ABSTRACT
In the active site of native BPU [21] (PDB code 2UBP, Fig. 3A), two nickel ions are bridged by the carbamylated Lysa220 and a hydroxide ion, with an Ni-Ni distance of 3.7 A. One Ni ion (Ni-1) is further bound to Hisa249 and Hisa275, while Ni-2 is bound to Hisa137, Hisa139, and Aspa363. The coordination geometry of each Ni ion (pseudosquare-pyramidal for the pentacoordinate Ni-1 and pseudooctahedral for the hexacoordinate Ni-2) is completed by one water molecule. A fourth water molecule is bound to the Ni-bound solvent molecules by H-bonds and completes a tetrahedral cluster of water/hydroxide molecules. Several structures of KAU, mutated at the carbamylated lysine residue bridging the two Ni ions in the active site, have revealed that the importance of such a ligand does not rest in the peculiar electron donor capability of the carbamate functional moiety; instead it simply serves to hold the metal ions close to each other for optimal bimetallic catalysis [19].
