Enzymes. Protein catalysts. (A few RNA molecules also possess catalytic activity). In an enzyme-mediated reaction, an enzyme binds to reactants (substrates) to form an enzyme-substrate complex, which breaks down to release products and the enzyme.The region of the enzyme to which the substrate binds is called the active site, the shape of which determines the chemical specificity of the enzyme.Cofactors. Substances that bind to enzymes to alter their conformations and make them active. Some cofactors are trace elements.hemical reactions involve: (1) the breaking of chemical bonds in reactant molecules and (2) making of new chemical bonds to form product molecules. Energy is either added or released as heat during chemical reactions.