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Amino-Acid Catabolism: Transamination and Ammonia

Tier 2 · #14 of 33 · high yield · topic 4.1 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    What obligate coenzyme is required by ALL aminotransferase (transaminase) reactions?
    Back
    Pyridoxal phosphate (PLP)
  2. Front
    Pyridoxal phosphate (PLP) , the coenzyme for all transaminations , is the active form of which vitamin?
    Back
    Vitamin B6
  3. Front
    In transamination , what molecule is the near-universal acceptor of the amino group, becoming glutamate ?
    Back
    α-ketoglutarate
  4. Front
    Transamination funnels nitrogen from nearly all amino acids into which single hub amino acid?
    Back
    Glutamate
  5. Front
    Which two amino acids are the exceptions that do NOT undergo classical transamination ?
    Back
    Lysine and threonine
  6. Front
    The two linked reactions of nitrogen removal — transamination followed by oxidative deamination — together are called ______.
    Back
    Transdeamination
  7. Front
    Which mitochondrial enzyme oxidatively deaminates glutamate , releasing free NH4+ and regenerating α-ketoglutarate ?
    Back
    Glutamate dehydrogenase (GDH)
  8. Front
    What is the primary source of free ammonium ions (NH4+) in the liver ?
    Back
    Glutamate dehydrogenase (GDH) deaminating glutamate
  9. Front
    Glutamate dehydrogenase (GDH) is allosterically ACTIVATED by which nucleotide?
    Back
    ADP
  10. Front
    Glutamate dehydrogenase (GDH) is allosterically INHIBITED by which nucleotide?
    Back
    GTP
  11. Front
    Physiologically, why does ADP activate and GTP inhibit glutamate dehydrogenase (GDH) ?
    Back
    Low energy (high ADP ) turns on amino acid catabolism ; high energy (high GTP ) suppresses it
  12. Front
    Which transaminase is considered MORE SPECIFIC for liver damage ?
    Back
    ALT (alanine aminotransferase)
  13. Front
    Which transaminase is MORE SENSITIVE for liver injury due to its very high activity inside hepatocytes ?
    Back
    AST (aspartate aminotransferase)
  14. Front
    AST transfers an amino group from glutamate to which acceptor, producing the urea-cycle nitrogen donor aspartate ?
    Back
    Oxaloacetate
  15. Front
    What does an elevated serum level of intracellular enzymes ALT and AST signal?
    Back
    Cell damage ( hepatocyte necrosis )
  16. Front
    What is the most abundant amino acid in plasma, reflecting its role as the primary nitrogen shuttle ?
    Back
    Glutamine
  17. Front
    Which enzyme adds free NH4+ to glutamate (using 1 ATP) to make non-toxic glutamine ?
    Back
    Glutamine synthetase
  18. Front
    In the CNS , which cells use glutamine synthetase as the major mechanism to remove ammonia and protect neurons?
    Back
    Astrocytes
  19. Front
    Which mitochondrial enzyme hydrolyzes glutamine back to glutamate + NH4+ in the liver ?
    Back
    Glutaminase
  20. Front
    Hepatic glutaminase activity increases in response to what dietary change?
    Back
    A high-protein diet
  21. Front
    Renal glutaminase activity increases in which acid-base disturbance, helping buffer urinary pH?
    Back
    Metabolic acidosis
  22. Front
    In the kidney , what happens to the NH4+ released by glutaminase (unlike in the liver)?
    Back
    It is excreted directly into the urine
  23. Front
    Which amino acid is the specific nitrogen carrier exported from MUSCLE to the liver ?
    Back
    Alanine
  24. Front
    In muscle , the amino group is collected onto which molecule (via ALT in reverse) to form alanine for export?
    Back
    Pyruvate
  25. Front
    What is the name of the loop that ferries nitrogen from muscle to liver while recycling carbon back as glucose?
    Back
    Glucose-alanine cycle
  26. Front
    In the glucose-alanine cycle , what is the fate of the pyruvate regenerated in the liver ?
    Back
    It enters gluconeogenesis to make glucose
  27. Front
    Besides carrying nitrogen, what additional resource does the glucose-alanine cycle deliver to the liver that the glutamine shuttle does not?
    Back
    Pyruvate carbon ( gluconeogenic substrate )
  28. Front
    Why is the liver 's first-pass handling of portal blood essential for ammonia? (Consider gut glutaminase .)
    Back
    Intestinal enterocytes release NH4+ into the portal circulation, which the liver must clear before it reaches systemic blood
  29. Front
    Which liver enzyme commits NH4+ to urea synthesis by condensing HCO3- , NH4+ , and 2 ATP into carbamoyl phosphate ?
    Back
    Carbamoyl phosphate synthetase I (CPS-I)
  30. Front
    CPS-I is allosterically activated by which molecule, without which it is essentially inactive?
    Back
    N-acetylglutamate
  31. Front
    Synthesis of the CPS-I activator N-acetylglutamate is stimulated by which amino acid (rising after a protein meal)?
    Back
    Arginine
  32. Front
    The two nitrogen atoms of urea arrive as NH4+ (via carbamoyl phosphate ) and as which amino acid?
    Back
    Aspartate
  33. Front
    How many ATP are consumed per molecule of urea produced in the urea cycle?
    Back
    4 ATP
  34. Front
    What is the most common inherited urea-cycle defect ?
    Back
    Ornithine transcarbamoylase (OTC) deficiency
  35. Front
    What is the normal plasma ammonium concentration range (in µg/dL)?
    Back
    10–20 µg/dL (≈5–35 µM)
  36. Front
    How does excess ammonia impair neuronal energy production via the Krebs cycle?
    Back
    It drives GDH /glutamine synthesis in reverse, depleting α-ketoglutarate from the Krebs cycle
  37. Front
    Accumulation of which amino acid inside astrocytes causes the osmotic swelling and cerebral edema of hyperammonemia?
    Back
    Glutamine
  38. Front
    Excess ammonia disrupts the balance of which excitatory neurotransmitter and its inhibitory derivative GABA?
    Back
    Glutamate
  39. Front
    Transamination is reversible — what biosynthetic role does this allow aminotransferases to perform?
    Back
    Synthesis of amino acids (from α-keto acids)
  40. Front
    Which coenzyme intermediate is formed when PLP accepts an amino group during transamination ?
    Back
    Pyridoxamine phosphate
  41. Front
    In a patient with liver failure , confusion, and asterixis with high ammonia, why does portal ammonia reach the systemic circulation?
    Back
    Liver failure disables the urea cycle , so gut-derived portal ammonia passes through unchecked
  42. Front
    Are amino acids stored as a dedicated reserve the way glycogen and triacylglycerol are?
    Back
    No — excess amino acids are rapidly degraded
  43. Front
    During oxidative deamination of glutamate , glutamate dehydrogenase (GDH) reduces which redox cofactor to NADH?
    Back
    NAD+ (or NADP+ )
  44. Front
    In a healthy, well-fed adult whose protein synthesis and degradation are balanced, the nitrogen balance is ______.
    Back
    Zero
  45. Front
    Which nitrogen-balance state occurs during growth or recovery (intake exceeds loss)?
    Back
    Positive nitrogen balance
  46. Front
    Which nitrogen-balance state occurs in starvation, trauma, or burns, accompanied by loss of muscle mass?
    Back
    Negative nitrogen balance
  47. Front
    Besides transdeamination and glutaminase , name the two additional sources of free NH4+ the chapter lists.
    Back
    Amino acid oxidases and deamination of purines and pyrimidines
  48. Front
    Which process is the DOMINANT (major) route of nitrogen removal from amino acids in the body?
    Back
    Transdeamination
  49. Front
    Write the catabolic-direction reaction catalyzed by alanine aminotransferase (ALT) .
    Back
    Alanine + α-ketoglutarate → pyruvate + glutamate
  50. Front
    In liver disease or urea-cycle defects , plasma ammonia can rise above approximately what concentration?
    Back
    ~1,000 µM
  51. Front
    Why is even brief hyperammonemia especially dangerous in newborns and young children?
    Back
    It causes permanent CNS damage
  52. Front
    At what daily rate does the liver produce urea ?
    Back
    20–30 g/day
  53. Front
    Running in reverse, GDH performs what biosynthetic reaction to capture free ammonia ?
    Back
    α-ketoglutarate + NH4+ → glutamate (consuming NADH/NADPH)
  54. Front
    List the three mechanisms by which excess ammonia is neurotoxic to the CNS .
    Back
    (1) Depletes α-ketoglutarate from the Krebs cycle (impairs neuronal energy), (2) disrupts glutamate/GABA neurotransmitter balance, (3) glutamine accumulation in astrocytes causes osmotic swelling/ cerebral edema
  55. Front
    Roughly how large is the body's free amino acid pool at any moment?
    Back
    ~90–100 g
  56. Front
    Each day the body delivers amino acids to catabolism from two sources: ~100 g from the diet and how much from endogenous protein turnover?
    Back
    ~200–300 g of endogenous protein
  57. Front
    Glutamine synthetase (which captures free NH4+ onto glutamate ) is active primarily in which tissues?
    Back
    Muscle and liver (and critically the CNS/astrocytes )
  58. Front
    In the intestine , why do enterocytes release NH4+ into the portal circulation?
    Back
    They use glutamine as a major fuel, and glutaminase liberates its NH4+
  59. Front
    Name the three reactions/enzymes that CONSUME free NH4+ (its metabolic fates).
    Back
    GDH running in reverse (→ glutamate ), glutamine synthetase (→ glutamine ), and CPS-I (→ carbamoyl phosphate / urea cycle)
  60. Front
    In transdeamination , transamination occurs in which subcellular compartment, and oxidative deamination ( GDH ) in which?
    Back
    Transamination in the cytosol ; oxidative deamination by GDH in the mitochondria

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