Enzymes are Biological Catalysts. They increase the rate of Metabolic reactions. Almost all Biological Reactions involve Enzymes. All enzymes are Globular Proteins with a specific Tertiary Shape. They are usually specific to only one reaction.The part of the Enzyme that acts a Catalyst is called the Active Site. The rest of the Enzyme is much larger and is involved in maintaining the specific shape of of the Enzyme.When a reaction involving an Enzyme occurs, a Substrate is turned into a Product. The Substrate can be one or more molecules. The Active Site of an Enzyme is Complementary to the Substrate it catalyses.