Change number
The turnover number (engl. Turnover frequency , abbreviated. TOF ) describes in the catalysis the number of formula sales (or catalytic cycles) that a certain catalyst per period and catalytically active center can accelerate. The turnover rate is a measure of the efficiency of a catalytic converter .
Homogeneous organometallic complex catalysis
In homogeneous organometallic complex catalysis, every complex molecule is usually catalytically active. The number of turns is measured e.g. B. as the volumetric reaction rate per catalytically active complex ; the SI unit of the turnover number is thus s −1 .
Heterogeneous catalysis
In heterogeneous catalysis , the turnover number is determined as the conversion per active center per catalyst volume.
The number of active metal centers is z. B. by hydrogen - or carbon monoxide adsorption can be determined, the number of active acid centers z. B. by the adsorption of ammonia .
Biocatalysis
In biochemistry , the quotient of the turnover number (k cat ) and the Michaelis-Menten constant (K m ) characterizes the performance or catalytic efficiency of an individual enzyme molecule .
In the case of enzymatic reactions, instead of the total number of possible conversions, the number of conversions per time period is determined. In this context, the turnover rate is the rate constant of the enzymatic reaction. This is why we speak of molecular activity (k cat ) here. From a mathematical point of view, the value describes the maximum enzyme turnover per time which can no longer be increased by adding more substrate .
Examples
| enzyme | Substrate | Alternation number (k cat ) in s −1 |
k cat / K m in s −1 mol −1 l |
source |
|---|---|---|---|---|
| Carbonic anhydrase | CO 2 | 1,000,000 | 76,000,000 | |
| " | HCO 3 - | 200,000 | 10,000,000 | " |
| Catalase | H 2 O 2 | 10,000,000? | 40,000,000 | |
| Acetylcholinesterase | AcCh | 25,000 | 50,000,000 | |
| Urease | urea | 3,000 | 1,250,000 | |
| Fumarase | Fumarate | 1150 | 5,560,000 | |
| " | S-malate | 600 | 700,000 | " |
| Kinases | ATP | 1,000 | - | ? |
| Trypsin , chymotrypsin | Proteins | 100-1,000 | - | ? |
| Dehydrogenases | NADH, H + , FADH 2 | 1,000 | - | ? |
| DNA polymerases | DNA, NTP | 10-10,000 | - | ? |
| - Pol I | " | 15th | - | ? |
| - Pol III | " | 10,000 | - | ? |
| Myosin -ATPase | ATP | 100 | - | ? |
| human aldolase A | Fructose 1,6 bisphosphate | 60 | 1,150,000 | |
| " | Fructose-1-phosphate | 1.3 | 50 | " |
| human aldolase B | Fructose 1,6 bisphosphate | 13 | 1,100,000 | " |
| " | Fructose-1-phosphate | 11 | 3,000 | " |
| human aldolase C | Fructose 1,6 bisphosphate | 20th | 1,500,000 | " |
| " | Fructose-1-phosphate | 2.7 | 150 | " |
| Lysozyme | Murein | 0.5 | - | ? |
See also
Individual evidence
- ↑ M. Boudart: Turnover Rates in Heterogeneous Catalysis. In: Chemical Reviews. 95, 1995, pp. 661-666, doi : 10.1021 / cr00035a009 .
- ↑ "The Catalytic Properties of Murine Carbonic Anhydrase VII"; Earnhardt, JN et al., Biochemistry 37 (1998), pp. 10837-10845 doi : 10.1021 / bi980046t (English).
- ↑ Stryer Biochemistry. Elsevier, 2006, ISBN 978-3-8274-1800-5 , p. 246
- ↑ "Kinetic and Structural Characterization of Urease Active Site Variants"; Matthew A. Pearson et al., Biochemistry 39 (2000), pp. 8575-8584, doi : 10.1021 / bi000613o (English).
- ↑ "X-ray crystallographic and kinetic correlation of a clinically observed human fumarase mutation"; Marcel Estévez et al., Protein Science 11 (2002), pages 1552-1557 doi : 10.1110 / ps.0201602 (English).
- ↑ "Human Aldolase C: Characterization of the Recombinant Enzyme Expressed in Escherichia coli"; Takahiro Kusakabe et al., J. Biochem. 115 (1994), number 6, pages 1172-1177 PMID 7982900 (English).