An enzyme called Huc that the soil bacterium Mycobacterium smegmatis uses to produce energy from hydrogen has been analysed and now we know how it works.
An enzyme that can generate energy from hydrogen in the air could power future fuel cells or small generators.
Soil bacteria that have evolved to consume hydrogen to make their energy take in about 60 million tonnes of the gas globally each year, but how exactly they do it has been a bit of a mystery.
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| A reconstruction of the Huc enzyme Chris Greening et al |
Rhys Grinter at Monash University in Melbourne and his colleagues have previously identified that an iron and nickel-based enzyme called Huc in the soil bacterium Mycobacterium smegmatis plays a key role in this hydrogen-cycling process, but it was unclear how it worked.
Now, Grinter and his team have analysed Huc using an ultracool electron microscope and shown that it can produce electrons when it is in a hydrogen-filled test tube and also power specially designed circuits.
“What makes our enzyme really special is its affinity for the hydrogen,” says Grinter. “It can break down hydrogen into electrons at a much lower concentration than any other catalyst that’s been identified, and it’s also completely resistant to any inhibition by gases like oxygen and carbon monoxide.”
To get enough of the Huc enzyme to analyse, Grinter and his team modified some M. smegmatis bacteria to produce more of it, then isolated the enzyme using a sticky resin that binds to it.
After mapping its structure and running simulations of how it processes gas, Grinter and his team realised that Huc’s ability to harvest hydrogen even at the low concentrations found in the air is due to the narrow channels in it. These only allow hydrogen, not other gases, to pass through to its centre, where electrons are removed from it.
They also found that the enzyme worked in temperatures from freezing all the way to 80°C (176°F).
To test Huc’s hydrogen-harvesting abilities, the researchers put some of the purified enzyme in a small vial with hydrogen and a dye that changes colour when electrons are present. They found that the dye changed colour and the hydrogen levels decreased until the gas was no longer detectable.
They also constructed simple electrical circuits in which Huc was attached to an electrode and found that it could generate small currents.
Huc could be used in fuel cells or for generators that power low-energy devices like remote sensors, if it could be produced in sufficiently large volumes. That would require scaling up the production process to hundreds of thousands of litres of bacteria from the roughly 10 litres possible now, which isn’t straightforward, says Grinter. There are also aspects of how Huc converts hydrogen that need further investigation, he says.
The work is still important for future fuel cell technology, says Simone Morra at the University of Nottingham, UK. “It’s a big technical challenge to understand the structure of this enzyme, because it’s a large complex made of many subunits.”
Although additional research would be needed to see if use of Huc can be scaled up for industrial applications, the detail with which we can now see the enzyme means that it could inspire the design of similar artificial enzymes that are more robust, he says.
Journal reference