In enzymology, the turnover number (kcat) is defined as the limiting number of chemical conversions of substrate molecules per second that a single active site will execute for a given enzyme concentration [ET] for enzymes with two or more active sites.
[1] For enzymes with a single active site, kcat is referred to as the catalytic constant.
[2] It can be calculated from the limiting reaction rate Vmax and catalyst site concentration e0 as follows:
An ideal catalyst would have an infinite turnover number in this sense, because it would never be consumed.
For most relevant industrial applications, the turnover frequency is in the range of 10−2 – 102 s−1 (103 – 107 s−1 for enzymes).
[4] The enzyme catalase has the largest turnover frequency, with values up to 4×107 s−1 having been reported.