New insights in uranium bioremediation by cytochromes of the bacterium G. uraniireducens

The bacterium Geotalea uraniireducens, commonly found in uranium-contaminated environments, plays a key role in bioremediation strategies by converting the soluble hexavalent form of uranium (U^VI) into less soluble forms (e.g. U^IV.). While most of the reduction and concomitant precipitation of uranium occur outside the cells, there have been reports of important reduction processes taking place in the periplasm. In any case, the triheme periplasmic cytochromes are crucial players, either by ensuring an effective interface between the cell´s interior and exterior or by directly participating in the reduction of the metal. Therefore, understanding the functional mechanism of the highly abundant G. uraniireducens' triheme cytochromes is crucial to assist the elucidation on the respiratory pathways in this bacterium. In this work, a detailed functional characterization of the triheme cytochromes PpcA and PpcB from G. uraniireducens was conducted using NMR and visible spectroscopy techniques. Despite sharing high amino acid sequence and structural homology with their counterparts from G. sulfurreducens, the results obtained showed that the heme reduction potential values are less negative, the order of oxidation of the hemes is distinct, and the redox and redox-Bohr network of interactions revealed unprecedented functional mechanisms of the G. uraniireducens cytochromes. In these cytochromes, the reduction potential values of the three heme groups are much more similar, hence covering a narrow range of values, features that facilitate the directional electron flow from the inner membrane, thereby favouring the optimal reduction of uranium.