Turnover number has two different meanings: In

enzymology Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

, turnover number (also termed ''k''_cat) is defined as the maximum number of chemical conversions of substrate molecules per second that a single

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

will execute for a given

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

concentration

_T /math> for enzymes with two or more active sites. For enzymes with a single active site, ''k''

_cat is referred to as the catalytic constant. It can be calculated from the maximum reaction rate

V_\max

and catalyst site concentration

_T /math> as follows:

: k_\mathrm = \frac (See

Michaelis–Menten kinetics In biochemistry, Michaelis–Menten kinetics is one of the best-known models of enzyme kinetics. It is named after German biochemist Leonor Michaelis and Canadian physician Maud Menten. The model takes the form of an equation describing the rate ...

). In other chemical fields, such as

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

catalysis Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

, turnover number (abbreviated ''TON'') has a 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 would never be consumed. The term turnover frequency (abbreviated ''TOF'') is used to refer to the turnover per unit time, equivalent to the meaning of turnover number in enzymology. For most relevant industrial applications, the turnover frequency is in the range of 10⁻² – 10² s⁻¹ (10³ – 10⁷ s⁻¹ for enzymes). The enzyme catalase has the largest turnover frequency, with values up to 4 × 10⁷ s⁻¹ having been reported. :

TON = \frac

TOF = \frac

Turnover number of diffusion-limited enzymes

AChE is a

serine hydrolase Serine hydrolases are one of the largest known enzyme classes comprising approximately ~200 enzymes or 1% of the genes in the human proteome. A defining characteristic of these enzymes is the presence of a particular serine at the active site, which ...

with a reported catalytic constant > 10,000/s. This implies that AChE 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 the

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

rate of its substrates. Typical catalytic constants for the different forms of this enzyme range between 10⁴ and 10⁶ reactions per second.

References

Enzyme kinetics Units of catalytic activity {{Science-stub

Turnover number of diffusion-limited enzymes

See also

References