Turnover number has two related meanings:
In enzymology, turnover number (also termed kcat) is defined as the maximum number of molecules of substrate that an enzyme can convert to product per catalytic site per unit time and can be calculated as follows: kcat = Vmax/[E]T (see Michaelis-Menten kinetics). For example, carbonic anhydrase has a turnover number of 400,000 s-1, which means that each carbonic anhydrase molecule can produce up to 400,000 molecules of product (CO2) per second.
In more chemical fields, such as organometallic catalysis, turnover number (abbreviated TON) is used with a slightly different meaning: the number of moles of substrate that a mole of catalyst can convert before becoming inactivated. An ideal catalyst would have an infinite turnover number in this sense, because it wouldn't ever be consumed, but in actual practice one often sees turnover numbers which go from 100 to a million or more. The term turnover frequency (abbreviated TOF) is used to refer to the turnover per unit time, as in enzymology.
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