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Enzyme Kinetics & Inhibition

Tier 1 · #5 of 33 · highest yield · topic 1.9 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    What does Km equal in terms of substrate concentration ?
    Back
    The [S] at which V₀ = ½Vmax
  2. Front
    A LOW Km means the enzyme has ___ affinity; a HIGH Km means ___ affinity for the substrate .
    Back
    Low Km = high affinity ; high Km = low affinity
  3. Front
    What is V₀ (initial velocity) , and why is it measured at time zero?
    Back
    Velocity measured before product accumulates enough to drive the reverse reaction
  4. Front
    Write the Michaelis-Menten equation .
    Back
    V₀ = (Vmax × [S]) / (Km + [S])
  5. Front
    What shape is the plot of V₀ vs [S] for a Michaelis-Menten enzyme ?
    Back
    A hyperbola
  6. Front
    Is Vmax ever actually reached at finite substrate concentration?
    Back
    No — it is approached asymptotically only at theoretically infinite [S]
  7. Front
    The Michaelis-Menten model assumes [ES] is constant because formation equals breakdown. What is this condition called?
    Back
    Steady-state conditions
  8. Front
    Besides substrate and enzyme concentration, which two physical factors affect V₀ ?
    Back
    pH and temperature
  9. Front
    Why does serum CK , ALT , and AST activity reflect how much enzyme protein is present?
    Back
    When [S] >> Km (saturating substrate) , measured rate is proportional to enzyme concentration
  10. Front
    Hexokinase I has a Km for glucose of ~0.05 mM. What does this low Km let it do?
    Back
    Grab glucose even at low concentrations , so tissues never starve
  11. Front
    What does the equation Vmax = kcat × [E]total tell you about how Vmax scales with enzyme amount?
    Back
    Vmax scales directly with enzyme amount — double the enzyme, double the Vmax
  12. Front
    Glucokinase (Hexokinase IV) has a Km for glucose of about ___ mM.
    Back
    ~5 mM
  13. Front
    What does kcat (the turnover number) measure?
    Back
    Substrate molecules converted per second by one fully-saturated enzyme molecule
  14. Front
    Why does Glucokinase only kick in after a carbohydrate meal?
    Back
    Its high Km (~5 mM) means it acts only when portal glucose is high — a glucose sensor/buffer
  15. Front
    Which is an intrinsic property of the enzyme, independent of enzyme concentration: kcat or Vmax ?
    Back
    kcat
  16. Front
    Which two tissues express Glucokinase (Hexokinase IV) ?
    Back
    Liver and pancreatic beta-cells
  17. Front
    What ratio expresses catalytic efficiency , reflecting enzyme performance at low substrate concentrations?
    Back
    kcat/Km
  18. Front
    Unlike Hexokinase I–III , Glucokinase is NOT inhibited by which product, and shows what type of kinetics?
    Back
    Not inhibited by glucose-6-phosphate ; shows sigmoidal kinetics
  19. Front
    For a diffusion-limited (catalytically perfect) enzyme, kcat/Km approaches what value?
    Back
    10⁸–10⁹ M⁻¹s⁻¹
  20. Front
    The Lineweaver-Burk plot is also called the ___ plot.
    Back
    Double-reciprocal plot
  21. Front
    On a Lineweaver-Burk plot , what does the y-intercept equal?
    Back
    1/Vmax
  22. Front
    On a Lineweaver-Burk plot , what does the x-intercept equal?
    Back
    −1/Km
  23. Front
    On a Lineweaver-Burk plot , what does the slope equal?
    Back
    Km/Vmax
  24. Front
    On a Lineweaver-Burk plot , a HIGHER y-intercept means a ___ Vmax.
    Back
    Lower Vmax (since y-intercept = 1/Vmax)
  25. Front
    How does an irreversible inhibitor bind the enzyme, and how is activity recovered?
    Back
    Forms a stable covalent bond ; activity recovers only by synthesizing new enzyme
  26. Front
    Aspirin irreversibly inhibits cyclooxygenase (COX) by acetylating which residue in the active site?
    Back
    A serine residue
  27. Front
    Aspirin blocks COX , preventing synthesis of which two classes of molecules?
    Back
    Prostaglandins and thromboxanes
  28. Front
    Penicillin is a suicide inhibitor of which bacterial enzyme?
    Back
    Glycopeptide transpeptidase
  29. Front
    What does bacterial glycopeptide transpeptidase normally do (the target of penicillin )?
    Back
    Cross-links bacterial cell wall peptidoglycans
  30. Front
    How does penicillin 's beta-lactam ring trap the transpeptidase enzyme?
    Back
    It mimics the reaction's transition state ; the enzyme attacks it and becomes irreversibly acylated at an active-site serine
  31. Front
    Allopurinol is a structural analog of ___, processed by xanthine oxidase into oxypurinol (alloxanthine) .
    Back
    Hypoxanthine
  32. Front
    Allopurinol inhibits xanthine oxidase to block synthesis of what, treating which disease?
    Back
    Blocks uric acid synthesis ; treats gout
  33. Front
    What is a suicide inhibitor ?
    Back
    An inhibitor that enters the active site as a substrate , then forms a permanent covalent adduct
  34. Front
    In competitive inhibition , what happens to Km and Vmax ?
    Back
    Km appears to increase ; Vmax is unchanged
  35. Front
    Why can competitive inhibition be overcome by excess substrate ?
    Back
    Flooding with substrate outcompetes the inhibitor for the active site, restoring the same Vmax
  36. Front
    On a Lineweaver-Burk plot , competitive inhibition changes the intercepts how?
    Back
    Same y-intercept; x-intercept shifts right toward zero (less negative), slope increases
  37. Front
    Methotrexate is a competitive inhibitor of which enzyme?
    Back
    Dihydrofolate reductase (DHFR)
  38. Front
    What reaction does DHFR normally catalyze, and why is its product indispensable?
    Back
    Reduces folate to tetrahydrofolate (THF) ; THF is required for one-carbon transfer in nucleotide synthesis
  39. Front
    Methotrexate is formally competitive, yet works almost irreversibly at therapeutic doses. Why?
    Back
    It binds DHFR ~1,000 times more tightly than the normal substrate, so physiologic substrate cannot outcompete it
  40. Front
    ACE inhibitors (e.g., captopril) competitively inhibit angiotensin-converting enzyme , blocking conversion of ___ to ___.
    Back
    Angiotensin I to angiotensin II
  41. Front
    How do ACE inhibitors lower blood pressure?
    Back
    Less angiotensin II → less vasoconstriction → lower blood pressure
  42. Front
    In noncompetitive inhibition , where does the inhibitor bind?
    Back
    An allosteric site (not the active site) , binding whether or not substrate is present
  43. Front
    In noncompetitive inhibition , what happens to Km and Vmax ?
    Back
    Km is unchanged ; Vmax decreases
  44. Front
    Why can't excess substrate rescue noncompetitive inhibition ?
    Back
    You cannot outcompete an inhibitor that is not competing for the active site
  45. Front
    On a Lineweaver-Burk plot , noncompetitive inhibition changes the intercepts how?
    Back
    Same x-intercept (−1/Km unchanged); y-intercept rises (lower Vmax)
  46. Front
    In uncompetitive inhibition , the inhibitor binds only to which form of the enzyme?
    Back
    Only the ES complex (not free enzyme)
  47. Front
    In uncompetitive inhibition , what happens to Km and Vmax ?
    Back
    Both decrease by the same factor
  48. Front
    What is the Lineweaver-Burk signature of uncompetitive inhibition ?
    Back
    Parallel line (slope unchanged), shifted up and to the left
  49. Front
    On a kinetics plot, irreversible inhibition resembles which reversible type (Vmax down, Km same)?
    Back
    Noncompetitive inhibition
  50. Front
    What experimental test distinguishes reversible noncompetitive from irreversible inhibition ?
    Back
    Washing out the inhibitor restores activity ( noncompetitive ) but not ( irreversible )
  51. Front
    Allosteric enzymes show ___ kinetics rather than hyperbolic, due to subunit cooperativity.
    Back
    Sigmoidal (S-shaped)
  52. Front
    On a sigmoidal curve, a positive allosteric modulator shifts the curve which direction?
    Back
    Left (lower K₀.₅ — activation at lower [S])
  53. Front
    What is the fastest reversible covalent switch for regulating enzyme activity?
    Back
    Phosphorylation (covalent modification)
  54. Front
    Protein kinases phosphorylate which three residues, and which enzymes reverse it?
    Back
    Serine, threonine, or tyrosine ; protein phosphatases remove the phosphate
  55. Front
    Glycogen phosphorylase is allosterically activated by which molecule signaling low energy?
    Back
    AMP
  56. Front
    Glycogen phosphorylase is also activated by phosphorylation driven by which two hormones?
    Back
    Glucagon and epinephrine
  57. Front
    In inactive Protein Kinase A , what holds the catalytic (C) subunits inactive?
    Back
    Two regulatory (R) subunits
  58. Front
    What activates Protein Kinase A by causing the regulatory subunits to dissociate?
    Back
    cAMP (four molecules bind the two R subunits)
  59. Front
    Which enzyme regulation mechanism is the ONLY irreversible one?
    Back
    Zymogen activation (proteolytic cleavage)
  60. Front
    What is a zymogen (proenzyme) ?
    Back
    An inactive enzyme precursor activated by proteolytic cleavage
  61. Front
    Trypsinogen → trypsin and chymotrypsinogen → chymotrypsin . Pepsinogen → ___?
    Back
    Pepsin
  62. Front
    Why are digestive proteases synthesized as zymogens ?
    Back
    To prevent them from digesting the cell that synthesizes them ; they activate only at the correct site
  63. Front
    What are isoenzymes (isozymes) ?
    Back
    Distinct proteins from separate genes that catalyze the same reaction but differ in sequence, kinetics, and tissue distribution
  64. Front
    Which creatine kinase isoenzyme is the classic serum marker of myocardial infarction ?
    Back
    CK-MB
  65. Front
    Creatine kinase is a dimer of M and B subunits. Match the isoenzyme to its tissue: brain , skeletal muscle , myocardium .
    Back
    CK-BB = brain ; CK-MM = skeletal muscle ; CK-MB = myocardium
  66. Front
    Lactate dehydrogenase (LDH) is a tetramer built from which two subunit types?
    Back
    H (heart) and M (muscle)
  67. Front
    Which LDH isoenzyme (subunit composition) predominates in myocardium and red blood cells ?
    Back
    LDH1 (H₄)
  68. Front
    Which LDH isoenzyme predominates in liver and skeletal muscle ?
    Back
    LDH5 (M₄)
  69. Front
    In a myocardial infarction , does LDH1 or LDH5 rise first?
    Back
    LDH1 rises before LDH5
  70. Front
    The LDH H subunit favors which reaction direction; the M subunit favors which?
    Back
    H favors lactate → pyruvate (oxidative) ; M favors pyruvate → lactate (anaerobic)
  71. Front
    Which serum enzymes are the classic markers of liver damage ?
    Back
    ALT and AST (alanine and aspartate aminotransferases)
  72. Front
    Why does an enzyme have an optimal pH?
    Back
    Catalysis depends on correct ionization of active-site side chains ; extreme pH denatures it
  73. Front
    What is the algebraic definition of Km in terms of the three rate constants?
    Back
    Km = (k₋₁ + k₂)/k₁
  74. Front
    Write the Michaelis-Menten reaction scheme (enzyme, substrate, product) with its rate constants.
    Back
    E + S ⇌(k₁/k₋₁) ES →(k₂) E + P
  75. Front
    In the Michaelis-Menten scheme, what does each rate constant represent: k₁, k₋₁, k₂?
    Back
    k₁ = ES formation ; k₋₁ = ES dissociation back to E + S ; k₂ = ES breakdown to E + P
  76. Front
    Velocity ( V ) is defined as either the rate of ___ or the rate of ___.
    Back
    Rate of product formation (d[P]/dt) OR rate of substrate disappearance (−d[S]/dt)
  77. Front
    State the three assumptions underlying the Michaelis-Menten equation .
    Back
    [S] >> [E]; [ES] is at steady state; V₀ is measured before significant product forms
  78. Front
    On a sigmoidal curve, a NEGATIVE allosteric modulator shifts the curve which direction, and what happens to K₀.₅?
    Back
    Shifts the curve RIGHT (higher K₀.₅) , without necessarily changing Vmax
  79. Front
    Aspirin 's effect is dose-dependent: a LOW dose is ___, while a HIGHER dose is ___.
    Back
    Low dose = antithrombotic; higher dose = anti-inflammatory
  80. Front
    Allosteric enzymes are almost always ___ in structure (number of subunits).
    Back
    Multisubunit (multiple subunits)
  81. Front
    What kind of molecules typically serve as allosteric modulators , and what do they let a pathway do?
    Back
    Metabolites acting as feedback signals — letting a pathway sense its own output and self-regulate
  82. Front
    At pH 6, what happens to the histidine in trypsin 's active center, and to its activity?
    Back
    The active-site histidine becomes protonated; activity drops sharply
  83. Front
    Hexokinase I–III have what quaternary structure, and how hard do they run at normal blood glucose ?
    Back
    Dimeric ; they run at near-maximum capacity at normal blood glucose (constitutive feed into glycolysis)
  84. Front
    At LOW [S] a Michaelis-Menten reaction is ___-order (rate proportional to [S]); at saturating [S] it becomes ___-order.
    Back
    First-order at low [S]; zero-order (rate constant, near Vmax) at saturating [S]
  85. Front
    On the Michaelis-Menten hyperbola, what kind of kinetics is seen where the curve is nearly linear at low [S]?
    Back
    First-order kinetics — rate is directly proportional to [S]
  86. Front
    In uncompetitive inhibition , WHY does the apparent Km decrease (appear to bind substrate better)?
    Back
    By trapping ES as dead-end ESI , the inhibitor pulls more enzyme into the ES form, lowering apparent Km — but those complexes can't turn over, so Vmax also falls
  87. Front
    An allosteric enzyme has which two distinct binding sites?
    Back
    The active site (catalysis) and a separate regulatory site (where the modulator binds)
  88. Front
    Name the four key reversible-or-irreversible mechanisms that regulate enzyme activity from within the cell.
    Back
    Allosteric regulation, covalent modification (phosphorylation), regulatory subunits, and zymogen activation
  89. Front
    Write the Lineweaver-Burk (double-reciprocal) equation .
    Back
    1/V₀ = (Km/Vmax)(1/[S]) + 1/Vmax
  90. Front
    Covalent modification ( phosphorylation ) is reversible — but does the phosphate come off spontaneously?
    Back
    No — reversal requires enzymatic action (a protein phosphatase ); it does not happen spontaneously
  91. Front
    Why is CK-MB elevation specific for myocardial infarction (not skeletal-muscle or brain injury)?
    Back
    CK-MM dominates skeletal muscle and CK-BB doesn't readily cross the blood-brain barrier , so serum CK-MB points to myocardium
  92. Front
    Glycogen phosphorylase is the lecture example of ONE enzyme controlled by which TWO regulatory mechanisms at once?
    Back
    Allosteric activation (by AMP ) AND covalent modification (phosphorylation via glucagon / epinephrine ) — same message: mobilize glucose
  93. Front
    By blocking DHFR → THF → nucleotide synthesis, methotrexate selectively kills which cells, making it useful for what?
    Back
    Rapidly dividing cells (e.g., cancer cells ) — used as chemotherapy

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