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

In 1926 James Sumner showed that urease is a protein. Urease is found in bacteria, yeast and several higher plants.

Characteristics:

• Active site metal: nickel(II)
• 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

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 ($\backslash alpha\_\{12\}\backslash beta\_\{12\}$). 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.

As diagnostic test

Organisms that produce urease tend to be gastrointestinal or urinary tract pathogens, since urease enables them to neutralize the acid present in these acidic environments.

Urease-positive pathogens include:

• Helicobacter pylori
• Enteric bacteria including Proteus, Klebsiella and Serratia
• Ureaplasma urealyticum, a relative of the mycoplasma
• Cryptococcus, an opportunistic fungus

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