Enzymes work much like puzzle pieces. An enzyme has a specific shape with an indented portion where a substrate (a substance that an enzyme acts upon) can fit in. Because the enzyme has a specific shape, only one substrate can fit in. (In such a way, enzymes are substrate-specific. That is, one type of enzyme only acts on one type of substrate.) The substrate attaches with the enzyme for the enzyme-substrate complex to be formed, and then the enzyme acts on the substrate. (What does acting on the substrate mean? Changing configuration of a substrate or breaking the substrate up. For example, if the substrate is sucrose, the enzyme breaks sucrose into its monomers, glucose and fructose.) The enzyme then releases the altered substrate and is ready to attach to another substrate. The presence of an enzyme serves as a catalyst for a reaction. Catalysts speed up reaction rates by a significant amount. One way that they do so is by lowering the activation energy of the reaction.
