How Enzymes Lower the Activation Barrier
Enzymes lower activation barriers through strategies such as positioning reactants, stabilising developing charges and transferring protons. Preferential stabilisation of the transition state helps catalysis. An enzyme changes the reaction path without changing the equilibrium constant.
What a learner can do afterwards
- Names three catalytic strategies and matches each to what it does to the barrier.
- Explains why tighter binding of the transition state, not of the substrate, is what speeds the reaction.
- States that a catalyst leaves the equilibrium position unchanged and says why.
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An activation barrier is the energy gap to a transition state, a brief high-energy arrangement. A high barrier can make a reaction slow. An enzyme provides a lower-barrier path without changing the free-energy difference between reactants and products.
Proximity and orientation position reacting groups. Acid-base catalysis transfers protons. Electrostatic interactions stabilise developing charges. Covalent catalysis forms a temporary enzyme-substrate bond. Induced fit means binding changes enzyme shape, which can position catalytic groups or strain the substrate. The enzyme is regenerated after its catalytic cycle.
Subtract bound-substrate energy from transition-state energy to find the barrier. Levels of 0 and 60 give 60. Lowering only the transition state to 35 gives 35. Lowering only the substrate to -15 gives 75. Tight substrate binding can hinder catalysis when transition-state stabilisation does not keep pace.
At a fixed temperature, an enzyme leaves the equilibrium constant unchanged because it leaves the free-energy difference between free reactants and products unchanged. It accelerates approach to equilibrium in either direction. Forward and reverse fluxes are equal at equilibrium; away from equilibrium, concentrations also affect their rates.
Enzymes lower activation barriers and are regenerated after catalysis. Preferential transition-state stabilisation helps reaction speed without changing the equilibrium constant.
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