Turnover Number

The turnover number of an enzyme, kcat, gives a direct measure of the catalytic production of product under optimum conditions (saturated enzyme). The units of kcat are seconds^-1. The reciprocal of kcat can be thought of as the time required by an enzyme molecule to "turn over" one substrate molecule. Alternatively, kcat measures the number of substrate molecules turned over per enzyme molecule per second. Thus, kcat is sometimes called the turnover number. Some typical turnover numbers are shown in Table 11.1, as well.