids exceed the body's needs for protein synthesis
  • Cellular proteins are used as fuel because carbohydrates are either unavailable (starvation) or not properly utilized (disease, e.g., diabetes)
• Dietary proteins are enzymatically hydrolyzed into amino acids.
• The amino groups and the carbon skeleton take separate but interconnected pathways:
  • The amino group enters the urea cycle and nitrogen is excreted in the form of urea.
  • The carbon skeleton enters the citric acid cycle to be oxidized.

Metabolic Fates of Amino Groups

• Ammonia is toxic, and urea is far less toxic and has very high solubility.
• The amino group is transferred to α-ketoglutarate, forming L-glutamate, catalyzed by aminotransferases or transaminases, which use the pyridoxal phosphate (PLP) cofactor.
• Glutamate acts as a temporary storage of nitrogen (in its amino group) and can donate the amino group to form urea for amino acid biosynthesis when needed.

Urea Cycle and Nitrogen Excretion

• The urea cycle produces urea from ammonia in five enzymatic steps, requiring 4 molecules of ATP.
• The energy cost is made up by converting fumarate to malate (see citric acid cycle) and the GDH reaction.
• The aspartate-argininosuccinate shunt (linking the citric acid and urea cycles) effectively links the fates of the amino groups and the carbon skeletons of amino acids.
• The interconnections between the urea and citric acid cycles are even more elaborate than the arrows suggest.