AA 2

• What keeps amino acids from oxidative breakdown?

  The presence of α-amino group
• Why is removing the α-amino group essential?

  It is necessary for energy production from amino acids
• What can nitrogen be incorporated into after removal?

  Other compounds or excreted as urea
• What are the stages of amino acid catabolism?

  1. Removal of α-amino group ➔ Ammonia (NH3)
  2. Catabolism of remaining carbon skeleton ➔ Energy
• What is transamination?

  Transfer of an amino group between amino acids
• What role do transaminases play in transamination?

  They catalyze transamination reactions
• What cofactor is used in transamination reactions?

  Pyridoxal phosphate (PLP)
• Are transamination reactions reversible?

  Yes, they are readily reversible
• Which amino acid is not transaminated?

  Lysine
• What are the common amino acid/α-keto acid pairs in transamination?

  Glutamate and α-ketoglutarate
• What are the two important aminotransferases?

  Alanine aminotransferase (ALT) and aspartate aminotransferase (AST)
• What do elevated plasma levels of aminotransferases indicate?

  Damage to cells rich in these enzymes
• How does ALT specificity compare to AST for liver disease?

  ALT is more specific than AST for liver disease
• What does the AST/ALT ratio indicate?

  It is a biomarker for liver health
• What do oxidative deamination reactions liberate?

  The amino group as free ammonia
• Where do oxidative deamination reactions primarily occur?

  In the liver
• What form does ammonia primarily exist in aqueous solution?

  As ammonium (NH4+)
• What enzyme catalyzes oxidative deamination?

  Glutamate dehydrogenase (GDH)
• What coenzymes can GDH use for oxidative deamination?

  NAD+ or NADPH
• Which amino acid is deaminated by histidase?

  Histidine
• What do serine and threonine dehydratases require?

  Pyridoxal phosphate (PLP)
• What do glutaminase and asparaginase convert?

  Glutamine to glutamate and NH4+, asparagine to aspartate and NH4+
• What cycle allows NH4+ release in muscle and brain?

  The purine nucleotide cycle
• How is ammonia transported to the liver?

  In the form of alanine and glutamine
• What is the normal concentration of ammonia in blood?

  Less than 50 µmol/L
• What happens when blood ammonia levels increase to 100 µmol/L?

  Disturbance of consciousness occurs
• What is ammonia intoxication?

  Toxicity of the brain due to increased ammonia
• How does increased ammonia affect the citric acid cycle?

  It interferes with ATP production in brain cells
• What are the two mechanisms for ammonia transport to the liver?

  1. Nitrogen transport by glutamine
  2. The glucose-alanine cycle
• What is the rate-limiting step in urea synthesis?

  Carbamoyl phosphate formation
• What enzyme synthesizes carbamoyl phosphate?

  Carbamoyl phosphate synthetase I (CPS I)
• What is formed when carbamoyl phosphate is transferred to ornithine?

  Citrulline
• What does argininosuccinate synthetase combine to form argininosuccinate?

  Citrulline and aspartate
• What drives the formation of argininosuccinate?

  Cleavage of ATP
• What does argininosuccinate lyase yield?

  Arginine and fumarate
• What happens to fumarate after argininosuccinate cleavage?

  It is hydrated to malate
• How is arginine cleaved to regenerate ornithine?

  By arginase hydrolyzing arginine
• Where is arginase primarily located?

  In the liver
• What activates carbamoyl phosphate synthetase I?

  N-Acetylglutamate (NAG)
• What regulates the urea cycle besides NAG?

  Substrate availability and enzyme induction