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Not just a load of gas

Date: 2019-05-16 15:34:42.0
Author: Jon Evans


Methane flaring from an oil well

Methane comes from many sources, including deposits of fossil fuels in the ground, the breakdown of organic matter in landfills by bacteria and farting cows. Although much of this methane is burnt to provide heat, either directly or to generate electricity, a lot is simply lost to the atmosphere, where it causes a problem because it is a potent greenhouse gas.

Methane has the potential, however, to be a solution rather than a problem, by acting as a feedstock for the production of useful chemicals and liquid fuels such as methanol. Unfortunately, catalytic processes for converting methane into chemicals and fuels are currently a bit of a non-starter, because they tend to require high pressures and temperatures. But in the same way there are bacteria that produce methane, there are also bacteria that consume and metabolize methane, collectively known as methanotrophs, and scientists are finding that they can efficiently convert methane into a whole range of useful compounds.

Under normal conditions, most methanotrophs metabolize methane in several steps, converting it first to methanol and then to formaldehyde, which is either incorporated into cell biomass or broken down further to produce energy. But scientists are finding that by altering the culture conditions, such as by reducing the supply of certain nutrients, they can direct this process, getting the bacteria to prioritize producing methanol or increasing biomass. Furthermore, by doing this with different species, scientists have found that they can get them to produce a wide range of different compounds.

For example, in a paper last year in Biotechnology for Biofuels, a team of scientists in China described how a strain of the methanotroph Methylomicrobium buryatense can convert methane into lipids that can be used for the production of green diesel. Furthermore, the scientists found that the rate of lipid production could be greatly increased by simply knocking out the gene for a glycogen-producing enzyme. Other studies have shown that, under the right conditions, certain methanotrophs can convert methane into the bioplastic polyhydroxyalkanoate and the organic compound ectoine, which is an active ingredient in skin care and sun protection products.

But it is the production of methanol that perhaps offers most promise, because methanol can be used as a liquid fuel in a certain type of fuel cell, where it is broken down to produce electricity. So, scientists are hard at work trying to understand more about the enzymatic process employed by methanotrophs for converting methane to methanol. While they’ve identified the enzyme responsible – methane monooxygenase – they don’t yet know exactly how it works.

This is because studies have revealed that methane monooxygenase contains several copper ions in different positions on the protein, any one of which could be the active site for the oxidation reaction that converts methane to methanol. Scientists have even postulated that two separate copper ions could take part in the process.

Now, in a paper in Science, a team of scientists from the US and the UK report identifying the active copper site on the methane monooxygenase produced by a methanotroph called Methylococcus capsulatus, and it is just the one site. Using a variety of analytical techniques, including electron paramagnetic resonance (EPR) spectroscopy and electron nuclear double resonance (ENDOR) spectroscopy, together with several experiments in which they modified the various copper sites, the scientists were able to pinpoint the precise copper site responsible for methane oxidation. Nevertheless, although only one copper site was responsible, a second copper site influenced how well it worked. When the scientists took out this second site, it reduced the enzyme’s activity by 80%.

This finding won’t just help in developing strains of methanotrophs that can produce more methanol, but it could also help in developing catalysts that can convert methane to methanol without requiring such high pressures and temperatures.

"While copper sites are known to catalyze methane-to-methanol conversion in human-made materials, methane-to-methanol catalysis at a monocopper site under ambient conditions is unprecedented," said team member Matthew Ross from Northwestern University. "If we can develop a complete understanding of how they perform this conversion at such mild conditions, we can optimize our own catalysts."

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.

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