Enzymes are catalysts. They are usually proteins, though some RNA molecules act as enzymes too.Enzymes lower the activation energy of a reaction – that is the required amount of energy needed for a reaction to occur. They do this by binding to a substrate and holding it in a way that allows the reaction to happen more efficiently.
Reaction coordinate diagram showing the course of a reaction with and without a catalyst. With the catalyst, the activation energy is lower than without.Image modified from OpenStax, CC BY 3.0.The part of the enzyme where the substrate binds is called the active site. Here, the enzyme changes shape slightly, fitting tightly with the substrate and forming the enzyme/substrate complex.