Urease reaction cycle

Fig. 3. Proposed reaction cycle for urease. For urea, R = —NH2. Step 1 urea is activated toward nucleophilic attack by O coordination to a nickel ion the =N+H2 is stabilized by interaction with a protein carboxylate. Step 2 nucleophilic attack by a hydroxide ion, coordinated to the second nickel, to form a tetrahedral intermediate. Step 3 breakdown of the tetrahedral intermediate to form a coordinated carbamate ion. Step 4 hydrolysis releases carbamate ion, the initial product of urease on urea. Reproduced, with permission, from Ref. 34.

Figure 1 Model of the structure of the active site of urease, after Jabri et al. [27], and reaction cycle. (After Refs. 21,161.)...

Enzyme activity (urease, amidase, dehydrogenase, pl-glucosidase, phosphatase, arylsulfatase, fluorescein diacetate hydrolysis) Laboratory incubation Indicates potential microbial activity and nutrient cycling reactions determined in nonstandard laboratory with specialized equipment highly spatially and temporally variable dependent upon organic inputs Dick et al. (1996) Parham et aL (2002)... 

Horseradish peroxidase (and urease) played an important role in the development of the modern concept of the nature of an enzyme and the role of metal ions (Sumner and Somers, 1943 Willstatter, 1965). The species now known as compound II (HRP-II) formed as a result of the reaction of HRP with H202, was discovered in 1937 (Keilen and Mann, 1937). Later compound I (HRP-I), formed prior to HRP-II was identified (Theorell, 1941). The spectra of HRP-I and HRP-II in the 400 nm (Soret band) region have been determined (Chance, 1949 a, b) and measurements have also been extended to the visible region (Chance, 1952). Formation of HRP-I is first order in H202 and HRP (Chance, 1943) and the -OOH group is essential for the oxidation of HRP by peroxide. The enzymatic cycle can be summarised by the following equations (George, 1952),... 

Many metabolic processes such as glycolysis, Krebs cycle reactions, photosynthesis, protein synthesis, and lipid metabolism are affected by exposure to F. Much of the action of F on these processes can be attributed to F-dependent inhibition of enzymes. Examples of enzymes shown to be inhibited by F include enolase, phosphoglucomutase, phosphatase, hexokinase, PEP carboxylase, pyruvate kinase, succinic dehydrogenase, malic dehydrogenase, pyrophosphatase, phytase, nitrate reductase, mitochondrial ATPase, urease (Miller et al. 1983), lipase (Yu et al. 1987), amylase (Yu et al. 1988), invertase (Yu 1996 Ouchi et al. 1999), and superoxide dismutase (SOD) (Wilde and Yu 1998). 

Fig. 19 Effect of reaction time and number of cycles on the enzymatic activity of urease immobilized in PNIPAAm cryogel and PNIPAAm hydrogel, incubated in 10 mL urea solution (3.2 g/L) at 50 °C. Reprinted from [34] with permission from John WUey Sons...

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