Enzymes/Reagents: Enzymes are macromolecular biological catalysts. Enzymes accelerate chemical reactions. The molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. A reagent is a substance or compound added to a system to cause a chemical reaction, or added to test if a reaction occurs.