Urease enzyme regulation

Several structures of ureases are available (2). In all cases, the active site contains two Ni(II) ions bridged by the carboxylate group of a carbamylated lysine and by a hydroxide ion (Fig. lA). Each Ni is also coordinated by two histidines and one water molecule, whereas Ni(2) is further bound to an aspartate, resulting in a pentacoordinate Ni(l) and hexacoordinate Ni(2). In the resting state of the enzyme from Bacillus pasteurii, the active site accommodates a fourth water molecule, completing a tetrahedral cluster of solvent molecules (12). The access to the active site is regulated by a flexible helix-loop-helix motif, the position of other amino acids involved in the catalysis being also critically affected by the flap movement. 

Several other biosensors for pesticide and toxic metal monitoring are also based on the inhibition of enzymes such as urease for heavy metals, tyrosinase for benzoic acid, thiourea and 2-mercaptoethanol, alcohol dehydrogenase for cyanides and heavy metal salts, amino oxidase for plant growth regulators, aldehyde dehydrogenase for fungicides, cytochrome c for cyanides, catalase for heavy metals, and peroxidase for cyanides and heavy metals. Thus, biosensors based on the inhibition of enzymes, suffer from false-positive results. Despite lack of selectivity, this type of biosensor is powerful when rapid toxicity screening is required. 

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