ACS Meeting News: Putting enzymes in a cage to clean up the environment

To clean up contaminated soil or water, environmental engineers can turn to Nature for help. This approach, called bioremediation, sometimes involves coaxing bacteria or fungi to use their inherent enzymatic machinery to chew up pollutants or transform the compounds into something less toxic.

Some engineers want to go around these microbial middlemen and deploy the enzymes alone. But naked enzymes aren’t stable in the environment for very long.

This week at the American Chemical Society national meeting in San Diego, researchers reported a way to protect bioremediation enzymes by packaging them in a protein cage.

Shaily Mahendra, an environmental engineer at the University of California, Los Angeles, who led the team, pointed out that bioremediation isn’t a new process engineers invented. Nature has always been cleaning up messes in the environment with organisms that repurpose molecules excreted by others. “If it weren’t for bioremediation, we’d be sitting on mountains of dinosaur waste right now,” she told C&EN.

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Engineers have exploited this natural process for various environmental clean-up jobs. For example, crews added fertilizers to Alaskan beaches to get soil bacteria to consume oil spilled by the Exxon Valdez.

But what happens if the microbes present at a site don’t have the right enzymes to do the job? Some engineers propose adding the needed bacteria or fungi. But, Mahendra told C&EN, finding the right strain that will thrive in a particular environment could be tricky, and some scientists worry about disrupting the area’s natural microbial communities.

Applying contaminant-degrading enzymes to sites could be an alternative strategy.

On Monday, at a symposium in the Division of Environmental Chemistry, Mahendra’s graduate student Meng Wang described a way to possibly keep these enzymes folded and active in the environment. The strategy involves 70-nm-long protein-RNA complexes called vault particles, which are produced by fungi, birds, mammals, and most other eukaryotes. The function of these complexes in cells isn’t completely understood, but Mahendra’s biochemistry colleagues at UCLA have tested the particles as a way to deliver drugs or vaccines.

In the new work, the team put the enzyme manganese peroxidase inside vault particles. Peroxidases are known to oxidize and break down organic contaminants.

The researchers modified the gene for the peroxidase so that when cells synthesized the enzyme it had an added domain that helped the enzyme bind to the inside of the vault particles. The team used insect cells to produce the vaultcomponents, as well as the modified enzyme.

The particles enhanced the peroxidase’s activity in lab tests involving contaminants in solution. After 24 hours, the vault-packaged peroxidase consumed >90% of bisphenol A, a contaminant from plastic production, while the free enzyme broke down just 40%. Also, compared with free enzyme, the packaged peroxidase was more stable between 20 ^oC and 40 ^oC.

The team published some of these results last year (ACS Nano 2015, DOI: 10.1021/acsnano.5b04073).

So far the UCLA team has tested the particles on just a handful of contaminants, but Mahendra would like to eventually work with other enzymes that can break down different classes of pollutants, as well as test the strategy under field-like conditions.

Lee Blaney, an environmental engineer at the University of Maryland, Baltimore County, said the vaults are a neat and novel strategy to increase the stability of enzymes for bioremediation. He thinks the team next needs to test the approach under more realistic conditions to see how other substances found in the environment, such as dissolved organic matter, might affect the stability and activity of the vaults and their enzymatic cargo.

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