Turnover number

Inchemistry,the term "turnover number"has two distinct meanings.

Inenzymology,the turnover number (k_cat) is defined as the limiting number of chemical conversions ofsubstratemolecules per second that a singleactive sitewill execute for a givenenzymeconcentration[E_T]for enzymes with two or more active sites.For enzymes with a single active site,k_catis referred to as thecatalytic constant.It can be calculated from the limiting reaction rateV_maxand catalyst site concentratione₀as follows:

$$k_{\mathrm {cat} }={\frac {V_{\max }}{e_{0}}}$$(SeeMichaelis–Menten kinetics).

In other chemical fields, such asorganometalliccatalysis,turnover number (TON) has a different meaning: the number ofmolesof substrate that a mole of catalyst can convert before becoming inactivated:

$$\mathrm {TON} ={\frac {n_{\mathrm {product} }}{n_{\mathrm {cat} }}}$$

An ideal catalyst would have an infinite turnover number in this sense, because it would never be consumed. The termturnover frequency(TOF) is used to refer to the turnover per unit time, equivalent to the meaning of turnover number in enzymology.

$$\mathrm {TOF} ={\frac {\mathrm {TON} }{t}}$$

For most relevant industrial applications, the turnover frequency is in the range of10⁻²– 10²s⁻¹(10³– 10⁷s⁻¹for enzymes).The enzymecatalasehas the largest turnover frequency, with values up to 4×10⁷s⁻¹having been reported.

Turnover number of diffusion-limited enzymes

See also:Diffusion limited enzyme

Acetylcholinesteraseis aserine hydrolasewith a reported catalytic constant greater than 10⁴s⁻¹.This implies that this enzyme reacts with acetylcholine at close to the diffusion-limited rate.

Carbonic anhydrase is one of the fastest enzymes, and its rate is typically limited by thediffusionrate of itssubstrates.Typical catalytic constants for the different forms of this enzyme range between 10⁴s⁻¹and 10⁶s⁻¹.

See also

• Catalysis