Urease

Helicobacter Pylori Urease drawn from PDB: 1E9Z​.

Urease (EC 3.5.1.5) is an enzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia. The reaction occurs as follows:

(NH₂)₂CO + H₂O → CO₂ + 2NH₃

More specifically, urease catalyzes the hydrolysis of urea to produce ammonia and carbamate, the carbamate produced is subsequently degraded by spontaneous hyrdolysis to produce another ammonia and carbonic acid. Urease activity tends to increase the pH of the environment in which it is as it produces ammonia, as it is a basic molecule.Ureases are are found in numerous bacteria, fungi, algae, plants and some invertebrates, as well as in soils as a soil enzyme. They are nickel-containing metalloenzymes of high molecular weight.

In 1926, James Sumneran assistant professor at Cornell college showed that urease is a protein by examining its crystallized form. Sumner's work was the first demonstration that a pure protein can function as an enzyme, and led eventually to the recognition that most enzymes are in fact proteins. The structure of urease was first solved by P. A. Karplus in 1995.Urease was the first ever enzyme crystallized.

Characteristics

• Active site requiring nickel in Jack Beans and several bacteria.
• Molecular weight: 480 kDa or 545 kDa for Jack Bean Urease (calculated mass from the amino acid sequence).
• Optimum pH: 7.4
• Optimum Temperature: 60 degrees Celsius
• Enzymatic specificity: urea and hydroxyurea
• Inhibitors: heavy metals (Pb^- & Pb²⁺)

The multi-subunit enzyme usually has a 3:3 (alpha:beta) stoichiometry with a 2-fold symmetric structure (note that the image above gives the structure of the asymmetric unit, one-third of the true biological assembly). An exceptional urease is found in Helicobacter pylori, which combines four of the regular six-subunit enzymes in an overall tetrahedral assembly of 24 subunits ($\alpha _{12}\beta _{12}$). This supra-molecular assembly is thought to confer additional stability for the enzyme in this organism, which functions to produce ammonia in order to neutralise gastric acid. The presence of urease is used in the diagnosis of Helicobacter species.

Urease Activity

The kcat/Km of urease in the processing of urea is 10¹⁴ times greater than the rate of the uncatalyzed elimination reaction of urea.There are many reasons for this observation in nature. The proximity of urea to active groups in the active site along with the correct orientation of urea allow hydrolysis to occur rapidly. Urea alone is very stable due to the resonance forms it can adopt. The stability of urea is understood to be due to its resonance energy, which has been estimated at 30-40 kcal/mol.This is beacuase the zwitterionic resonance forms all donate electrons to the carbonyl carbon making it less of a electrophile making it less reactive to nucleophilic attack.

Action of Urease in Pathogenesis

Bacterial ureases are most often the mode of pathogenesis for many medical conditions. They are associated with hepatic encephalopathy / Hepatic coma, infection stones, and peptic ulceration.

Infection Stones

Infection induced urinary stones are a mixture of struvite (MgNH₄PO₄•6H₂O) and carbonate apatite [Ca₁₀(PO₄)6•CO₃].These polyvalent ions are soluble but become insoluble when ammonia is produced from microbial urease during urea hydrolysis, as this increases the surrounding environments pH from roughly 6.5 to 9. The resultant alkalinization results in stone crystallization. In humans the microbial urease, Proteus mirabilis, is the most common in infection induced urinary stones.

Urease in Hepatic Encephalopathy/ Hepatic coma

Studies have shown that Helicobacter pylori along with cirrhosis of the liver cause hepatic encephalopathy and hepatic coma. Heliobacter pylori are microbial ureases found in the stomach. As ureases they hydrolyze urea to produce ammonia and carbonic acid. As the bacteria are localized to the stomach ammonia produced is readily taken up by the circulatory system from the gastric lumen. This results in elevated ammonia levels in the blood and is coined as hyperammonemia, eradication of Heliobacter pylori show marked decreases in ammonia levels.

Urease in Petic Ulcers

Helicobacter pylori is also the cause of peptic ulcers with its manifestation in 55%-68% reported cases. . This was confirmed by decreased ulcer bleeding and ulcer reoccurrence after eradication of the pathogen. In the stomach there is an increase in pH of the mucosal lining as a result of urea hydrolysis this prevents movement of hydrogen ions between gastric glands and gastric lumen. In addition, the high ammonia concentrations have an effect on intercellular tight junctions increasing permeability and also disrupting the gastric mucous membrane of the stomach.

As diagnostic test

Main article: Rapid urease test

Many gastrointestinal or urinary tract pathogens produce urease, enabling the detection of urease to be used as a diagnostic to detect presence of pathogens.

Urease-positive pathogens include:

• Proteus mirabilis and Proteus vulgaris
• Ureaplasma urealyticum, a relative of Mycoplasma spp.
• Nocardia
• Campylobacter ureolyticus
• Cryptococcus spp., an opportunistic fungus
• Helicobacter pylori
• Certain Enteric bacteria including Proteus spp., Klebsiella spp., Morganella, Providencia, and possibly Serratia spp.
• Brucella

Other uses

Urease conductometric biosensors for detection of heavy-metal ions

Urease conductometric biosensors for detection of heavy-metal ions consisting of interdigitated gold electrodes and enzyme membranes formed on their sensitive parts have been used for a quantitative estimation of general water pollution with heavy-metal ions. The measurements of the urease residual activity have been carried out in Tris-HNO_3> buffer after preincubation in model metal-salt solution. The detection limits, depending on preincubation time and dynamic ranges, have been determined in model solutions of heavy-metal ions. The sequence of metals ions relative to their toxicity toward urease is: Hg²⁺ > Cu²⁺ > Cd²⁺ > Co²⁺ > Pb²⁺ > Sr²⁺ > . The conditions for practical applications of the biosensors have been investigated and critically evaluated for optimization. Urease reactivation by EDTA after inhibition by heavy-metal ions has been demonstrated. The performance characteristics of the conductometric biosensor are discussed by G. A. Zhylyaka, S. V. Dzyadevichb, Y. I. Korpana, A. P. Soldatkina and A. V. El'skayaa in their paper.

See also

• Urea carboxylase
• Allophanate hydrolase

References

• Zimmer, M. (2000). Molecular mechanics evaluation of the proposed mechanisms for the degradation of urea by urease. Journal of Biomolecular Structure and Dynamics, 17(5), 787-797.
• Krajewska, B., Van Eldik, R., & Brindell, M. (2012). Temperature- and pressure-dependent stopped-flow kinetic studies of jack bean urease. implications for the catalytic mechanism. Journal of Biological Inorganic Chemistry, 17(7), 1123-1134.