How Enzymes Lower the Activation Barrier · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How enzymes lower the activation barrier

Science · Biochemistry & Molecular Biology · ages 18-20
Name ______________________   Date ____________

  1. An active-site group donates a proton during bond cleavage. Which catalytic strategy is this?

    • Covalent catalysis through a temporary enzyme-substrate bond
    • Acid-base catalysis through proton transfer
    • Proximity catalysis through positioning two reacting groups

  2. A reaction is slow at body temperature because it has a high activation barrier. How can an enzyme help?

    • By raising the temperature throughout the cell
    • By providing a path with a lower barrier
    • By changing the equilibrium constant toward products

  3. An enzyme may form a temporary covalent bond during catalysis and still be regenerated after the cycle.

    Circle one:   True   False

  4. A change stabilises the transition state but leaves the bound-substrate energy unchanged. What happens to the barrier?

    • It increases because the starting state has become lower
    • It stays fixed because transition-state binding affects only equilibrium
    • It decreases because the transition state is lower

  5. A change stabilises the bound substrate but leaves the transition-state energy unchanged. What happens to the barrier from the bound state?

    • It increases because the starting state is lower
    • It decreases because the transition state is lower
    • It stays fixed because binding never affects energy barriers

  6. Why does an enzyme leave the equilibrium constant unchanged at a fixed temperature?

    • It leaves the reactant-product free-energy difference unchanged
    • It prevents products from reacting in the reverse direction
    • It changes product concentrations until every substrate molecule disappears

  7. Using one common energy scale, site A has bound-substrate energy -10 and transition-state energy 20. Site B has levels -25 and 20. Which has the lower barrier from its bound state?

    • Site B: its barrier is 45, below site A's barrier of 30
    • Both sites: their barriers are equal because their transition states match
    • Site A: its barrier is 30, below site B's barrier of 45

  8. Substrate binding changes an enzyme's shape and positions its catalytic groups. Which model describes this?

    • Rigid lock-and-key binding without any shape change
    • Induced fit during substrate binding
    • Competitive inhibition by a molecule that replaces the substrate
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Answer key

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How enzymes lower the activation barrier W1-mt_uwB5TXF3Nb-s1

  1. Acid-base catalysis through proton transfer · Donating or accepting a proton is acid-base catalysis.
  2. By providing a path with a lower barrier · The enzyme provides a lower-barrier reaction path without heating the cell or changing the equilibrium constant.
  3. True · A temporary chemical change is compatible with catalysis if the enzyme is regenerated.
  4. It decreases because the transition state is lower · Lowering the transition-state energy while holding the starting state fixed reduces the activation barrier.
  5. It increases because the starting state is lower · The barrier is the difference between transition-state and bound-substrate energies. Lowering only the substrate increases that difference.
  6. It leaves the reactant-product free-energy difference unchanged · The equilibrium constant depends on the free-energy difference between reactants and products, which the enzyme does not change.
  7. Site A: its barrier is 30, below site B's barrier of 45 · Site A has barrier 20 - (-10) = 30. Site B has barrier 20 - (-25) = 45.
  8. Induced fit during substrate binding · Induced fit describes a change in enzyme shape associated with substrate binding.
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