Urease

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Urease ( Klebsiella aerogenes )
Urease (Klebsiella aerogenes)
Ribbon model with 2 Ni-ions (green) according to PDB  2KAU

Existing structural data: see UniProt entry

Mass / length primary structure 2319 = 3 * (567 + 106 + 100) amino acids
Secondary to quaternary structure 3 * (α + β + γ)
Cofactor 2 Ni 2+
Identifier
External IDs
Enzyme classification
EC, category 3.5.1.5 hydrolase
Response type hydrolysis
Substrate Urea + H 2 O
Products CO 2 + 2 NH 3
Occurrence
Homology family Urease alpha
Parent taxon bacteria

The urease is the enzyme , the urea in ammonia and carbon dioxide , or in ammonium - and carbonate ions splits; Since the reaction takes place in an aqueous solution, hydrogen carbonate ions are also partly formed :

or

Urease belongs to the group of amidases and is often found in plant seeds, bacteria , crustaceans and seashells. Plant urease acts as an insecticide independent of its urea-cleaving function and generates platelet aggregation in mammalian blood .

In particular, the urease in soil bacteria plays an important role in the nitrogen cycle , because without it, nitrogen fertilization using urea , a component of liquid manure, which is decomposable in the absence of enzymes , would not be possible. On the other side are urease-positive bacteria in the feces the cause of unwanted ammonia - emissions in livestock farming (in Germany in 2005, an estimated 490,000 of 590,000 tons of total ammonia emissions).

The rate of reaction is increased by a factor of 10 14 as a result of the enzyme's catalytic activity. The urease from beans (and probably also from bacteria) is a metalloenzyme and contains nickel .

The urease from jack beans was the first enzyme that could be purified and crystallized in 1926 by James Batcheller Sumner . For this there was the Nobel Prize in Chemistry in 1946.

Due to the structure of the nickel-containing center in organometallic chemistry, urease is regarded as a possible model system for the catalytic activation of carbon dioxide. Complex N-carbamates (binding of the electron-poor carbon atom of carbon dioxide to the electron-rich nitrogen atom of imines / amines) with nickel (0 or 2-valent) as the central atom exist and are structurally characterized; Infrared spectroscopic investigations have shown an energy reduction (activation) of the thermodynamically very stable C = O double bonds. However, evidence of the transfer of such “activated” carbon dioxide to other substrates is still pending.

Medical importance

Organisms (e.g. Helicobacter pylori ) that have urease are often pathogens in the digestive system because the ammonia released allows them to survive in the acidic environment of the stomach. The same applies to the pathological species of the genus Proteus that colonize the urogenital tract . This alkaline reaction of ammonia is also used to detect these organisms (see: Helicobacter urease test ).

Importance for agriculture

Urease in soil bacteria is the prerequisite for the effect of excrement as nitrogen fertilizer, since only the hydrolytic breakdown of urea makes nitrogen available as ammonium ions . The intermediate product ammonia is, however, gaseous and only small amounts of the gas can be caught by the moisture in the soil. A portion of the ammonia therefore always escapes and is lost as fertilizer. This proportion is greater, the more concentrated the raw material urea is present and that is why most of the ammonia is lost from stable floor and manure. Since ammonia is also harmful to the environment in several ways and there has been international agreement to limit emissions, attempts are being made to use urease inhibitors to solve the problem and to reduce emissions from stables.

kinetics

The reaction of urease with urea is suitable for illustrating the dependence of the reaction rate on time when a urea solution with a high concentration is selected.
Since ions are formed when urea decays in aqueous solution , the increase in ions can be followed by measuring the conductivity . The conductivity increases linearly over a longer period of time.
The reaction can also be followed
photometrically . In this case, bromothymol blue is added as an indicator , which turns increasingly blue in the course of the reaction. The extinction , a measure of the concentration, also increases linearly over a longer period of time.
When urea decomposes, the reaction rate is constant over a longer period of time. There is a zero order reaction for this.
However, it is crucial that the substrate concentration, here that of urea, is not too low. For smaller substrate concentrations, these experiments can be used to illustrate the dependence of the reaction rate on the concentration, with the time being kept constant, in accordance with the Michaelis-Menten theory .

history

The effects of urease were discovered by Fourcroy and Vauquelin in 1798 and associated with microorganisms by Louis Pasteur in 1861 , which were first described by Van Tieghem in 1864 as Micrococcus ureae . In 1876 bacterial urease was isolated by Musculus and the catalytic reaction was defined. It was only with the discovery of soybean urease by T. Takeuchi in 1909 and its subsequent production in large quantities that more detailed studies were possible.

See also

literature

  • L. Holm, C. Sander: An evolutionary treasure: unification of a broad set of amidohydrolases related to urease . In: Proteins . tape 28 , no. 1 , May 1997, p. 72-82 , PMID 9144792 .

Individual evidence

  1. C. Follmer, R. Real-Guerra, GE Wasserman, D. Olivera-Severo, CR Carlini: Jackbean, soybean and Bacillus pasteurii ureases: biological effects unrelated to ureolytic activity . In: Eur. J. Biochem. tape 271 , no. 7 , April 2004, p. 1357-1363 , doi : 10.1111 / j.1432-1033.2004.04046.x , PMID 15030486 .
  2. U. Dämmgen: Emissions from German Agriculture - National Emissions Report (NIR) 2007 for 2005. Introduction, methods and data. Agricultural research Völkenrode, SH 304
  3. U. Dämmgen: Emissions from German Agriculture - National Emissions Report (NIR) 2007 for 2005. Tab.n, Landbauforschung Völkenrode, SH 304
  4. ^ The Isolation and Crystallization of the Enzyme Urease. In: Journal of biological Chemistry. No. 69, pp. 435-441.
  5. ^ Nobel Prize Winners in Chemistry 1946 , Nobelprize.org , accessed November 1, 2012.
  6. Martin Leinker: Development of a principle solution to reduce ammonia emissions from livestock stalls with the help of urease inhibitors. Diss. Martin Luther Univ. Halle-Wittenberg 2007. urn : nbn: de: gbv: 3-000012809 (PDF)
  7. H. Hassinger, R.-D. Wiebusch: Experimental Enzymology. Diesterweg Salle & Sauerländer, 1977, ISBN 3-425-05345-0 .
  8. Dr. Bruno Lange GmbH: manual for the Lange photometer, experiment instructions.
  9. ^ William Robert Fearon: Urease. In: Biochemical Journal . Volume 17, No. 1, 1923, pp. 84-93, especially p. 84, (online) (PDF; 1.1 MB).

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