Enzyme doesn’t need oxygen to make ethylene

The chemical industry makes about 25 million metric tons of ethylene a year and uses the compound to manufacture plastics, polymers, and longer chain carbon compounds. Scientists want to engineer bacteria to make this vital chemical feedstock to avoid using fossil fuels. But so far they can’t do so on a large enough scale to satisfy industry’s needs. Also the enzymes currently used by engineered bacteria to make ethylene need oxygen to function, which creates a safety issue, because O₂ plus ethylene is an explosion risk.

A group from Ohio State University, Oak Ridge National Laboratory, Pacific Northwest National Laboratory, and Colorado State University has discovered a bacterial enzyme that can produce ethylene and methane without needing O₂ (Science, 2020, DOI: 10.1126/science.abb6310). This ethylene-making enzyme could potentially offer a safer path toward synthesizing the valuable chemical.

The bacteria come from both soil and freshwater. Basically, these microbes take bits of leftover metabolic waste that contain sulfur, and break down those molecules to (2-methylthio)ethanol, says Justin North, a research microbiologist at Ohio State and the first author on the paper. Then the methylthio-alkane reductase takes the sulfur from that molecule and releases ethylene (shown). The enzyme can produce methane from dimethyl sulfide in the environment, he says. The bacteria only use these reactions in low-sulfur conditions as a way to scavenge for the element.

Cells need sulfur to make important biomolecules such as methionine and adenosyl-methionine. Scientists didn’t know where bacteria got the sulfur to make these compounds in low-oxygen conditions, because in such environments there isn’t much sulfate, the bacteria’s preferred sulfur source.

“People have been interested in the observation for years that something out there in the environment can make ethylene in the absence of oxygen,” North says.

The ethylene-O₂ safety issue is a large concern for finding a biosynthetic route to the olefin, says Jianping Yu, a molecular biologist at the National Renewable Energy Laboratory in Golden, Colorado. “The discovery of an enzyme/pathway for ethylene synthesis under anaerobic conditions bypasses this safety risk, and could open up a new research direction for biological ethylene production.”

North says that the group is now working on engineering the bacteria to make more ethylene. “We are applying synthetic biology, we’re applying directed evolution and all manner of things to try to get these organisms to naturally produce ethylene, to the levels they need to be competitive,” he says.