Mechanism of Reduction of Aqueous U(V)-dpaea and Solid-Phase 2 U(VI)-dpaea Complexes: The Role of Multiheme c‑Type Cytochromes

The biological reduction of soluble U(VI) complexes to form immobile U(IV) species has been proposed to remediate contaminated sites. It is well established that multiheme c-type cytochromes (MHCs) are key mediators of electron transfer to aqueous phase U(VI) complexes for bacteria such as Shewanella oneidensis MR-1. Recent studies have onfirmed that the reduction proceeds via a first electron transfer forming pentavalent U(V) species that readily disproportionates. However, in the presence of the stabilizing aminocarboxylate ligand, dpaea2− (dpaeaH2bis(pyridyl-6-methyl-2-carboxylate)-ethylamine), biologically produced U(V) persisted in aqueous solution at pH 7. We aim to pinpoint the role of MHC in the reduction of U(V)-dpaea and to establish the mechanism of solid-phase U(VI)-dpaea reduction. To that end, we investigated U-dpaea reduction by two deletion mutants of S. oneidensis MR-1−one lacking outer membrane MHCs and the other lacking all outer membrane MHCs and a transmembrane MHC−and by the purified outer membrane MHC, MtrC. Our results suggest that solid-phase U(VI)-dpaea is reduced primarily by outer membrane MHCs. Additionally, MtrC can directly transfer electrons to U(V)-dpaea to form U(IV) species but is not strictly necessary, underscoring the primary involvement of outer membrane MHCs in the reduction of this pentavalent U species but not excluding that of periplasmic MHCs.