Turnover number
Encyclopedia
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 of 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 to 600,000 s−1, which means that each carbonic anhydrase molecule can produce up to 600,000 molecules of product (bicarbonate
ions) per second.
In other 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 up to 40 million for Catalase
. The term turnover frequency (abbreviated TOF) is used to refer to the turnover per unit time, as in enzymology. For most relevant industrial applications, the turnover frequency is in the range of 10−2 - 102 s−1 (enzymes 103 - 107 s−1).[1]
(AChE) may be one of the fastest enzymes. It hydrolyzes acetylcholine
to choline
and an acetate
group. One of the earliest values of the turnover number was 3 x 107 (molecules of acetylcholine) per minute per molecule of enzyme. A more recent value at 25°C, pH = 7.0, acetylcholine concentration of 2.5 x 10−3 M, was found to be 7.4 x 105 min−1.
There may be some 30 active centers per molecule. AChE is a serine hydrolase
that reacts with acetylcholine at close to the diffusion-controlled rate.
In enzymology, turnover number (also termed kcat) is defined as the maximum number of molecules of substrate
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
that an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
can convert to product per catalytic site per unit of time and can be calculated as follows: kcat = Vmax/[E]T (see Michaelis-Menten kinetics
Michaelis-Menten kinetics
In biochemistry, Michaelis–Menten kinetics is one of the simplest and 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 of enzymatic reactions, by relating...
). For example, carbonic anhydrase
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...
has a turnover number of 400,000 to 600,000 s−1, which means that each carbonic anhydrase molecule can produce up to 600,000 molecules of product (bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
ions) per second.
In other chemical fields, such as organometallic
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
, 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 up to 40 million for Catalase
Catalase
Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...
. The term turnover frequency (abbreviated TOF) is used to refer to the turnover per unit time, as in enzymology. For most relevant industrial applications, the turnover frequency is in the range of 10−2 - 102 s−1 (enzymes 103 - 107 s−1).[1]
Turnover number of acetylcholinesterase
AcetylcholinesteraseAcetylcholinesterase
"Acetylcholinesterase, also known as AChE or acetylcholine acetylhydrolase, is an enzyme that degrades the neurotransmitter acetylcholine, producing choline and an acetate group. It is mainly found at neuromuscular junctions and cholinergic nervous system, where its activity serves to terminate...
(AChE) may be one of the fastest enzymes. It hydrolyzes acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
to choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....
and an acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
group. One of the earliest values of the turnover number was 3 x 107 (molecules of acetylcholine) per minute per molecule of enzyme. A more recent value at 25°C, pH = 7.0, acetylcholine concentration of 2.5 x 10−3 M, was found to be 7.4 x 105 min−1.
There may be some 30 active centers per molecule. AChE is a serine hydrolase
Serine hydrolase
The serine hydrolase superfamily is one of the largest known enzyme families comprising approximately ~200 enzymes or 1% of the genes in the human proteome. A characteristic defining feature of this superfamily is the presence of an active site nucleophilic serine that is used for the hydrolysis of...
that reacts with acetylcholine at close to the diffusion-controlled rate.