Nothing is superfluous in nature. When trees topple in the forest or straw is left lying in the field after harvest, bacteria and fungi immediately start converting the dead biomass. Using enzymes, they are able to separate the molecules so they can access the sugars they feed on.
We humans have also recognized this process. Initially, like the microorganisms, we have focused on separating the sugar—also called cellulose—which makes up about 50 per cent of a tree trunk—and its more complex variant, hemicellulose. We have identified the enzymes fungi and bacteria use to break down the long polymeric structures into the single component—glucose. Now, researchers are looking at the last 20-30 per cent of the plant, which is composed of lignin.
Lignin acts as a kind of ‘glue’ that binds together the long fibre structures of sugar molecules that enable the plant to stay upright even in windy conditions, transport water, grow and expand—while making it resistant to pathogenic microorganisms. Lignin acts as a barrier against microorganisms’ use of carbohydrates. Separating lignin will thus provide a much better basis for exploiting the carbohydrates.
Lignin is a so-called polyaromatic compound—the only one found in nature as it happens. An aromat is a six-pronged carbon ring and the lignin consists of many of these rings linked together. Oil also contains aromatic compounds, but it is not renewable, so researchers are trying to find alternative materials with similar properties.
Another quality of lignin is that it is water-repellent. It is a so-called hydrophobic polymer, which helps to ensure that the plants do not disintegrate when it rains. This property is destroyed when lignin, for example, is recovered in the paper making process where it is separated from the cellulose.
A great deal can therefore be gained by finding alternative methods for cleaning the lignin, and nature has in fact already created the remedy in the form of an enzyme which has now been found and described by a group of researchers at DTU Bioengineering, headed by Assistant Professor Jane Agger.
Fascinating enzymes
An enzyme is a protein that can accelerate a chemical reaction, causing it to occur faster than it would spontaneously—without itself being altered in the process and thus being consumed. All biological organisms—humans, and in particular, plants and microorganisms—use enzymes to regulate and control the biological processes that occur within them. The enzymes are, so to speak, the tool that ensures the process takes places quickly and effectively.
There is pretty much an enzyme for each reaction—so many of them that finding the ideal candidate is easier said than done.
“It’s like looking for a needle in a haystack, even though we have developed good tools to aid us in our search,” says Jane Agger. “A fungal genome contains perhaps 10-15,000 genes, 200- 300 of which typically encode enzymes used for biomass degradation. We don’t yet know them all and still have much to learn about how the fungal degradation of wood pulp occurs.”
In an article in the scientific journal Nature Communications published in February, Jane Agger’s research team described an enzyme that is interesting in this context. The enzyme is found in the group ‘esterases’—so called because they work on certain ester bonds in the plant mass which go by the name lignin-carbohydrate complexes—abbreviated to LCC.
The researchers have managed to explain how the enzyme works and looks, how it recognizes the lignin it needs to work on, and how it breaks the binding to the carbohydrate fibres. It is actually really amazing that it is even possible, says Jane Agger:
“An enzyme is a fairly small molecule compared to biomass, existing in a water phase. Scientifically speaking, it’s very interesting that a water-loving molecule is thus able to find and attach itself to a solid water-repellent surface like lignin and break some very specific bonds here.”
Working with real plant material in the laboratory can be difficult. Researchers therefore often simplify things with the help of model systems, but these are not always accurate, so DTU researchers have opted for a slightly more laborious approach:
“We’ve succeeded in detecting the enzyme on real biomass—birch wood—which hasn’t been done before with this kind of enzyme. It’s a laborious process. First, the biomass has to be prepared so that we can analyse what happens when we attach the enzymes. This is done in small tubes. And the next step is then to incorporate the enzyme into a process where we can run bigger pieces of the wood biomass through,” says Jane Agger.
Endless possibilities
Research is still at a fairly early stage, and Jane Agger believes that perhaps five years of intensive research is needed before a proper demonstration process is achieved. However, she sees great prospects in the newly discovered enzyme, which is likely to allow at least ten per cent of the lignin to be taken out of the biomass in such a pure form that it can be used for products of very high value.
For example, the purest carbon fibres could be incorporated into coatings that could make materials water-repellent—or give them other specific properties. They can be used in electronics as a substitute for chemicals for glue and in different types of biodegradable solid materials.
In a less clean form, the lignin can be incorporated into different kinds of plastic—e.g. foam or other flexible materials. It could also be used as a filling material in building materials such as concrete or epoxy—and it will be possible to use it for the part of chemical production that currently relies on the aromatic structures in oil. And the very last remnants can subsequently be burned off to provide heat energy.
“In this way, we will be able to use all the biomass, and thus assert that the research contributes to the two UN Sustainable Development Goals: ‘Responsible Consumption and Production’ and ‘Climate Action’,” says Jane Agger.
Basic scientific interest
Given that research into biomass-depleting enzymes began as many as 25-30 years ago, it may seem surprising that the lignin-optimized enzyme is only now being discovered. According to Jane Agger, one of the reasons is probably that the very bonds that this enzyme controls are found in fairly low concentrations in biomass. There are many more bonds in a cellulose molecule, and these are the ones that have mainly been focused on.
“Now we’re on our way to mapping which bonds in the biomass are essential for keeping the whole complex structure together. Funnily enough, it turns out that when the bonds between lignin and hemicellulose are cut, the entire structure disintegrates. It’s about cutting in the right places,” she says.
“The newly discovered enzymes thus become not only a key to opening up an industrially interesting process to exploit the lignin, but also an opportunity to learn more about the nature of the three-dimensional structure of the plant cell.”