Therefore, a catalyst in this case, sulfuric acid, can be used to accelerate a reversible reaction such as ester formation or its inverse, ester hydrolysis: 6. 6. 1: Acid-catalyzed reactions. Enzymes are powerful catalysts. The enormous catalytic activity of enzymes can perhaps best be expressed by a constant, k cat, which is known as the turnover rate, turnover frequency, or turnover number. This constant represents the number of substrate molecules that can be converted to product by a single enzyme molecule.