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 inframe 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 protoncoupled electron transfer processes. The new class of reductase, GolR, opens the chapter for the mechanistic understanding of Au(III) bioreduction.

Automated summary

Microorganisms contribute to the formation of secondary gold deposits through enzymatic reduction of Au(III) to Au(0). In this study, a previously unknown Au reductase, GolR, was identified in the cytoplasm of the bacterium Erwinia sp. IMH. The expression of golR was found to be up-regulated in response to increasing Au(III) concentrations and exposure time. The GolR protein was able to reduce Au(III) to Au nanoparticles in vitro, and molecular dynamic simulations suggested that GolR may selectively bind AuCl2(OH)(2)(-) and facilitate its reduction through proton-coupled electron transfer processes at the Fe-containing active site.