Binding of ruthenium and osmium at non­iron sites of transferrin accounts for their iron-independent cellular uptake.

Being identified with less toxic and generally showing selective effects for solid tumor metastases, ruthenium and osmium compounds are promising drug candidates for clinical uses. Human serum proteins, such as albumin and transferrin, play vital roles in the transportation and accumulation of ruthenium and osmium agents into target tissues. However, the molecular mechanism of how transferrin transport ruthenium and their osmium analogues at atomic level remains obscure. In this study, we uncovered that the cellular uptake of Os3+ or Ru3+ are not competed by Fe3+. To unveil the molecular mechanism behind the phenomena, we report the first crystal structures of human serum transferrin (hTF) in complex with ruthenium and osmium compounds bound to the non-conserved residues on the surface of hTF without altering its overall conformation. As for Ru3+ and Os3+, these binding sites by descending affinity are: His14/His289, His349-350 ~ His578/Arg581. Ruthenium drugs and their osmium analogues preferentially bind to His14/His289 with bipyridine or imidazole ligands leaving. These binding sites on hTF surface are also available in human lactoferrin and some transferrin family member of other species. The presence of these binding sites makes the cellular uptake of Ru3+ and Os3+ less affected by Fe3+, compare to Zr4+ or Hf4+. Collectively, these findings are critical for our understanding of the role of serum transferrin in cellular delivery of ruthenium and osmium anticancer agents.

Bismuth antimicrobial drugs serve as broad-spectrum metallo-ß-lactamase inhibitors.

Drug-resistant superbugs pose a huge threat to human health. Infections by Enterobacteriaceae producing metallo-ß-lactamases (MBLs), e.g., New Delhi metallo-ß-lactamase 1 (NDM-1) are very difficult to treat. Development of effective MBL inhibitors to revive the efficacy of existing antibiotics is highly desirable. However, such inhibitors are not clinically available till now. Here we show that an anti-Helicobacter pylori drug, colloidal bismuth subcitrate (CBS), and related Bi(III) compounds irreversibly inhibit different types of MBLs via the mechanism, with one Bi(III) displacing two Zn(II) ions as revealed by X-ray crystallography, leading to the release of Zn(II) cofactors. CBS restores meropenem (MER) efficacy against MBL-positive bacteria in vitro, and in mice infection model, importantly, also slows down the development of higher-level resistance in NDM-1-positive bacteria. This study demonstrates a high potential of Bi(III) compounds as the first broad-spectrum B1 MBL inhibitors to treat MBL-positive bacterial infection in conjunction with existing carbapenems.

"Anion clamp" allows flexible protein to impose coordination geometry on metal ions.

X-ray crystal structures of human serum transferrin (77 kDa) with Yb(III) or Fe(III) bound to the C-lobe and malonate as the synergistic anion show that the large Yb(III) ion causes the expansion of the metal binding pocket while octahedral metal coordination geometry is preserved, an unusual geometry for a lanthanide ion.

UreE-UreG complex facilitates nickel transfer and preactivates GTPase of UreG in Helicobacter pylori.

The pathogenicity of Helicobacter pylori relies heavily on urease, which converts urea to ammonia to neutralize the stomach acid. Incorporation of Ni(2+) into the active site of urease requires a battery of chaperones. Both metallochaperones UreE and UreG play important roles in the urease activation. In this study, we demonstrate that, in the presence of GTP and Mg(2+), UreG binds Ni(2+) with an affinity (Kd) of ∼0.36 µm. The GTPase activity of Ni(2+)-UreG is stimulated by both K(+) (or NH4 (+)) and HCO3 (-) to a biologically relevant level, suggesting that K(+)/NH4 (+) and HCO3 (-) might serve as GTPase elements of UreG. We show that complexation of UreE and UreG results in two protein complexes, i.e. 2E-2G and 2E-G, with the former being formed only in the presence of both GTP and Mg(2+). Mutagenesis studies reveal that Arg-101 on UreE and Cys-66 on UreG are critical for stabilization of 2E-2G complex. Combined biophysical and bioassay studies show that the formation of 2E-2G complex not only facilitates nickel transfer from UreE to UreG, but also enhances the binding of GTP. This suggests that UreE might also serve as a structural scaffold for recruitment of GTP to UreG. Importantly, we demonstrate for the first time that UreE serves as a bridge to grasp Ni(2+) from HypA, subsequently donating it to UreG. The study expands our horizons on the molecular details of nickel translocation among metallochaperones UreE, UreG, and HypA, which further extends our knowledge on the urease maturation process.