Which piece splits apart during a reaction that is catalyzed by an enzyme?

The piece which can be split apart during a reaction catalyzed by an enzyme is SUBSTRATE.
Many components are involved in enzyme-catalyzed reactions, these include the enzyme, the substrate, energy barrier and the products formed. The substrate is the chemical compound upon which the enzyme acts. The action of the enzyme on the substrate can be divided into two, it is either the enzyme bring two substrates together or the enzyme can split one substrate into two,depending on the type of reaction that is involved. The splitting or the addition of the substrate usually occur at the active site of the enzyme.