Enzyme protects bacteria from toxic gold

Certain bacteria protect themselves from a toxic form of gold by transforming it into high-purity nanoparticles. Researchers have now identified a gold-reducing enzyme called GolR that is a key part of this process. They hope the enzyme—the first gold reductase known to science, they say—could be used to harvest gold from electronic waste (JACS Au 2022, DOI: 10.1021/jacsau.2c00170).

The team found GolR in a bacterium called Erwinia sp. IMH, which thrives in gold ores. Like some other microbes, the bacterium reduces toxic gold(III) compounds, which can generate reactive oxygen species that harm the bacteria, into less-toxic gold nanoparticles. “Although this metal is precious to humans, bacteria hate it,” says team member Chuanyong Jing of the Research Center for Eco-Environmental Sciences at the Chinese Academy of Sciences. Researchers had previously identified other ways that bacteria deal with gold(III)—using peptides, polysaccharides, or glutathione, for example. But if gold(III) sneaks past these barriers, Jing says that GolR acts as the microbe’s last line of defense.

To find GolR, the team studied which of Erwinia’s genes were spurred into action by gold(III). When they deleted the most activated gene, the microbe became more sensitive to gold(III) toxicity. A trawl of other genomes found the gene in a diverse range of bacteria, suggesting that GolR could play an important role in forming the up to 99% pure gold granules found in natural deposits.

Molecular simulations suggest that GolR’s active site contains an iron atom that binds hydroxo ligands in AuCl₂(OH)₂, one of the main soluble forms of gold(III).

Jing suggests that bacteria modified to produce GolR might be harnessed to “mine” gold from wastewater or electronic waste. “These bacteria could collect gold ions and then reduce them,” he says. “We’re now trying to develop this kind of engineered bacteria.”