3D-structure rendering of a glucuronoyl esterase from the bacterium Opitutus terrae. The enzyme structure is colored turquoise, and shown binding to the researchers’ current model of a cell wall polysaccharide (orange) connected to a large lignin fragment (yellow).
Biomass from forests and agriculture can be used in fossil-free production of biofuels, environmentally friendly chemicals and different kind of materials. However, the raw material is hard to deconstruct into the simple sugars needed for production. The plant cell walls are built to be recalcitrant, a necessary property for survival in nature.
One way to deconstruct wood or other types of plant biomass, is to use enzymes, which in nature work as molecular scissors. Researchers from the Department of Biology and Biological Engineering at Chalmers University of Technology, together with the University of Copenhagen, have taken a closer look at the features of one specific group of enzymes with big potential.
– A factor that strongly complicates the deconstruction of the carbohydrate chains in plant cell wall to simple sugars, is a polymer called lignin, says Johan Larsbrink, Assistant Professor at the Division of Industrial Biotechnology.
The long carbohydrate chains stick together because of the lignin, which works as an adhesive.
– In some places of the cell wall, the lignin and carbohydrates not just stick together – they are directly connected by so-called covalent chemical bonds. If we cleave these bonds, the overall deconstruction would be simplified, since the entire plant cell wall network would be weakened, one could say.
This is where enzymes come into play. By using nature’s own scissors, the production chain can be made more sustainable, effective and, most likely, cheaper.
The enzymes that are able to cleave bonds between carbohydrates and lignin are called Glucuronoyl Esterases, or GEs. The vast majority of previous studies have focused on enzymes from fungi, but the knowledge about this type of enzyme is still very limited. Johan Larsbrink’s group have studied ten GEs from three different bacterial species, instead of fungi.
– We chose to focus on bacterial enzymes since they have a much larger diversity than the fungal counterparts, and basically no studies of these had been made. We characterized the enzymes biochemically on model substrates, and managed to solve their 3D structures on the atomic level. This means that we get an extremely detailed picture of how they work. They are designed for their purpose in nature, so there’s a lot to be learned by this, he says.
– We can also gain new knowledge about the plant cell wall itself by studying the enzymes. It’s like learning about the features of a hand by looking at the design of a glove.
Results from the study suggest that the GEs interact with lignin, which was somewhat surprising.
– Most of enzymes acting on carbohydrates are specific for those well-defined structures, while lignin has a more or less random structure that enzymes find hard to handle, Johan Larsbrink explains.
– To see how the enzymes may interact simultaneously with both carbohydrates and lignin makes sense, but it is a unique finding.
The research group also tested their enzymes on corn cob biomass, which is a common waste product in agriculture. They used an enzyme cocktail without GEs, and observed what happened after addition of their GE enzymes. The results were dramatic:
– With GEs in the mix, the amount of free sugars was greatly increased. With these results, we are able to conclude that GE enzymes really make a huge contribution in cleaving bonds that are of high importance in the plant cell wall.
Text: Mia Malmstedt
Photo: Johan Larsbrink (enzyme model), Silvia Hüttner