A molecular level mechanism for uranium (VI) toxicity through Ca2 + displacement in pyrroloquinoline quinone-dependent bacterial dehydrogenase

• UO22 + displaces Ca2 + on pyrroloquinoline quinone resulting in acute uranium toxicity.
• The properties that make this active site susceptible to uranium toxicity were investigated.
• The UO22 + displacement of other biologically important metal cations was examined.
• Results show that UO22 + has a distinctly higher binding affinity for the active site.
• Integrated atomic based models open the path to identify other potential uranium interactions.

Dipicolinic acid (DPA), a small molecule analogue for the pyrroloquinoline quinone (PQQ) bacterial dehydrogenase cofactor, was used to model displacement of the complexing ion, Ca2 +, by a uranium (VI) dioxo-cation, UO22 +. Complexation of UO22 + with DPA through the displacement of Ca2 + was examined with UV/visible spectroscopy, ESI (electrospray ionization)-Mass spectrometry, and density functional theory based-modeling. The UO22 + displacement of other biologically important metal cations (Zn2 +, Cu2 +, Ni2 +, and Fe3 +) from DPA was also examined. Results show that UO22 + has a distinctly higher binding affinity (logβ = 10.2 ± 0.1) for DPA compared to that of Ca2 + (logβ = 4.6 ± 0.1), and provide molecular level insight into the mechanism of uranium toxicity associated with the {ONO} site. These results support those of VanEngelen et al. (2011) where a key interaction between PQQ and UO22 + produced significant uranium toxicity in bacteria. The observed toxicity mechanism was determined to be the displacement of a Ca2 + cation bound to the {ONO} site on PQQ and was observed even at submicromolar UO22 + concentrations. Here we couple experimental findings with density functional theory (DFT) calculations to investigate the electronic and structural properties that make the {ONO} site so distinctively favorable for UO22 + binding. This novel approach using integrated experimental and fundamental atomic based models opens the path to identify a library of potential uranium interactions with critical biological molecules.

Dipicolinic acid (DPA), an analogue for pyrroloquinoline quinone (PQQ) was used to model displacement of the complexing ion, Ca2 +, by UO22 +. Results show that UO22 + has a distinctly higher binding affinity for DPA compared to that of Ca2 + and provide molecular insight into the mechanism of uranium toxicity.Figure optionsDownload as PowerPoint slide