Identification and Characterization of a Au(III) Reductase from Erwinia sp. IMH

Microorganisms contribute to the formation of secondary gold (Au) deposits through enzymatic reduction of Au­(III) to Au(0). However, the enzyme that catalyzes the reduction of Au­(III) remains enigmatic. Here, we identified and characterized a previously unknown Au reductase (GolR) in the cytoplasm of Erwinia sp. IMH. The expression of golR was strongly up-regulated in response to increasing Au­(III) concentrations and exposure time. Mutant with in-frame deletion of golR was incapable of reducing Au­(III), and the capability was rescued by reintroducing wild-type golR into the mutant strain. The Au­(III) reduction was determined to occur in the cytoplasmic space by comparing the TEM images of the wild-type, mutant, and complemented strains. In vitro assays of the purified GolR protein confirmed its ability to reduce Au­(III) to Au nanoparticles. Molecular dynamic simulations demonstrated that the hydrophobic cavity of GolR may selectively bind AuCl₂(OH)₂⁻, the predominant auric chloride species at neutral pH. Density functional theory calculations revealed that AuCl₂(OH)₂⁻ may be coordinated at the Fe-containing active site of GolR and is probably reduced via three consecutive proton-coupled electron transfer processes. The new class of reductase, GolR, opens the chapter for the mechanistic understanding of Au­(III) bioreduction.