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Ureazy : znaczenie, właściwości, kataliza

Krajewska B.

Wiadomości Chemiczne

2002

[Z] 56, 3-4

223-253

Urea is a ubiquitous compound in nature. It is formed in large amounts as the end product of the catabolism of nitrogen-containing compounds by most of the terrestial animals and also used in agriculture as the most common fertilizer. If it were not for its enzymatic hydrolysis that is 1014 times faster than the nonenzymatic degradation, urea would rapidly accumulate in nature causing serious environmental problems. The hydrolysis of urea is catalyzed by urease (urea amidohydrolase EC 3.5.1.5), an enzyme widely distributed in bacteria, fungi, algae, some invertebrates and higher plants, the first enzyme ever crystallized (1926) and the first one shown to contain nickel in its active site (1975). The products of the hydrolysis are ammonia and carbamate, the latter spontaneously decomposing to ammonia and carbonic acid in an uncatalyzed reaction. The products generate an increase in pH. Despite extensive research to date the mechanism of urease action has not been fully elucidated. The negative effects of both ammonia and the reaction-generated increase in pH brought about by urease activity, have profound medical and agricultural implications. Several ureolytic bacteria have been recognized as pathogenic factors in human/animal infections of urinary and gastrointestinal tracts. In the former they are involved in the urinary stone formation, catheter encrustation and pyelonephritis, and in the latter in chronic active gastritis, peptic ulcers, both caused by Helicobacter pylori, and in hepatic coma. In agriculture urease is essential for converting urea-based fertilizers to utilizable ammonia. Too rapid a hydrolysis, however, results both in plant damage by ammonia toxicity and in the alkalization of soil and finally in the loss of nitrogen by ammonia volatilization, thus creating severe environmental and economic problems. To combat these undesirable ureolytic effects the use of urease inhibitors has been proposed. Final elucidation of the structure of the urease active site and of its catalytic mechanism will substantially facilitate the design of effective urease inhibitors at a molecular level, thus replacing tedious kinetic screening. This paper summarizes the established knowledge on urease and reviews the recent findings on the enzyme.