Urease related enzymes

The dinuclear active site of urease (1) has been studied in great detail23-29 and has inspired manifold model studies—hence a separate section, Section 6.3.4.12.7, is dedicated to the coordination chemistry related to urease. E. coli Glx I is the first example of a Ni-dependent isomerase and contains a single Ni11 ion coordinated by two histidines, two axial carboxylates of glutamic acid, and two water molecules (2).30-32 It is not active with Zn bound, which is believed to result from the inability of the Zn-substituted enzyme to bind a second aqua ligand and to adopt a six-coordinate structure. 

Hydrogenases are not the only nickel-containing enzyme, and researchers must therefore compare the maturation of different nickel proteins to obtain an integrated picture of nickel metabolism. Indeed, similarities between some of the hydrogenase-related nickel-processing proteins with urease and carbon monoxide dehydrogenase maturation factors have been noted, and this has facilitated interpretations of the results for... 

The structure of the urease active center is similar to that of adenosine deaminase, an enzyme containing one zinc(II) per active site (though see 48). This enzyme catalyzes the deamination of adenosine to inosine and NH3 (see Scheme 9), a reaction mechanistically related... 

The Enzyme Commission catalog (EC 3.5.1.5) lists the urease reaction as urea + 2 H20 = C02 + 2 NH3. Since two C-N bonds are broken it is evident that the stoichiometric relation above is the result of two component reactions. Any conjecture concerning the mechanisms of these reactions and the nature of the intermediates must encompass the action of inhibitors and the spectrum of substrates. Some of the organic inhibitors that have been reported are shown in Table I. The substrates that have been shown to be hydrolyzed are listed in Table II. 

Urease catalyzes the hydrolysis of mea to NH3 and CO2 and incorporates nF in an octahedral environment. The function of the Ni center(s) in these enzymes is thought to be analogous to that of in carbonic anhydrase, to activate the mea substrate electrostatically to nucleophilic attack. The binuclear nF complex (108) displays mease activity, firstly via conversion of mea to ammonia and cyanate, followed by a second step involving the hydrolysis of the cyanate. " In addition, a related asymmetric complex reacts with mea and allows the structmal characterization of the isocyanate product. 

The sensitivity of this method is directly related to the apparent molar enthalpy of reaction, so that very endo- or exothermic reactions will be most readily followed. Examples of the application of this method to the determination of enzyme kinetic parameters include dihydrofolate reductase, creatine phosphokinase, hexo-kinase, urease, trypsin, HIV-1 protease, heparinase, and pyruvate carboxylase. 

In addition to variations in a biofilm s polymer content related to sucrose consumption, bacteria that metabolize salivary urea or dietary arginine also affect biofilm pH. The enzyme urease converts salivary urea to ammonia and carbon dioxide (Sect. 12.1.2),... 

These were pioneer steps indeed, and great strides forward were made in the hundred years between Wohler s synthesis of urea and the isolation of the first crystalline enzyme - also, by some strange chance, one related to urea, urease - by Sumner in America in 1926. But the really explosive growth of biochemistry has had to wait on the consolidation of chemical theory, and the pushing forward of the frontiers of biology to a region where the distinction between it and chemical physiology became obscure. By the... 

Table 10.1 gives values of rate constants, activation energies, and frequency factors for three enzyme-catalyzed reactions. For comparison, the values for other catalysts are included. Note that molecule for molecule, the enzymes are much more effective catalysts than the nonbiological catalysts. In urease and catalase this higher effectiveness is related to a much smaller activation energy, which is true for a number of other enzyme systems. Enzymes evidently exert their action by allowing the process to occur by a much more favorable reaction path. 

The catalyst intervening in the decoir sition of urea was discovered in 1874 by Musculus, who found that ammoniacal urine, filtered and evaporated in a vacuum, is capable of causing the fermentation of fresh urea, giving a thick and viscous product similar to that obtained by precipitating decomposed urine with alcohol. Musculus thus established that the production of ammonia is not due exclusively to the ferment, but claims that it results from the action of a special substance of enzymic nature secreted in the bladder. Thus, while finding the presence of the enzyme, Musculus did not comprehend the relation which exists between the bacterium and the active substance. It was Miquel who definitely demonstrated that the enzyme acting on urea is indeed secreted by a micro-organism. This special catalyst was first described under the name of urase, then under that of urease. 

Immimodiagnostics and enzyme biosensors are two of the leading technologies that have a greatest impact on the food industry. The use of these two systems has reduced the time for detection of pathogens such as Salmonella to 24 hr and has provided detection of biological compounds such as cholesterol or chymotrypsin [15]. Biosensors analyses Beta lactams in milk and presence of urea in milk that lead to production of synthetic milk, the biocomponent part of the urea biosensor is an immobilized urease yielding bacterial cell biomass isolated from soil and is coupled to the ammonium ions selective electrodes of a potentiometric transducer. The membrane potential of all types of potentiometric cell based probes is related to the activity of electrochemically-detected product [16]. 

Of the latter methyl- and halogen- substituted p-benzoquinones were the most effective. Heavy metal ions and quinones probably inhibited by reacting with enzyme SH groups shown to be essential for urease activity, whereas inhibition by urea derivatives was of tbe mixed type. Inhibition by p-benzoquinone and hydroquinone was inversely related to the organic C, organic N, clay and silt contents of surface soils. ... 

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