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Query: EC:6.3.4.6 (urease)
 7,490 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Four palladium(II) aqua complexes catalyze hydrolytic decomposition of urea into carbon dioxide and ammonia. The initial rates of carbon dioxide formation at 313 K and pH 3.3 fall in the range 6.7 x 10(-)(5) to 1.6 x 10(-)(4) M min(-)(1), depending on the catalyst. The pseudo-first-order rate constant for the formation of carbon dioxide is 1.7 x 10(-)(3) min(-)(1) in the presence of 0.30 M cis-[Pd(en)(H(2)O)(2)](2+) as the catalyst at 313 K and pH 3.3. This reaction is ca. 1 x 10(5) times faster than the uncatalyzed decomposition of urea. The reaction catalyzed by cis-[Pd(en)(H(2)O)(2)](2+) is monitored by (13)C and (15)N NMR spectroscopic methods. The following steps in the mechanism of this reaction are studied quantitatively: binding of urea to the catalyst, formation of carbamic acid (H(2)NCOOH) coordinated to palladium(II) via the nitrogen atom, and conversion of this intermediate into carbon dioxide and ammonia. These products are formed also by another pathway that does not involve carbamic acid. Kinetic effects of added acid and inhibition of the reaction by addition of thiourea and of bases are interpreted quantitatively. Ammonia inhibits the decomposition. When, however, this product is sequestered by metal cations, the reaction becomes relatively fast and catalytic turnover is achieved. The most effective of these sequestering agents is the silver(I) cation. Although the simple palladium(II) complexes are very different from the enzyme urease, which contains nickel(II) ions, the decomposition of urea catalyzed by both kinds of agents involves carbamic acid as the intermediate. Kinetic and mechanistic studies with metal complexes contribute to the understanding of the enzymatic mechanism.
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PMID:Kinetics and Mechanism of Urea Hydrolysis Catalyzed by Palladium(II) Complexes. 1167 Feb 15

The palladium(II) aqua complex cis-[Pd(en)(H(2)O)(2)](2+) catalyzes the alcoholysis of urea into alkyl carbamate and ammonia. The observed rate constants for the ester formation fall in the range from 1.8 x 10(-)(5) to 5.9 x 10(-)(1) min(-)(1) at 313 K and pH 3.3, depending on the alcohol. This catalyzed reaction is at least 10(5) times faster than the uncatalyzed alcoholysis of urea under the same conditions. This is the first example of catalytic, nonhydrolytic cleavage of the amide bond in urea. The following steps in the mechanism of the methanolysis reaction are studied quantitatively: binding of urea to the catalyst in the presence of various alcohols or various concentrations of water, direct methanolysis of O-bound and N-bound urea, formation of carbamic acid (NH(2)COOH) coordinated to palladium(II) via the nitrogen atom, methanolysis of this intermediate, and the fast dissociation resulting in free methyl carbamate. Ammonia, a product of alcoholysis, inhibits this reaction by binding to palladium(II). When, however, ammonia is sequestered by the silver(I) cation, alcoholysis becomes relatively fast, and catalytic turnover is achieved. Various alcohols are compared in their reactivity toward urea. The effects of nucleophilicity, steric bulk, size, and additional hydroxyl groups (in diols) are examined. The intramolecular alcoholysis in the 2,6-dithia-1,8-octanediol complex cis-[Pd(C(6)H(14)O(2)S(2))(H(2)O)(2)](2+) results in at least 100-fold rate enhancement relative to the intermolecular alcoholysis by cis-[Pd(en)(H(2)O)(2)](2+). Alkyl carbamates do not hydrolyze further into carbamic acid and alcohol. Aryl carbamates do hydrolyze further, and this reaction requires the palladium(II) aqua complex as a catalyst. Carbamic acid then spontaneously decomposes into carbon dioxide and ammonia. Observed rate constants for the appearance and disappearance of aryl carbamates agree with the relative nucleophilicities of aryl alcohols. This study of the catalysis by a metal complex may contribute to the understanding of the metalloenzyme urease. We propose a new method, alcoholysis, for cleaving amide bonds in peptides and proteins.
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PMID:Alcoholysis of Urea Catalyzed by Palladium(II) Complexes. 1167 May 66

The tetranuclear aggregate (enH(2))[Fe(4)(mu(3)-O)(heidi)(4)(mu-O,O'-O(2)CNHC(2)H(4)NH(3))] x 4H(2)O contains a novel bidentate zwitterionic carbamic acid ligand. Magnetic studies indicate that the unsymmetrical Fe(4) core is ferrimagnetic with an S=4 ground state. Similar ligands have been obtained on rectangular tetranuclear aggregates [M(4)(mu-O)(mu-OH)(hpdta)(2)(mu-X)(2)](n-) (M[double bond]Fe, Al, Ga). The carbamic acid ligands are considered to result from the hydrolytic activation (fixation) of atmospheric CO(2) by the aggregate precursor to give a carbonato intermediate, which then reacts with the organic diamine used as base in the synthesis. Similar aggregates with acetate ligands result from hydrolytic activation of the DMA used as cosolvent. Closely related mechanisms for these two activation processes are proposed, which are also related to the accepted mechanisms for carbonic anhydrase and urease.
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PMID:Biomimetic hydrolytic activation by Fe(III) aggregates: structures, reactivity and properties of novel oxo-bridged iron complexes. 1212 74

We present a high-level quantum chemical study of possible elimination reaction mechanisms associated with the catalytic decomposition of urea at the binuclear nickel active site cluster of urease. Stable intermediates and transition state structures have been identified along several possible reaction pathways. The computed results are compared with those reported by Suarez et al. for the hydrolytic catalyzed decomposition. On the basis of these comparative studies, we propose a monodentate coordination of urea in the active site from which both the elimination and hydrolytic pathways can decompose urea into CO2 and NH3. This observation is counter to what has been experimentally suggested based on the exogenous observation of carbamic acid (the reaction product from the hydrolysis pathway). However, this does not address what has occurred at the active site of urease prior to product release. On the basis of our computed results, the observation that urea prefers the elimination channel in aqueous solution and on the observation of Lippard and co-workers of an elimination reaction channel in a urease biomimetic model, we propose that the elimination channel needs to be re-examined as a viable reaction channel in urease.
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PMID:Competitive hydrolytic and elimination mechanisms in the urease catalyzed decomposition of urea. 1767 90

Urea decomposes to ammonia and carbon dioxide via carbamic acid, and amine groups can be introduced to the glassy carbon electrode surface during the electrode oxidation of carbamic acid. This modified carbon electrode has excellent catalytic activity of the oxidation of carbamic acid, and can be used to electrooxidize urea by combining urease reaction and electrode oxidation. We found that nitrogen gas is finally produced by the carbamic acid produced from urea. The production of nitrogen was confirmed by gas chromatography-mass spectrometry, and fragment pattern of hydrazine was also detected in the electrolyzed solution of urea. We intend to describe new electrochemical conversion system of urea to harmless nitrogen gas. The electrode oxidation current of urea was decreased by addition of radical trapping agent such as DMPO (5,5-dimethyl-1-pyrroline N-oxide), and this fact suggests that carbamic acid radical couples to form nitrogen-nitrogen bond, and this dimer is oxidized to nitrogen. The electrode oxidation current of urea became larger when oxygen was removed. This fact indicates that the intermediate species (probably hydrazine) produced by the electrolysis is oxidized by not only electrode reaction but also oxygen.
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PMID:Bioelectrochemical conversion of urea to nitrogen using aminated carbon electrode. 2508 44