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The Urea Cycle

Tier 3 · #25 of 33 · moderate yield · topic 4.2 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    What is the overall purpose of the urea cycle ?
    Back
    To package two nitrogen atoms into urea (water-soluble, non-toxic) for renal excretion, disposing of toxic ammonia from amino acid catabolism.
  2. Front
    In which organ does the urea cycle take place?
    Back
    The liver .
  3. Front
    Which enzyme catalyzes the committed first step of the urea cycle ?
    Back
    Carbamoyl phosphate synthetase I (CPS-I) .
  4. Front
    In which subcellular compartment is CPS-I located?
    Back
    The mitochondrial matrix .
  5. Front
    What is the obligatory allosteric activator of CPS-I , without which the enzyme is completely inactive?
    Back
    N-acetylglutamate (NAG) .
  6. Front
    Two-step generation of free NH4+ from amino acid nitrogen is called ______.
    Back
    Transdeamination (transamination then oxidative deamination).
  7. Front
    Which cofactor do all aminotransferases (transaminases) require?
    Back
    Pyridoxal phosphate (PLP) , the active form of vitamin B6 .
  8. Front
    Which subcellular compartment is CPS-II located in?
    Back
    The cytosol .
  9. Front
    In transamination , an amino group is transferred from an amino acid onto which acceptor to form glutamate ?
    Back
    α-ketoglutarate .
  10. Front
    What nitrogen donor does CPS-I use?
    Back
    Free NH4+ (ammonium).
  11. Front
    Which enzyme strips the amino group from glutamate to release free NH4+ in the mitochondrial matrix ?
    Back
    Glutamate dehydrogenase (GDH) .
  12. Front
    What nitrogen donor does CPS-II use?
    Back
    The amide nitrogen of glutamine .
  13. Front
    How does GTP affect glutamate dehydrogenase (GDH)?
    Back
    GTP inhibits it (high energy charge slows amino acid deamination).
  14. Front
    Which pathway does CPS-II supply carbamoyl phosphate for?
    Back
    Pyrimidine synthesis (not urea).
  15. Front
    How does ADP affect glutamate dehydrogenase (GDH)?
    Back
    ADP activates it (low energy charge ramps up amino acid catabolism).
  16. Front
    What is the allosteric activator of CPS-II ?
    Back
    PRPP .
  17. Front
    Which enzyme hydrolyzes glutamine to release NH4+ and glutamate in the liver and kidneys ?
    Back
    Glutaminase .
  18. Front
    What is the allosteric inhibitor of CPS-II ?
    Back
    UTP .
  19. Front
    Renal glutaminase activity is specifically upregulated during which condition?
    Back
    Metabolic acidosis (excreting NH4+ removes acid).
  20. Front
    CPS-I condenses NH4+ with which molecule to form carbamoyl phosphate ?
    Back
    Bicarbonate (HCO3−) .
  21. Front
    How many ATP are consumed by CPS-I to make carbamoyl phosphate ?
    Back
    2 ATP .
  22. Front
    N-acetylglutamate is synthesized from glutamate and which other substrate?
    Back
    Acetyl-CoA .
  23. Front
    Which urea cycle intermediate activates N-acetylglutamate synthase , creating a feed-forward loop?
    Back
    Arginine .
  24. Front
    Which carrier molecule shuttles between the mitochondria and cytosol to bridge the two compartments of the urea cycle?
    Back
    Ornithine (via the ornithine–citrulline antiporter).
  25. Front
    Reaction 1: Carbamoyl phosphate reacts with ornithine to form which product?
    Back
    Citrulline .
  26. Front
    Which enzyme catalyzes Reaction 1 ( carbamoyl phosphate + ornithine → citrulline )?
    Back
    Ornithine transcarbamoylase (OTC) .
  27. Front
    Which two steps of the urea cycle occur in the mitochondria ?
    Back
    Carbamoyl phosphate synthesis (CPS-I) and citrulline formation (OTC) .
  28. Front
    Which amino acid donates the second nitrogen of urea in Reaction 2?
    Back
    Aspartate .
  29. Front
    Which enzyme joins citrulline with aspartate to form argininosuccinate ?
    Back
    Argininosuccinate synthetase .
  30. Front
    How many ATP does argininosuccinate synthetase consume, and to what is the ATP hydrolyzed?
    Back
    1 ATP , hydrolyzed to AMP + PPi (≈ 2 high-energy bonds / ATP equivalents).
  31. Front
    Aspartate for Reaction 2 is produced when AST transfers an amino group from glutamate onto which acceptor?
    Back
    Oxaloacetate .
  32. Front
    Reaction 3: Argininosuccinase cleaves argininosuccinate into arginine and which TCA-linking molecule?
    Back
    Fumarate .
  33. Front
    Which enzyme cleaves argininosuccinate into arginine and fumarate ?
    Back
    Argininosuccinase (argininosuccinate lyase).
  34. Front
    Reaction 4: Arginase hydrolyzes arginine to release urea and regenerate which intermediate?
    Back
    Ornithine .
  35. Front
    Which urea cycle reaction is the only one that directly produces urea ?
    Back
    Reaction 4 — arginase cleaving arginine .
  36. Front
    What is the total energy cost of synthesizing one molecule of urea ?
    Back
    4 ATP equivalents (2 from CPS-I , 2 from argininosuccinate synthetase as ATP→AMP+PPi).
  37. Front
    What are the two sources of the two nitrogen atoms in urea ?
    Back
    One from NH4+ (via carbamoyl phosphate); one from aspartate .
  38. Front
    The fumarate → malate → oxaloacetate → aspartate loop linking the urea cycle to the TCA cycle is called the ______.
    Back
    Aspartate-argininosuccinate shunt .
  39. Front
    Which memory trick links CPS-I to its location and nitrogen source?
    Back
    CPS-I is in the mItochondria and uses NH4+ (inorganic nitrogen).
  40. Front
    What is the most common inherited urea cycle disorder?
    Back
    OTC (ornithine transcarbamoylase) deficiency .
  41. Front
    What is the inheritance pattern of OTC deficiency ?
    Back
    X-linked recessive .
  42. Front
    Which mediator provides short-term (acute) regulation of the urea cycle?
    Back
    N-acetylglutamate (NAG) , the obligatory activator of CPS-I .
  43. Front
    How is the urea cycle regulated long-term in response to a chronically high-protein diet?
    Back
    Induced transcription of all five urea cycle enzymes (capacity scales up over days to weeks).
  44. Front
    What is the normal blood ammonium concentration?
    Back
    Approximately 5–35 µM .
  45. Front
    In hyperammonemia , NH4+ depletes which TCA cycle intermediate, impairing brain energy production?
    Back
    α-ketoglutarate .
  46. Front
    In hyperammonemia , accumulation of which molecule in astrocytes draws in osmotic water to cause cerebral edema ?
    Back
    Glutamine .
  47. Front
    Besides urea cycle enzyme defects, what acquired condition is a major cause of hyperammonemia ?
    Back
    Severe liver disease / hepatic failure .
  48. Front
    Which two drugs provide alternative nitrogen excretion routes in hyperammonemia by conjugating with amino acids?
    Back
    Benzoate and phenylacetate .
  49. Front
    A newborn has encephalopathy , hyperammonemia , and elevated urinary orotic acid . What is the defect?
    Back
    OTC deficiency .
  50. Front
    In OTC deficiency , why is orotic acid elevated?
    Back
    Accumulated carbamoyl phosphate leaks into the cytosol and is shunted into CPS-II -driven pyrimidine synthesis , overproducing orotic acid .
  51. Front
    Arginine is the direct precursor of which signaling gas (via nitric oxide synthase )?
    Back
    Nitric oxide (NO) .
  52. Front
    Arginine combines with glycine in the kidney to form which high-energy storage compound?
    Back
    Creatine .
  53. Front
    In the glucose-alanine cycle , which amino acid carries nitrogen from muscle to the liver ?
    Back
    Alanine .
  54. Front
    Which class of cofactors do amino acid oxidases use for direct oxidative deamination?
    Back
    Flavins ( FAD or FMN ).
  55. Front
    What is the primary nitrogen carrier in the blood, delivering amino-acid nitrogen to the liver ?
    Back
    Glutamine .
  56. Front
    In severe urea cycle enzyme deficiencies, blood ammonia can exceed ______ µM, constituting a medical emergency.
    Back
    1,000 µM .
  57. Front
    Arginine is classified as ______ in neonates and during recovery from illness.
    Back
    Conditionally essential .
  58. Front
    Why is arginine conditionally essential in neonates and recovering patients?
    Back
    Demand for its products ( NO , creatine , polyamines , ornithine ) can exceed the cycle's synthetic capacity .
  59. Front
    In hyperammonemia , excess NH4+ drives which enzyme in REVERSE, fixing ammonia onto α-ketoglutarate to form glutamate ?
    Back
    Glutamate dehydrogenase (GDH) .
  60. Front
    In hyperammonemia , excess NH4+ drives which enzyme to overproduce glutamine ?
    Back
    Glutamine synthetase .
  61. Front
    Which two enzyme activities (and their direction) jointly deplete α-ketoglutarate in hyperammonemia ?
    Back
    GDH running in reverse plus glutamine synthetase — both consume α-ketoglutarate -derived glutamate to mop up NH4+ .
  62. Front
    Which three reactions of the urea cycle occur in the cytosol ?
    Back
    Argininosuccinate synthetase , argininosuccinase , and arginase (Reactions 2, 3, and 4).
  63. Front
    Beyond depleting α-ketoglutarate and swelling astrocytes , how else does ammonia injure the CNS?
    Back
    It disrupts glutamate and GABA neurotransmitter metabolism (perturbed precursor pools).
  64. Front
    Besides benzoate/phenylacetate, what two dietary measures treat hyperammonemia ?
    Back
    Restrict dietary protein and provide non-protein energy (carbohydrate) to slow amino acid catabolism.
  65. Front
    Arginine supplies ornithine for the synthesis of which class of polycationic molecules?
    Back
    Polyamines .
  66. Front
    When glutamate dehydrogenase (GDH) deaminates glutamate , which cofactor(s) does it reduce?
    Back
    Either NAD+ or NADP+ (GDH can use both).
  67. Front
    Why does the transamination step (amino acid + α-ketoglutarate ⇌ glutamate ) work in both catabolism and biosynthesis?
    Back
    Because transamination is entirely reversible .
  68. Front
    Complete the CPS-II memory trick: CPS-II is cytosol__c and uses glutam__ne.
    Back
    CytosolIIc and uses glutamIIne (two I's = CPS-II).
  69. Front
    In the glucose-alanine cycle , what happens to the alanine carbon vs. its nitrogen once the liver takes it up?
    Back
    Carbon → pyruvate → glucose (gluconeogenesis); nitrogen → glutamate → NH4+ → urea.

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