Engineering a Metal Reductase for the Bioremediation of Anthropogenic Electronic Wastes: From Hg(II) to Au(III) and Ag(I) Enzymatic Reduction

• Published in: JACS Au Vol. 4; no. 6; pp. 2335 - 2342
• Main Authors: Chua, Jasmine Puay Suan, Rajasabhai, Rashmi, Teo, Wei Zhe, Xue, Bo, Yew, Wen Shan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 24.06.2024
• Subjects: gold nanoparticles biosynthesis; e-waste; enzyme engineering; bioremediation; Au(III) reductase; precious metal recovery
• ISSN: 2691-3704; 2691-3704
• DOI: 10.1021/jacsau.4c00297

Recovering precious metals from electronic waste (e-waste) using microbes presents a sustainable methodology that can contribute toward the maintenance of planetary health. To better realize the potential of bioremediation using engineered microbes, enzymes that mediate the reduction of Au­(III) to Au(0) have been the subject of intense research. In this study, we report the successful engineering of a metal reductase, MerA, whose cognate substrate is mercury­(II), toward other precious metals such as Au­(III) and Ag­(I). The engineered variant, G415I, exhibited a 15-fold increase in catalytic efficiency (k cat/K M) in Au­(III) reduction to Au(0) and a 200-fold increase in catalytic efficiency in Ag­(I) reduction to Ag(0) with respect to the wild-type enzyme. The apparent shift in preference toward noncognate metal ions may be attributed to the energetics of valency preference. The improved Au­(III) reductase has an apparent increased preference toward monovalent cations such as Au­(I) and Ag­(I), with respect to divalent cations such as Hg­(II), the cognate substrate of the progenitor MerA (an increase in K M of 5.0-fold for Hg­(II), compared to a decrease in K M of 5.8-fold for Au­(III) and 1.8-fold for Ag­(I), respectively). This study further extends the mechanistic understanding of Au­(III) bioreduction that could proceed through the stabilization of Au­(I) en route to Au(0) and suggests that the biosynthesis of Au nanoparticles with high efficiency can be realized through the engineering of promiscuous metal reductases for precious metal recovery from e-wastes.