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Enzymes: Catalysis and Classification

Tier 3 · #21 of 33 · moderate yield · topic 1.8 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    Mechanistically, HOW does an enzyme speed up a reaction?
    Back
    It binds and stabilizes the transition state , lowering the activation energy (Ea)
  2. Front
    When an enzyme is added to a reaction, what happens to ΔG°' (standard free energy change)?
    Back
    Unchanged — enzymes do not alter thermodynamics
  3. Front
    When an enzyme is added to a reaction, what happens to the equilibrium constant (Keq)?
    Back
    Unchanged
  4. Front
    When an enzyme is added to a reaction, what happens to the activation energy (Ea)?
    Back
    Decreased
  5. Front
    When an enzyme is added to a reaction, what happens to the initial velocity?
    Back
    Increased
  6. Front
    By what factor do enzymes increase reaction rates over the uncatalyzed rate?
    Back
    10^6 to 10^14-fold
  7. Front
    Cloze: A negative ΔG tells you a reaction is ______ but says ______ about its rate.
    Back
    spontaneous ; nothing
  8. Front
    Why does a bowl of sugar NOT spontaneously burn despite combustion having a very negative ΔG°' (−5,693 kJ/mol)?
    Back
    There is no catalyst to lower the activation energy barrier , so the rate is negligible ( spontaneity ≠ speed )
  9. Front
    Define a catalyst (what is the key feature that makes an enzyme one)?
    Back
    Speeds up a reaction without being consumed or permanently altered
  10. Front
    What is the catalytic cycle of an enzyme (E, S, P)?
    Back
    E + S → ES → EP → E + P
  11. Front
    Roughly how many H2O2 molecules does one catalase enzyme convert per minute?
    Back
    ~7 million
  12. Front
    Besides speed, what TWO properties separate enzymes from a metal catalyst (e.g. why glucokinase ignores galactose , why only L-amino acids react)?
    Back
    Specificity and stereospecificity
  13. Front
    What is the inactive protein backbone of an enzyme , stripped of all cofactors , called?
    Back
    Apoenzyme
  14. Front
    What is the complete, active enzyme WITH all its cofactors called?
    Back
    Holoenzyme
  15. Front
    What is a cofactor that is tightly or covalently bound to the enzyme called?
    Back
    Prosthetic group (e.g. biotin in pyruvate carboxylase )
  16. Front
    What model describes the active site changing shape upon substrate binding (vs. a rigid lock-and-key)?
    Back
    Induced fit
  17. Front
    What enzyme is the textbook example of induced fit, closing its two lobes around glucose ?
    Back
    Hexokinase
  18. Front
    Why must the active site create a hydrophobic microenvironment that excludes water?
    Back
    Water would otherwise interfere with the reaction chemistry
  19. Front
    EC Class 1 enzymes catalyze what reaction type?
    Back
    Oxidation-reduction ( oxidoreductases )
  20. Front
    EC Class 2 ( transferases ) catalyze what reaction type?
    Back
    Transfer of functional groups from one molecule to another
  21. Front
    EC Class 3 ( hydrolases ) catalyze what reaction type?
    Back
    Cleaving bonds by adding water (hydrolysis)
  22. Front
    EC Class 4 ( lyases ) catalyze what reaction type?
    Back
    Adding/removing small molecules across double bonds (or forming double bonds), without water or oxidation
  23. Front
    EC Class 5 ( isomerases ) catalyze what reaction type?
    Back
    Interconversion of structural or geometric isomers
  24. Front
    EC Class 6 ( ligases ) catalyze what reaction type, and at what cost?
    Back
    Joining two molecules together at the cost of ATP hydrolysis
  25. Front
    TRAP: In which EC class is a PHOSPHATASE (e.g. glucose-6-phosphatase )?
    Back
    Hydrolase (uses water to cleave the phosphate)
  26. Front
    TRAP: In which EC class is a PHOSPHORYLASE (e.g. glycogen phosphorylase )?
    Back
    Transferase (transfers phosphate from inorganic phosphate , not water)
  27. Front
    TRAP: In which EC class is a KINASE (e.g. hexokinase )?
    Back
    Transferase (transfers phosphate from ATP )
  28. Front
    TRAP: A SYNTHETASE belongs to which EC class, and does it use ATP?
    Back
    Ligase (Class 6); yes, requires ATP — e.g. pyruvate carboxylase , glutamine synthetase
  29. Front
    TRAP: A SYNTHASE belongs to which EC class, and does it use ATP?
    Back
    Transferase (Class 2); no ATP — e.g. citrate synthase , glycogen synthase
  30. Front
    Protein kinases phosphorylate which three amino acid residues?
    Back
    Serine, threonine, and tyrosine
  31. Front
    Protein kinases belong to which EC class? (classic trap)
    Back
    Transferases (Class 2) — NOT oxidoreductases or ligases
  32. Front
    Which coenzymes do dehydrogenases ( oxidoreductases ) mainly use to accept/donate electrons?
    Back
    NAD+, NADP+, FAD, and FMN
  33. Front
    Cloze: A ______ oxidizes a substrate by removing hydrogen atoms; alcohol dehydrogenase ( ethanol → acetaldehyde ) uses ______.
    Back
    dehydrogenase ; NAD+
  34. Front
    What do OXIDASES transfer electrons to, often producing H2O2 ?
    Back
    O2 (molecular oxygen)
  35. Front
    Which oxidoreductase uses H2O2 as the oxidant and contains selenocysteine ?
    Back
    Glutathione peroxidase
  36. Front
    Catalase destroys H2O2 using which prosthetic group?
    Back
    Heme
  37. Front
    Aminotransferases ( transaminases ) require which coenzyme , and from which vitamin?
    Back
    Pyridoxal phosphate (PLP) , from vitamin B6
  38. Front
    Which EC Class 4 enzyme converts fumarate to malate in the TCA cycle?
    Back
    Fumarase (a hydratase/dehydratase )
  39. Front
    Pyruvate carboxylase converts pyruvate + CO2 + ATP into what product?
    Back
    Oxaloacetate (+ ADP + Pi)
  40. Front
    Which vitamin-derived prosthetic group does pyruvate carboxylase use as its CO2 carrier?
    Back
    Biotin (vitamin B7)
  41. Front
    NAD+ and NADP+ are derived from which vitamin?
    Back
    Vitamin B3 (niacin)
  42. Front
    FAD and FMN are derived from which vitamin?
    Back
    Vitamin B2 (riboflavin)
  43. Front
    Tetrahydrofolate (THF) is derived from which vitamin?
    Back
    Folic acid (folate)
  44. Front
    Coenzyme A is derived from which vitamin?
    Back
    Pantothenic acid (vitamin B5)
  45. Front
    Vitamin B12 (cobalamin) carries which type of group in transfer reactions?
    Back
    Methyl / alkyl groups (e.g. homocysteine → methionine )
  46. Front
    How many electrons and protons does NAD+ accept to become NADH ?
    Back
    Two electrons and one H+ (effectively a hydride, H−)
  47. Front
    How many electrons and protons does FAD accept to become FADH2 ?
    Back
    Two electrons and two H+
  48. Front
    Which enzyme reduces dihydrofolate to tetrahydrofolate , and what is its electron donor?
    Back
    Dihydrofolate reductase , using NADPH
  49. Front
    Dihydrofolate reductase is the target of which antifolate drug?
    Back
    Methotrexate
  50. Front
    What is the relationship between ΔG°' and the equilibrium constant?
    Back
    ΔG°' = −RT ln Keq
  51. Front
    What is the canonical coupling partner cells use to drive thermodynamically unfavorable reactions forward?
    Back
    ATP hydrolysis (ΔG°' = −30.5 kJ/mol)
  52. Front
    Coenzyme A carries which type of chemical group (as in acetyl-CoA )?
    Back
    Acyl groups
  53. Front
    What is the aminoacyl-tRNA synthetase reaction (a ligase )?
    Back
    amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi
  54. Front
    PLP transamination uses which kinetic mechanism, forming pyridoxamine phosphate as an intermediate?
    Back
    Ping-pong mechanism
  55. Front
    Cloze: ΔG < 0 means a reaction is ______, ΔG > 0 means it is ______, and ΔG = 0 means the system is at ______.
    Back
    exergonic/spontaneous ; endergonic/non-spontaneous ; equilibrium
  56. Front
    Which Class 1 ( oxidoreductase ) subclass INCORPORATES oxygen into a substrate ?
    Back
    Oxygenases
  57. Front
    What does a DECARBOXYLASE (a lyase ) remove from its substrate ?
    Back
    CO2
  58. Front
    What does a DEHYDRATASE (a lyase ) remove, and what does it form?
    Back
    Removes water (H2O) to form a double bond
  59. Front
    Clinically, why does vitamin B6 deficiency impair synthesis of most non-essential amino acids?
    Back
    Without PLP you cannot transaminate , so you cannot make the amino acids that depend on transamination
  60. Front
    What is the one exception to the rule 'an enzyme is a catalytic protein'?
    Back
    Ribozymes — catalytic RNA molecules
  61. Front
    A strongly NEGATIVE ΔG°' corresponds to a Keq that is ______, meaning equilibrium lies toward ______.
    Back
    much greater than 1 (Keq >> 1) ; products
  62. Front
    For a coupled pair of reactions, how do you find the total ΔG, and what does this allow?
    Back
    Algebraic SUM of the individual ΔG values ; a sufficiently favorable reaction can pull an unfavorable (ΔG > 0) one forward
  63. Front
    Which two lyase subtypes are named in the chapter, and what does each do?
    Back
    Decarboxylases (remove CO2) and dehydratases (remove water to form a double bond)
  64. Front
    Name the three isomerase reaction subtypes (no new bonds to other molecules are made or broken).
    Back
    Cis-trans isomerization, aldose-ketose interconversion, and epimerization
  65. Front
    Which kind of isomerase interconverts an ALDOSE and a KETOSE?
    Back
    An aldose-ketose isomerase (e.g., interconverting aldose and ketose sugars)
  66. Front
    Name the five group types transferred by transferases (Class 2).
    Back
    Amino, acyl, phosphate, one-carbon, and glycosyl groups
  67. Front
    Glycogen synthase is a glucosyltransferase — what EC class does that place it in?
    Back
    Transferase (Class 2) — it transfers glucose units, no ATP
  68. Front
    Under typical CELLULAR conditions (not standard state), the ΔG of ATP hydrolysis is closer to what value?
    Back
    About −47 to −50 kJ/mol (vs ΔG°' = −30.5 kJ/mol at standard state)
  69. Front
    Specificity example: glucokinase phosphorylates glucose but NOT which closely related sugar (differing by one hydroxyl orientation)?
    Back
    Galactose
  70. Front
    Stereospecificity: enzymes catalyze reactions of which amino acid stereoisomer, leaving the other untouched?
    Back
    L-amino acids (D-amino acids are untouched)
  71. Front
    What is the hexokinase reaction (the paradigm kinase / phosphate transfer)?
    Back
    Glucose + ATP → glucose-6-phosphate (+ ADP)
  72. Front
    What is the glucose-6-phosphatase reaction (the chapter's classic hydrolase)?
    Back
    Glucose-6-phosphate + H2O → glucose + Pi
  73. Front
    Cofactors come in two forms — name them with examples.
    Back
    Metal ions (Fe, Cu, Zn) and organic molecules called coenzymes
  74. Front
    Which two coenzymes/vitamins cooperate to convert homocysteine to methionine ?
    Back
    Tetrahydrofolate (folate) and vitamin B12 (cobalamin)
  75. Front
    Under what standard conditions is the biological standard free energy ΔG°' defined?
    Back
    pH 7, 25°C, 1 atm, 1 M concentrations
  76. Front
    How many EC classes are there now, and what was the 7th class added?
    Back
    Seven — translocases were added as Class VII (exam focus remains the original six)
  77. Front
    Which oxidoreductase subclass uses NADPH as its electron donor (e.g. dihydrofolate reductase )?
    Back
    Reductases ( NADPH is the electron donor)
  78. Front
    Tetrahydrofolate (THF) carries one-carbon units — name its two essential metabolic functions.
    Back
    Nucleotide synthesis and remethylation of homocysteine to methionine

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