in a catalyzed reaction the reactants are often called
II. Most enzymes are proteins. A. Protein enzymes have specific three-dimensional shapes, which are determined by the amino acid sequence and, ultimately, by the genes. B. The reactants in an enzyme-catalyzed reaction, called the substrates of the enzyme, fit into a specific pocket in the enzyme called the active site.he activation energy is the amount of energy needed by the reactant molecules to participate in a reaction. 2. In the absence of a catalyst, only a small proportion of the reactants possess the activation energy to participate in the reaction. 3.