ABSTRACT
An alternative mechanistic proposal is based on different roles played by the two Ni ions: Ni-1 binding and activating urea, Ni-2 binding and activating the nucleophile hydroxide [17,128-130] (Fig. 12B). However, this mechanism raises two problems [130]: one about the missing general base, which would deprotonate the Ni-2 -bound water molecule at the optimum pH for enzyme activity (pH 8), and the second about the role of Hisa323 as general acid, which must be protonated at pH 8 even though it has a pKa of 6.5 [27]. Therefore, according to this mechanism, only 0.3% of all urease molecules would be in the optimal protonation state for catalysis, an inconsistency justified by the “ reverse protonation hypothesis” [130].
