How enzymes speed reactions without changing equilibrium
An enzyme lowers the activation energy (the energy hurdle for a reaction), so equilibrium (the point where forward and reverse reactions balance) is reached sooner but not moved.
Remember
Enzymes speed up a reaction by lowering the activation energy through stabilising the transition state (the unstable halfway arrangement between the starting molecules and the products), and they are not used up and do not change the equilibrium or the overall energy change (the net energy difference between the substrates and the products).
Memory aid
an enzyme is like a tunnel through a hill
The tunnel lowers the climb, while the start and finish stay at the same height.
Every reaction has an energy hurdle, the activation energy: the minimum energy the starting molecules (the substrates) need before they can turn into the products. Enzymes are biological catalysts, so they speed a reaction up without being used up, and they do it by lowering that hurdle.
The enzyme binds the substrate in its active site, a small cleft where the reaction takes place. There it stabilises the transition state, the unstable, high-energy arrangement that the substrate passes through on its way to becoming product. A lower barrier means a larger share of molecules can react in a given time, so the reaction runs faster.
A catalyst changes the speed, not the destination. The equilibrium is the point at which the forward and reverse reactions balance, so the ratio of products to substrates stops changing. That ratio depends on the free energy (the energy available to do work) of the substrates and the products, and the enzyme leaves both unchanged. The barrier is lowered in both directions, so the forward and reverse reactions speed up equally. The mixture reaches the same ratio sooner.
The enzyme itself is not changed by the reaction, so it can be used again in further reactions. If one amino acid lining the cleft is altered, the transition state can be stabilised less well, so the reaction slows, yet the equilibrium ratio stays the same.
Key facts
- Activation energy
- the energy hurdle reactants have to clear, which an enzyme lowers so the reaction is faster.
- Transition state
- the unstable halfway arrangement between substrates and products, which the enzyme stabilises.
- Both directions
- the forward and reverse reactions are both accelerated, so equilibrium is reached sooner.
- Equilibrium
- unchanged by an enzyme, because the free energy of the substrates and products is unchanged.
- Reuse
- an enzyme is released unchanged after each reaction, so it is not used up.
- Active site
- a small cleft built from several amino acids, so one changed amino acid can slow the reaction markedly.
Exam traps
- Enzymes do not increase the total energy released by a reaction, because that depends on the free energy of the substrates and the products, which the enzyme leaves unchanged.
- Enzymes do not make a non-spontaneous reaction spontaneous (able to proceed without an energy input) by changing the equilibrium, because they speed up the forward and reverse reactions alike and leave the equilibrium where it was.
- Enzymes lower the barrier for the reverse reaction as well as the forward one; if just the forward reaction were sped up, the equilibrium ratio would change.
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