Delivering a toxic metal to the active site of urease.

Urease is a nickel (Ni) enzyme that is essential for the colonization of Helicobacter pylori in the human stomach. To solve the problem of delivering the toxic Ni ion to the active site without diffusing into the cytoplasm, cells have evolved metal carrier proteins, or metallochaperones, to deliver the toxic ions to specific protein complexes. Ni delivery requires urease to form an activation complex with the urease accessory proteins UreFD and UreG. Here, we determined the cryo-electron microscopy structures of H. pylori UreFD/urease and Klebsiella pneumoniae UreD/urease complexes at 2.3- and 2.7-angstrom resolutions, respectively. Combining structural, mutagenesis, and biochemical studies, we show that the formation of the activation complex opens a 100-angstrom-long tunnel, where the Ni ion is delivered through UreFD to the active site of urease.

Moving nickel along the hydrogenase-urease maturation pathway.

Hydrogenases and ureases play vital metabolic functions in all three domains of life. However, nickel ions are cytotoxic because they can inactivate enzymes that require less competitive ions (e.g. Mg2+) in the Irving-Williams series to function. Life has evolved elegant mechanisms to solve the problem of delivering the toxic metal to the active site of nickel-containing enzymes inside the cells. Here, we review our current understanding of nickel trafficking along the hydrogenase and urease maturation pathways. Metallochaperones and accessory proteins (SlyD, HypA, HypB, UreD, UreE, UreF, and UreG) form specific protein complexes to allow the transfer of nickel from one protein to another without releasing the toxic metal into the cytoplasm. The role of SlyD is not fully understood, but it can interact with and transfer its nickel to HypB. In the hydrogenase maturation pathway, nickel is transferred from HypB to HypA, which can then deliver its nickel to the hydrogenase large subunit precursor. In Helicobacter pylori, the urease maturation pathway receives its nickel from HypA of the hydrogenase maturation pathway via the formation of a HypA/UreE2 complex. Guanosine triphosphate (GTP) binding promotes the formation of a UreE2G2 complex, where UreG receives a nickel from UreE. In the final step of the urease maturation, nickel/GTP-bound UreG forms an activation complex with UreF, UreD, and apo-urease. Upon GTP hydrolysis, nickel is released from UreG to the urease. Finally, some common themes learned from the hydrogenase-urease maturation pathway are discussed.