The enormous catalytic activity of enzymes can perhaps best be expressed by a constant, kcat, that is variously referred to 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 per unit time (usually per minute or per second). Examples of turnover rate values are listed in Table 1. For example, a single molecule of carbonic anhydrase can catalyse the conversion of over half a million molecules of its substrates, carbon dioxide (CO2) and water (H2O), into the product, bicarbonate (HCO3−), every second—a truly remarkable achievement.
Table 1.
Turnover rate of some common enzymes showing wide variation.
| Enzyme | Turnover rate (mole product s−1 mole enzyme−1) |
|---|---|
| Carbonic anhydrase | 600 000 |
| Catalase | 93 000 |
| β–galactosidase | 200 |
| Chymotrypsin | 100 |
| Tyrosinase | 1 |