RECENT ADVANCES IN THE CHEMISTRY OF AN OLD ENZYME, UREASE

Urease was the first enzyme to be crystallized and shown to be a protein. Some 50 years after its crystallization, it was shown to be the first example of the nickel metalloenzymes. Whereas urea is degraded by an elimination mechanism in aqueous solution, the enzyme urease catalyzes the hydrolysis of the substrate to carbamate and ammonia as the initial products. This change of mechanism clearly points to the involvement of nickel ion in the chemistry of the enzyme, and this has been independently established. An active-site amino acid residue undergoes a decrease in pK′a on the binding of the bidentate inhibitor, acylhydoxamate; the mechanism of this acid-strengthening is discussed in detail. The now known list of substrates is large, and special interest attaches to the derivatives of phosphoric acid which are (among others) simultaneously substrates and inhibitors of the enzyme. The mechanism of this inhibition has been elucidated. An active-site peptide from the jack bean enzyme shows very high sequence homology with that from Klebsiella oxytoca in accordance with predictions based on the involvement of nickel in the chemistry of the enzyme. A method has been developed to enable the nickel ion at the active site of the enzyme to be replaced by other bivalent metal ions. This relies on the fact that although the enzyme is very stable in 20-50 mm sulfite, the metal ion is slowly lost in very dilute sulfite; the system remains completely soluble, and the electrophoretic and ultracentrifugal properties of the nickel-depleted enzyme are the same as those of the native enzyme. © 1991.