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Amino-Acid Metabolism: PLP, One-Carbon, PKU & B12

Tier 1 · #8 of 33 · highest yield · topic 4.3 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    Which two amino acids are PURELY ketogenic ?
    Back
    Leucine (Leu) and lysine (Lys)
  2. Front
    Metabolically, what does it mean that an amino acid is ' ketogenic '?
    Back
    Its carbon skeleton yields only acetyl-CoA or acetoacetate , which cannot be converted to net glucose in mammals
  3. Front
    Glucogenic amino acids yield which intermediates that can feed gluconeogenesis? (name the five)
    Back
    Pyruvate , oxaloacetate , fumarate , succinyl-CoA , or α-ketoglutarate
  4. Front
    Which FIVE amino acids are BOTH glucogenic and ketogenic ?
    Back
    Threonine (Thr), isoleucine (Ile), tryptophan (Trp), phenylalanine (Phe), and tyrosine (Tyr)
  5. Front
    Why are Phe and Tyr classified as both glucogenic and ketogenic ? (name the two products)
    Back
    Tyr is degraded to fumarate (glucogenic) PLUS acetoacetate (ketogenic)
  6. Front
    What is the active coenzyme form of vitamin B6 ?
    Back
    Pyridoxal phosphate (PLP)
  7. Front
    In transamination , the amino group is transferred onto which acceptor almost every time?
    Back
    α-ketoglutarate (generating glutamate )
  8. Front
    Cloze: Transamination followed by oxidative deamination is called ___, the major route for nitrogen removal from amino acids.
    Back
    Transdeamination
  9. Front
    Which enzyme oxidatively deaminates glutamate in the mitochondria to release free NH4+?
    Back
    Glutamate dehydrogenase
  10. Front
    Which two amino acids do NOT undergo transamination ?
    Back
    Lysine and threonine
  11. Front
    Which aminotransferase is MORE liver-specific, ALT or AST ?
    Back
    ALT (alanine aminotransferase)
  12. Front
    PLP-dependent DOPA decarboxylase converts DOPA to which product?
    Back
    Dopamine
  13. Front
    PLP-dependent histidine decarboxylase converts histidine to which product?
    Back
    Histamine
  14. Front
    PLP-dependent glutamate decarboxylase converts glutamate to which product?
    Back
    GABA (the main inhibitory neurotransmitter of the CNS)
  15. Front
    Serine–threonine dehydratase (PLP-dependent) deaminates serine to which product?
    Back
    Pyruvate + NH4+
  16. Front
    From which vitamin is tetrahydrofolate (THF) derived?
    Back
    Folate (vitamin B9)
  17. Front
    Which enzyme reduces dietary folate to THF (via DHF), using NADPH ?
    Back
    Dihydrofolate reductase (DHFR)
  18. Front
    Methotrexate kills rapidly dividing cells by inhibiting which enzyme?
    Back
    Dihydrofolate reductase (DHFR) — emptying the THF pool
  19. Front
    The major source of one-carbon units for THF comes from which reaction?
    Back
    Serine → glycine (by serine hydroxymethyltransferase )
  20. Front
    Which TWO cofactors does serine hydroxymethyltransferase (Ser→Gly) require?
    Back
    PLP and THF
  21. Front
    The methylene group stripped from serine ends up on THF as which specific form?
    Back
    5,10-methylene-THF
  22. Front
    Which enzyme uses 5,10-methylene-THF to convert dUMP to dTMP ?
    Back
    Thymidylate synthase
  23. Front
    Why does folate deficiency cause megaloblastic anemia ?
    Back
    THF is required for dTMP synthesis; without dTMP there is no thymidine for DNA replication in erythroid precursors
  24. Front
    Which THF form is the methyl donor used to regenerate methionine from homocysteine ?
    Back
    5-methyl-THF
  25. Front
    Cloze: ___ is the principal methyl-group donor for over 100 methylation reactions in the cell.
    Back
    S-adenosylmethionine (SAM)
  26. Front
    From which amino acid is SAM formed?
    Back
    Methionine
  27. Front
    After SAM donates its methyl group it becomes SAH , which is then hydrolysed to which compound?
    Back
    Homocysteine (Hcys)
  28. Front
    Which enzyme remethylates homocysteine back to methionine ?
    Back
    Methionine synthase
  29. Front
    Methionine synthase requires which methyl donor AND which vitamin cofactor?
    Back
    5-methyl-THF (methyl donor) and vitamin B12 / methylcobalamin (cofactor)
  30. Front
    In the transsulfuration pathway, which PLP-enzyme condenses homocysteine with serine to form cystathionine ?
    Back
    Cystathionine beta-synthase (CBS)
  31. Front
    Cysteine gets its SULFUR from which amino acid and its CARBON skeleton from which amino acid?
    Back
    Sulfur from methionine (via homocysteine ); carbon from serine
  32. Front
    Deficiency of which enzyme causes homocystinuria ?
    Back
    Cystathionine beta-synthase (CBS)
  33. Front
    Name three classic clinical features of homocystinuria (besides elevated plasma homocysteine ).
    Back
    Lens dislocation, skeletal abnormalities, and markedly increased vascular thrombosis risk
  34. Front
    What is the methyl-folate trap ?
    Back
    In B12 deficiency , THF is trapped as 5-methyl-THF (cannot be reconverted without methionine synthase ), depleting other THF forms needed for dTMP/purine synthesis
  35. Front
    Vitamin B12 is required for exactly how many reactions in human metabolism, and which two?
    Back
    Two: methionine synthase and methylmalonyl-CoA mutase
  36. Front
    Which cobalamin form does methylmalonyl-CoA mutase use, and what reaction does it catalyse?
    Back
    5'-deoxyadenosylcobalamin ; converts methylmalonyl-CoA → succinyl-CoA
  37. Front
    What is the MOST sensitive biochemical marker of B12 deficiency ?
    Back
    Elevated urinary methylmalonate (methylmalonic aciduria) — more sensitive than serum B12 itself
  38. Front
    Which neurological complication occurs in B12 deficiency but NOT in folate deficiency ?
    Back
    Subacute combined degeneration of the spinal cord (demyelination)
  39. Front
    Why must you rule out B12 deficiency before treating megaloblastic anemia with folate alone?
    Back
    Folate can correct the anemia while masking (allowing progression of) the B12 neurological damage
  40. Front
    Which liver enzyme converts phenylalanine to tyrosine ?
    Back
    Phenylalanine hydroxylase
  41. Front
    Which cofactor does phenylalanine hydroxylase require?
    Back
    Tetrahydrobiopterin (BH4)
  42. Front
    After phenylalanine hydroxylase oxidises BH4 to BH2 , which enzyme regenerates BH4 (using NADH )?
    Back
    Dihydropteridine reductase
  43. Front
    Classical phenylketonuria (PKU) is caused by deficiency of which enzyme?
    Back
    Phenylalanine hydroxylase
  44. Front
    Which accumulated metabolite gives the urine/sweat its characteristic musty (mousey) odour in PKU ?
    Back
    Phenylacetate
  45. Front
    Why does tyrosine become conditionally essential in PKU ?
    Back
    Phenylalanine can no longer be converted to tyrosine , so it must be supplied in the diet
  46. Front
    Why do BH4-deficient PKU patients NOT respond to a low-phenylalanine diet alone?
    Back
    BH4 is also required for hydroxylation of tyrosine and tryptophan (catecholamine and serotonin synthesis), which restriction can't fix
  47. Front
    When must a low-phenylalanine diet be started in classical PKU to prevent irreversible neurological damage?
    Back
    Within the first two weeks of life
  48. Front
    Tyrosine degradation (via homogentisate) yields which two products?
    Back
    Fumarate (glucogenic) and acetoacetate (ketogenic)
  49. Front
    In catecholamine synthesis, which cofactor sequence is used from tyrosine to adrenaline ?
    Back
    BH4 (hydroxylation) → PLP (decarboxylation to dopamine) → ascorbic acid (to noradrenaline) → SAM (to adrenaline)
  50. Front
    Which enzyme methylates noradrenaline to adrenaline , and what is its methyl donor?
    Back
    PNMT , using SAM
  51. Front
    Which enzyme converts tyrosine toward melanin in skin and hair , and its deficiency causes which disorder?
    Back
    Tyrosinase ; deficiency causes albinism
  52. Front
    Tryptophan is the precursor of which three notable products?
    Back
    Serotonin, melatonin, and nicotinamide (NAD+/NADP+)
  53. Front
    Where is melatonin synthesised and what does it regulate?
    Back
    In the pineal gland ; regulates the sleep–wake cycle
  54. Front
    Why does BH4 deficiency impair BOTH catecholamine and serotonin synthesis?
    Back
    Both the tyrosine →catecholamine and tryptophan →serotonin pathways begin with a BH4 -dependent hydroxylase
  55. Front
    Arginine is used by nitric oxide synthase (NOS) to produce which signalling molecule?
    Back
    Nitric oxide (NO) — a potent vasodilator
  56. Front
    Creatine is synthesised from which two amino acids (plus a SAM methylation)?
    Back
    Arginine (donates guanidino group) and glycine
  57. Front
    Cloze: Creatine spontaneously and irreversibly cyclises to ___, excreted proportionally to muscle mass.
    Back
    Creatinine
  58. Front
    Glycine plus succinyl-CoA initiate the synthesis of which molecule (via ALA)?
    Back
    Heme
  59. Front
    Glutathione is a tripeptide of which three residues?
    Back
    Glutamate–cysteine–glycine (Glu-Cys-Gly)
  60. Front
    Which three essential amino acids are the branched-chain amino acids (BCAAs) ?
    Back
    Valine, leucine, and isoleucine
  61. Front
    In which tissue are BCAAs primarily catabolised?
    Back
    Muscle (not liver)
  62. Front
    Which enzyme complex performs oxidative decarboxylation of all three branched-chain α-ketoacids ?
    Back
    Branched-chain α-ketoacid dehydrogenase complex
  63. Front
    Which five cofactors does the branched-chain α-ketoacid dehydrogenase complex require?
    Back
    TPP, lipoamide, CoA, FAD, and NAD+
  64. Front
    Maple syrup urine disease (MSUD) is caused by deficiency of which enzyme?
    Back
    Branched-chain α-ketoacid dehydrogenase complex
  65. Front
    Glutamate dehydrogenase is allosterically INHIBITED by ___ and ACTIVATED by ___.
    Back
    Inhibited by GTP ; activated by ADP (runs faster when the cell is energy-poor)
  66. Front
    How does ammonia toxicity deplete GABA and impair the TCA cycle?
    Back
    Excess NH4+ drives glutamate toward glutamine synthesis, depleting both α-ketoglutarate (TCA) and the glutamate needed for GABA
  67. Front
    Cysteine is non-essential only when which amino acid is available to supply sulfur?
    Back
    Methionine (remove it from the diet and cysteine becomes essential)
  68. Front
    What is the general rule for whether a non-essential amino acid can be synthesised?
    Back
    You can make an amino acid only if you can make its corresponding α-ketoacid
  69. Front
    Alanine is synthesised by transamination of which α-ketoacid?
    Back
    Pyruvate
  70. Front
    Which α-ketoacid is the precursor of the glutamate family (Glu, Gln, Pro, Arg)?
    Back
    α-ketoglutarate
  71. Front
    Aspartate and asparagine are synthesised from which α-ketoacid?
    Back
    Oxaloacetate
  72. Front
    Serine is synthesised from which glycolytic intermediate?
    Back
    3-phosphoglycerate
  73. Front
    Which aminotransferase is MORE sensitive (not more specific) for hepatocyte damage, and why?
    Back
    AST — because its activity in liver is very high
  74. Front
    Serine–threonine dehydratase (PLP) deaminates threonine to which products?
    Back
    α-ketobutyrate + NH4+
  75. Front
    Name three physiological actions of histamine .
    Back
    Contracts bronchial smooth muscle, stimulates gastric acid + pepsin secretion, and dilates blood vessels
  76. Front
    Antihistamines relieve allergy by what mechanism?
    Back
    Blocking histamine receptors (NOT by inhibiting histamine synthesis)
  77. Front
    Serotonin is chemically identical to which compound (give the full name)?
    Back
    5-hydroxytryptamine (5-HT)
  78. Front
    Besides being a neurotransmitter, what other function does serotonin serve?
    Back
    Platelet activator
  79. Front
    Name the five products that SAM methylation generates.
    Back
    Adrenaline, creatine, lecithin (phosphatidylcholine), melatonin, and methylated DNA/RNA
  80. Front
    Roughly what fraction of collagen residues are glycine ?
    Back
    One-third (1/3)
  81. Front
    Glycine is conjugated to bile acids to form which bile acid?
    Back
    Glycocholic acid
  82. Front
    Besides heme, glutathione, collagen, and bile acids, glycine is a building block for which class of nucleotides?
    Back
    Purine nucleotides
  83. Front
    Methylmalonyl-CoA mutase processes propionyl-CoA arising from which four sources?
    Back
    Odd-chain fatty acids, branched-chain amino acids (Val and Ile), methionine, and threonine
  84. Front
    Besides catecholamines and melanin, tyrosine is the precursor of which hormones?
    Back
    Thyroid hormones
  85. Front
    What is the function of phosphocreatine (creatine phosphate) in muscle ?
    Back
    A rapid-turnover high-energy phosphate store that regenerates ATP during intense contraction
  86. Front
    By what mechanism does high phenylalanine cause brain damage in untreated PKU ?
    Back
    It competes with other large neutral amino acids for transport across the blood–brain barrier
  87. Front
    Besides mousey odour, decreased pigmentation, and intellectual disability, name two other clinical features of untreated PKU .
    Back
    Tendency to vomit and eczematoid rash
  88. Front
    The glycine cleavage system releases which three products?
    Back
    CO2, NH4+, and a methylene group ( 5,10-methylene-THF ) onto the one-carbon pool
  89. Front
    Even below frank homocystinuria , elevated homocysteine is recognised as an independent risk factor for what?
    Back
    Cardiovascular disease / vascular thrombosis
  90. Front
    Among the branched-chain amino acids, which one is PURELY ketogenic ?
    Back
    Leucine (Leu)
  91. Front
    Valine and isoleucine are glucogenic because their carbon enters the TCA cycle as which intermediate?
    Back
    Succinyl-CoA (via propionyl-CoA )
  92. Front
    Which single branched-chain amino acid is BOTH glucogenic and ketogenic ?
    Back
    Isoleucine (Ile) — yields both succinyl-CoA and acetyl-CoA
  93. Front
    Glycine is synthesised from which amino acid?
    Back
    Serine
  94. Front
    The branched-chain α-ketoacid dehydrogenase complex is analogous to which other multi-enzyme complex?
    Back
    Pyruvate dehydrogenase
  95. Front
    Which enzyme transaminates the branched-chain amino acids , and in which tissue is it highly expressed and induced during fasting?
    Back
    Branched-chain aminotransferase ; in muscle
  96. Front
    Nitric oxide synthase (NOS) exists as which two isoforms named in the chapter?
    Back
    Endothelial and inducible (eNOS and iNOS)
  97. Front
    In creatine synthesis, the guanidino group of arginine is donated to glycine in the kidney to form which intermediate?
    Back
    Guanidinoacetate (then methylated by SAM in the liver to creatine)
  98. Front
    Nicotinamide (derived from tryptophan) is which B vitamin ?
    Back
    Vitamin B3 (incorporated into NAD+ and NADP+ )
  99. Front
    Catecholamines are released from the adrenal medulla in response to which four stimuli (fight-or-flight)?
    Back
    Fright, exercise, cold, and hypoglycaemia
  100. Front
    In the catecholamine pathway, which enzyme (and cofactor) converts dopamine to noradrenaline ?
    Back
    Dopamine beta-hydroxylase , using ascorbic acid (vitamin C)
  101. Front
    Which characteristic urine finding identifies maple syrup urine disease (MSUD) ?
    Back
    A sweet, maple-syrup odour (from accumulated branched-chain amino acids and their α-ketoacids)
  102. Front
    Which enzyme catalyses the first (rate-limiting) step of catecholamine synthesis, converting tyrosine to DOPA ?
    Back
    Tyrosine hydroxylase (requires BH4 )

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