Ancient enzyme structure reveals new path to sustainable ethylene production

Researchers have made a significant advance toward the goal of using bacteria—rather than fossil fuels—to produce ethylene, a key chemical in the production of many plastics.

In a new study, scientists identified the enzyme that certain bacteria use to break down organic sulfur compounds to create ethylene. They also, for the first time, were able to extract the enzyme from bacteria to study its function and structure.

“What we wanted to know is how the enzymes in these bacteria worked to make ethylene so we can in the future harness them for sustainably making the everyday plastics we need,” said Justin North, senior author of the study and assistant professor of microbiology at The Ohio State University.

The study on this ancient enzyme—called methylthio-alkane reductase (MAR)—appears in Nature Catalysis.

This new work builds on a 2020 study by North’s lab that was published in Science.

North’s research group at Ohio State teamed up with Hannah Shafaat, professor of chemistry and biochemistry at UCLA, and researchers from the U.S. Department of Energy (DOE) Berkeley Lab Joint Genome Institute (JGI) and the Laboratory for Biomolecular Structure at Brookhaven National Lab to understand how MAR makes ethylene.

“At the onset, we only knew the genes responsible in our bacteria for the methylthio-alkane reductase that turned organic sulfur compounds into ethylene. And curiously, they looked a lot like the ancient nitrogenase enzymes from bacteria that take nitrogen gas out of the air and turn it into biological nitrogen fertilizer,” North said.

To convert these genes into proteins that could be studied for their potential in biofuel production, North’s group first turned to the Joint Genome Institute. Yasuo Yoshikuni, DNA synthesis lead at JGI and co-author on the paper, constructed multiple versions of MAR genes for Ohio State researchers to test in their soil bacterium, Rhodospirillum rubrum, for making the MAR protein.

“We were so excited when we isolated MAR from our bacteria as a pure enzyme to study,” North said. “This is a feat that had never been accomplished before.”

Srividya Murali, a research associate in North’s Lab and co-lead author of the study, was responsible for finding a way to isolate MAR.

This breakthrough opened the door for the research team to finally begin to understand how these enzymes worked.

The biggest surprise came when the researchers discovered just how close MAR is to nitrogenase. Nitrogenase is an enzyme that has one of the most complicated iron and sulfur-containing metal cofactors in nature, which is at the core of its catalytic function. Previously, scientists thought nitrogenases might be the only enzymes to have these sophisticated metal catalysts.

However, detailed spectroscopy measurements from Shafaat’s group on how the metal cofactors of MAR gain and use electrons for extracting sulfur and making ethylene from organic sulfur compounds quickly revealed just how complex MAR is.

“Looking at the metal centers in MAR is like looking into a mirror and seeing an older relative of nitrogenase on the other side,” said Shafaat. “The way that the enzyme moves electrons through the massive protein complex to perform a very specific reaction is so elegant, but is also notably different from how nitrogenase can fix nitrogen into fertilizer.”

These discoveries were bolstered when Brookhaven National Laboratory researchers Guobin Hu from the Laboratory for BioMolecular Structure, also a co-lead author on the study, and Dale Kreitler from the National Synchrotron Light Source II were the first scientists to reveal the structure of MAR and its complex metal cofactor through cryogenic electron microscopy.

They found that MAR is structurally built like nitrogenase and uses similar but unique metal cofactors for achieving its chemistry. But there are differences in the metals—and the area around the metals—that MAR prefers for extracting sulfur and making ethylene from organic sulfur compounds, compared to how nitrogenase fixes nitrogen.

“These discoveries help us start thinking about how MAR’s structure allows it to function like it does,” North said.

Now that they know the structure and function of MAR, the researchers are working to engineer a MAR enzyme that is even better at producing ethylene than those found naturally, North said. The ultimate goal is to make the process of using MAR to produce ethylene cost-effective enough to replace the use of fossil fuels.

“We are making progress and the findings in this study were an important milestone toward that goal,” North said.