An enzyme is a biomolecule that acts as a catalyst to speed up a reaction, without undergoing any change itself. 

The transition state is a high-energy, unstable state that molecules must pass through for a chemical reaction to take place. It represents the point of maximum energy in a reaction. 

Enzymes play a crucial role in helping molecules reach the transition state by lowering the activation energy, which is the energy molecules require to reach the transition state. By lowering this energy barrier, enzymes help to initiate the reaction and also accelerate the rate of the reaction.