Dictionary Active Energy Enzyme The minimum quantity of energy that the reacting species must possess in order to undergo a specified reaction. A substance produced by a living organism that acts as a catalyst to bring about a specific biochemical reaction. Product Reactant Is a substance that is formed when two or more chemicals react. When a chemical reaction takes place, a new substance is often created from the atoms or molecules of the original substances. There are often multiple productsformed in a reaction. The substances that result from this recombination of atoms are called the products of the reaction. When chemical reactions are written down, the reactants​ are written on the left side, in a similar manner to a math equation; one ​reactant plus another. Substrate Active Site The material or substance on which an enzyme acts. See more at enzyme. The surface on or in which plants, algae, or certain animals, such as barnacles or clams, live or grow. A substrate​ may serve as a source of food for an organism or simply provide support. The part of an enzyme molecule at which catalysis of the substrate occurs. active site​ in ​Science Expand. ​active site​. The part of an enzyme where the catalytic activity on the substrate takes place.