20. Enzymes are globular proteins that act as biological catalysts. Every chemical reaction requires that a certain amount of energy, called activation energy, be absorbed to prime the reaction. The activation energy pushes the reactants to an energy level where their random collisions are forceful enough to ensure interaction. Enzymes allow reactions to occur at normal body temperature by decreasing the amount of activation energy required. Three steps appear to be involved in the mechanism of enzyme action. First, the enzyme's active site must bind with the substance on which it acts. These substances are the substrates of the enzyme. This causes the active site to change shape so that the substrate and the active site fit together. Then, the enzyme-substrate complex undergoes internal rearrangements that form the product. The enzyme next releases the product of the reaction. If the enzyme became part of the product it would be a reactant and not a catalyst.