3. Enzymes, and their activities

Enzymes are potent catalysts

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.

EnzymeTurnover rate (mole product s−1 mole enzyme−1)
Carbonic anhydrase600 000
Catalase93 000

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