Structure-based rationalization of urease inhibition by phosphate: novel insights into the enzyme mechanism

The structure of Bacillus pasteurii urease (BPU) inhibited with phosphate was solved and refined using synchrotron X-ray diffraction data from a vitrified crystal (1.85 Angstrom resolution, 99.3% completeness, data redundancy 4.6 R-factor 17.3%, PDB code 6UBP). A distance of 3.5 Angstrom separates the two Ni ions in the active site. The binding mode of the inhibitor involves the formation of four coordination bonds with the two Ni ions: one phosphate oxygen atom symmetrically bridges the two metal ions (1.9-2.0 Angstrom), while two of the remaining phosphate oxygen atoms bind to the Ni atoms at 2.4 Angstrom. The fourth phosphate oxygen is directed into the active site channel. Analysis of the H-bonding network around the bound inhibitor indicates that phosphate is bound as the H2PO4 anion, and that an additional proton is present on the O delta2 atom of Asp(alpha 363), an active site residue involved in Ni coordination through O delta1. The flexible flap flanking the active site cavity is in the open conformation. Analysis of the complex reveals why phosphate is a relatively weak inhibitor and why sulfate does not bind to the nickels in the active site. The implications of the results for the understanding of the urease catalytic mechanism are reviewed. A novel alternative for the proton donor is presented.