Turnover number

In chemistry, the term "turnover number" has two distinct meanings.

In

It can be calculated from the limiting reaction rate V_max and catalyst site concentration e₀ as follows:

$${\displaystyle k_{\mathrm {cat} }={\frac {V_{\max }}{e_{0}}}}$$

(See

In other chemical fields, such as organometallic catalysis, turnover number (TON) has a different meaning: the number of moles of substrate that a mole of catalyst can convert before becoming inactivated:^[3]

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

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

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

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.^[5]

Turnover number of diffusion-limited enzymes

Acetylcholinesterase is a serine hydrolase with a reported catalytic constant greater than 10⁴ s⁻¹. This implies that this enzyme reacts with acetylcholine at close to the diffusion-limited rate.^[6]

Carbonic anhydrase is one of the fastest enzymes, and its rate is typically limited by the

• Catalysis

References

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