NREL values nature

Two recent studies from researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) describe two very different new approaches for deriving value from natural materials. One approach is chemical and involves obtaining a widely-used industrial chemical known as acrylonitrile from natural materials for the first time. The second is biological and offers a fresh way to enhance the enzymes required to convert cellulose into fermentable sugars.

Acrylonitrile is used in the production of a wide range of plastics, rubbers and resins, making it one of the most widely-used monomers in the chemical industry. It is also the main feedstock in the production of carbon fibers, which are experiencing rapid growth as an additive for enhancing the strength of plastics. At the moment, acrylonitrile can only be produced from propylene derived from fossil fuels.

That could soon change, though, thanks to a team of US and UK scientists led by Gregg Beckham at NREL. They have found a way to produce acrylonitrile from 3-hydroxypropionic acid (3-HP), which is produced by genetically-engineered microbes and already used in the production of various industrial chemicals. Their inspiration came from recent work showing that carboxylic acids can be converted into nitriles by passing them over solid acids in the presence of ammonia.

As they report in Science, the process they came up with involves first chemically converting 3-HP to ethyl 3-HP, which is then passed over a titanium dioxide solid acid catalyst in the presence of ammonia at temperatures of up to 320°C. In tests, they found this process could convert more than 90% of 3-HP into acrylonitrile. Further investigations revealed that the conversion likely proceeds in three stages: first, ethyl 3-HP undergoes a dehydration reaction to form ethyl acrylate; second, this reacts with the ammonia to form acrylamide; and finally another dehydration reaction converts acrylamide to acrylonitrile.

Next, Beckham and his colleagues modelled a feasible industrial version of this process, which calculations showed should be able to convert 98% of 3-HP to acrylonitrile. This is much more efficient than the current process for producing acrylonitrile, which can convert at most 83% of the propylene feedstock into acrylonitrile, and should help to make the process economically competitive.

Improving the economics of cellulosic biofuels, meanwhile, tends to require improving the efficiency with which cellulose is converted into fermentable sugars. The main approach for doing this currently involves screening wood-degrading bacteria and other microbes for new and improved enzymes. A team of US scientists, again led by Gregg Beckham at NREL, decided to take a different approach, by looking at the properties an enzyme needs for the industrial production of cellulosic biofuels.

In particular, they focused on small sugar molecules called glycans, which are attached at various places on enzymes in a process known as glycosylation. Scientists already knew that glycans can help prevent enzymes from being broken down by other enzymes. But Beckham wanted to know whether they conferred any other properties as well and whether the precise positioning of the glycans on the enzyme determined these properties.

To find out, he and his team generated numerous mutant versions of a cellulose-degrading enzyme known as Cel7A, with these mutants differing in the number of glycans they possessed. They then identified the location of the missing glycans on each of the mutants and investigated what effect these missing glycans had on the mutants’ properties, including their cellulose-degrading abilities.

As they report in the Proceedings of the National Academy of Sciences, this revealed that the location of the glycans did indeed influence the mutants’ properties. These included many that would be of use in the production of cellulosic biofuels, such as rate of cellulose conversion and thermal stability.

“In confirming their locations, we not only discovered new glycans, we also elucidated the structures of the glycans at each specific site,” says Antonella Amore, a postdoctoral researcher at NREL. “This provides insight into how the microbe is protecting and decorating its enzymes for optimal activity, which in turn provides clues on how to improve them for industrial applications.”

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