Ancient enzyme structure reveals new pathway for 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 have identified an enzyme that some bacteria use to break down organic sulfur compounds to make ethylene. They were, for the first time, 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 that we could use them in the future to make the everyday plastics that we need sustainably,” said Justin North, senior author and assistant professor of microbiology at The Ohio State University.

Studies of this ancient enzyme—called methylthio-alkaline reductase (MAR) Nature catalysis.

The new work builds on a 2020 study from North’s lab that was published Science.

North’s research group at Ohio State collaborated with Hannah Shift, professor of chemistry and biochemistry at UCLA, and researchers from the US Department of Energy’s (DOE) Berkeley Lab Joint Genome Institute (JGI) and the Laboratory for Biomolecular Structures at Brookhaven National Lab to understand how ethylene is made.

“At the beginning, we only knew the genes responsible in our bacteria for the methylthio-alkane reductase that converts organic sulfur compounds to ethylene. And curiously, they look very similar to the ancient nitrogenase enzymes from bacteria that extract nitrogen gas from the air and convert it into biological nitrogen.”

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

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

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

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

The biggest surprise came when the researchers discovered how close Mar is to nitrogenase. Nitrogenase is an enzyme that has a highly complex iron- and sulfur-containing metal cofactor in nature, which is central to its catalytic function. Previously, scientists believed that nitrogenases might be the only enzymes in which these are noble metal catalysts.

However, detailed spectroscopic measurements by Shafat’s group on how MAR’s metal cofactors capture and use electrons to extract sulfur and make ethylene from organic sulfur compounds soon revealed just how complex MAR is.

“Seeing the metal centers in Mar is like looking in a mirror and seeing an ancient relative of nitrogenase on the other side,” Shafaat said. “The way an enzyme moves electrons through a large protein complex to carry out a very specific reaction is very elegant, but it’s also very different from how nitrogenase can fix nitrogen in fertilizer.”

These discoveries were reinforced when Brookhaven National Laboratory researchers Gubin Ho for Biomolecular Structures, also a co-leader of the study, and Dale Kreitler of the National Synchrotron Light Source II were the first scientists to reveal the structure of Mar and its complex metal cofactors by cryogenic electron microscopy.

They found that Mar is structurally similar to nitrogenase and uses a similar but unique metal cofactor to achieve its chemistry. But there are differences in the metals and the region around the metals that Mar prefers to extract sulfur and make ethylene from organic sulfur compounds, compared to how nitrogenase fixes nitrogen.

“These findings help us start to think that the structure of the mar allows it to function as it does,” North said.

Now that they know the structure and function of Mar, 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 GARM process to produce ethylene cost-effective enough to replace fossil fuel use.

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