How Enzymes Lower Activation Energy Without Changing Equilibrium

Binding favors the transition state

Chemical reactions must pass through a high-energy transition state. The energy gap between the reactants and that transition state is the activation energy. Enzymes accelerate reactions by providing a pathway with a lower activation barrier.

An enzyme binds one or more substrates at an active site. The active site can orient reacting groups, create a useful chemical microenvironment, strain particular bonds, or stabilize charge that develops in the transition state. These interactions make the transition state easier to reach.

A simplified catalytic cycle is

$$E+S\rightleftharpoons ES\rightarrow EP\rightleftharpoons E+P.$$

The enzyme is available again after product release, so it acts as a catalyst rather than a consumed reactant.

Rate changes; thermodynamics do not

Lower activation energy increases the rates of both forward and reverse reactions. It does not change the free energies of the starting materials or products, the overall $\Delta G$, or the equilibrium constant. The system reaches the same equilibrium faster.

Why conditions affect enzyme activity

• Temperature: Moderate warming can increase collision frequency, but excessive heat can disrupt the enzyme's three-dimensional structure.
• pH: Changes in protonation can alter active-site charge and shape; extreme pH can denature the enzyme.
• Substrate concentration: More substrate can increase rate until available active sites become saturated.
• Inhibitors: Competitive inhibitors occupy the active site, while other inhibitors alter activity from a different binding site.

The key distinction is between kinetics and thermodynamics: enzymes change how quickly a reaction proceeds, not where the reaction's equilibrium lies.