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Ureases (), functionally, belong to the superfamily of amidohydrolases and phosphotriesterases. It 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 hydrolysis to produce another ammonia and carbonic acid. Urease activity tends to increase the pH of its environment as it produces ammonia, a basic molecule. Ureases 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 B. Sumner, an assistant professor at Cornell University, showed that urease is a protein by examining its crystallized form.〔Karplus, P. A., Pearson, M. A., & Hausinger, R. P. (1997). 70 years of crystalline urease: What have we learned? Accounts of Chemical Research, 30(8), 330–337〕 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, and the award of the Nobel prize in chemistry to Sumner in 1946.〔(The Nobel Prize in Chemistry 1946 )〕 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.〔(nickel in biology )〕 However, in vitro activation also has been achieved with manganese and cobalt * Molecular weight: 480 kDa or 545 kDa for jack- bean Urease (calculated mass from the amino acid sequence). 840 amino acids per molecule, of which 90 are cysteines. * Optimum pH: 7.4 * Optimum Temperature: 60 degrees Celsius * Enzymatic specificity: urea and hydroxyurea * Inhibitors: heavy metals (Pb− & Pb2+) Bacterial ureases are composed of three distinct subunits, one large (α 60–76kDa) and two small (β 8–21 kDa, γ 6–14 kDa) commonly forming (αβγ)3 trimers 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), they are cysteine-rich enzymes, resulting in the enzyme molar masses between 190 and 300kDa.〔 An exceptional enzyme is the urease of Helicobacter species, which is composed of two subunits, α(26–31 kDa)-β(61–66 kDa), and has been shown to form a supramolecular dodecameric complex. of repeating α-β subunits, each coupled pair of subunits has an active site, for a total of 12 active sites.〔 (). It plays an essential function for survival, neutralizing gastric acid by allowing urea to enter into periplasm via a proton-gated urea channel. The presence of urease is used in the diagnosis of ''Helicobacter'' species. All bacterial ureases are solely cytoplasmic, except for Helicobacter pylori urease, which along with its cytoplasmic activity, has external activity with host cells. In contrast, all plant ureases are cytoplasmic.〔 Fungal and plant ureases are made up of identical subunits (~90 kDa each), most commonly assembled as trimers and hexamers. For example, Jack Bean urease has two structural and one catalytic subunit. The α subunit contains the active site, it is composed of 840 amino acids per molecule (90 cysteines), its molecular mass without Ni(II) ions amounting to 90.77 kDa. The mass of the hexamer with the 12 nickel ions is 545.34 kDa. It is structurally related to the (αβγ)3 trimer of bacterial ureases. Other examples of homohexameric structures of plant ureases are those of soybean, pigeon pea and cotton seeds enzymes.〔 It is important to note, that although composed of different types of subunits, ureases from different sources extending from bacteria to plants and fungi exhibit high homology of amino acid sequences.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「urease」の詳細全文を読む スポンサード リンク
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