And that offers hope for the future.

When you think of greenhouse gases, you probably immediately think of carbon dioxide. But after CO2, methane is the most important greenhouse gas that contributes to global warming. Methane is even 84 times worse for the environment than carbon dioxide over a period of 20 years. Now there are promising bacteria that can convert the dangerous greenhouse gas into usable fuel. And researchers are trying that process in a new study to understand better.

More about methane
Since the Industrial Revolution, a lot of methane has been pumped into the atmosphere. The atmospheric concentration has more than doubled since then. And that while methane is a polluting greenhouse gas that affects the health of people and ecosystems. Although the gas is present on Earth to a lesser extent than CO2, it is much more powerful; as a greenhouse gas it is even 28 times stronger. Therefore, smaller amounts already have a large effect on the atmospheric temperature. Most of the methane on Earth is produced by microorganisms that convert organic matter into methane in low-oxygen areas. This happens, for example, in wetlands, in the stomachs of cows or on rice fields. Also, a significant amount is produced by human activities, including agriculture (such as rice paddies and livestock), fossil fuels, landfills and biomass combustion.

‘Methane-eating’ bacteria can absorb up to 30 million tons (!) of methane from their environment every year. They then convert this into methanol, a substance that can serve well as fuel. Although that sounds promising, we actually know very little about how the bacteria do that exactly. However, if we want to learn this trick from these bacteria, we will have to know the finer points about it.

Enzyme

It is known that the methane-eating bacteria stimulate the reaction with the help of an enzyme – called pMMO. But we don’t know exactly how this enzyme works. And that’s a crucial piece of the puzzle. “If we don’t understand exactly how the enzyme does the difficult chemistry, we can’t develop and optimize it for biotech applications,” said researcher Amy Rosenzweig.

Previous studies

Scientists have already made frantic efforts to study the enzyme. However, this is a particularly difficult protein to analyze because it is embedded in the cell membrane of the bacteria. When scientists examine the bacteria, they usually use a rather harsh technique, in which the proteins are ripped out of the cell membranes with a certain toxic solution. While this effectively isolates the enzyme, it also kills all enzyme activity and limits the amount of information researchers can gather.

New technique

In the new study, the researchers decided to take a different approach. For example, they wondered whether inserting the enzyme into a membrane that resembles its natural environment might work. The team used lipids from the bacteria to form a membrane in a protected particle called a nanodisc — a synthetic model that helps study membrane proteins — and then embedded the enzyme in that membrane. “By mimicking the enzyme’s natural environment in the nanodisc, we were able to restore the enzyme’s activity,” explains researcher Christopher Koo.

Atomic structures

Thanks to their ingenious technique, the researchers succeeded in visualizing the structure of the active enzyme in high resolution. “We were able to see even atomic details,” Rosenzweig said. It means that the researchers have now discovered important structures that drive the conversion process. And by exposing those structures, engineers can eventually mimic the entire process.

To ask

It means that the newly discovered structures are a new starting point to answer questions that until now just kept piling up. Once those questions have been answered, the team plans to study the enzyme directly in the bacterial cell using an advanced imaging technique. If successful, scientists can see exactly how the enzyme is arranged in the cell membrane, determine how it operates in its truly natural environment, and learn whether other proteins around the enzyme also play a role. These discoveries could ultimately be the important missing link for engineers.

All in all, the bacterium that converts the dangerous greenhouse gas methane into fuel is slowly but surely starting to reveal its secrets. The study therefore brings the development of man-made biological catalysts that convert methane gas into methanol one step closer.