2.3 Energetics of enzyme-catalyzed reactions

With biochemical reactions, as with all chemical reactions, energy comes in with two questions:

Will the reaction proceed at all in the desired direction?
If it does, will it at a sufficient rate?

The first question is decided by the free energy of the reaction, ΔG, which when negative will make the reaction go forward. The second question depends on the activation energy, ΔG^*, which forms a barrier between the two states of the reactants. The very short-lived, energy-rich state at the top of this barrier is called the transition state. Enzymes can make a big difference to the activation energy and thus accelerate reactions, but they cannot change the free energy (and therefore, the direction) of the reaction.