In an enzyme-catalyzed reaction, the reactant is called the substrate. The substrate finds and bonds to the active site of the enzyme, and it then forms an enzyme-substrate complex.
When an enzyme catalyzes a chemical reaction, the enzyme helps the reaction take place and often speeds up the rate of reaction. The active site of an enzyme is a specific area that the substrate fits into. When the substrate fits into the active site and forms the enzyme-substrate complex, the substrate molecules are oriented and bonds are weakened. This catalyzes the reaction because new bonds are then able to form more easily.