Plant cell walls contain a tangled, branched sugar polymer called xylan that locks up a fifth to a third of the energy in grass and wood, and current methods to break it down are too expensive or create toxic byproducts. This matters because turning plant biomass into biofuel requires releasing those sugars for fermentation. Xylan’s branched structure blocks enzymes from reaching the cellulose underneath, and acid treatments meant to loosen it produce compounds that kill the microbes needed for fermentation. The programme will map the genetic machinery that builds xylan in the first place, identify the protein complexes that assemble it, and find or engineer enzymes that can dismantle it cleanly into fermentable sugars. If successful, the work could cut the cost of processing woody and grassy feedstocks for biofuel production, making lignocellulosic ethanol and butanol economically viable. It will also deliver high-throughput tools for analysing biomass composition and enzyme activity, which other researchers and industry can use to optimise feedstocks and processing. Shell and Novozymes are collaborating to ensure the findings translate into real-world industrial processes.
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To achieve the goal of producing biofuel from plant biomass (lignocellulose), the plant cell wall can be degraded by a cocktail of hydrolase enzymes that generate monosaccharide sugars (saccharification). Biomass feedstocks are pretreated to increase enzyme accessibility of the cellulose and hemicelluloses prior to addition of the enzyme cocktails. The released sugars are then industrially fermented to generate biofuels such as ethanol and butanol. The viability of lignocellulosic biofuel technology will depend on maximising the fermentable sugar from biomass, and minimising the costs of processing. Currently, it is difficult to use the pentose-rich hemicellulose xylan component which constitutes 20- 30% of most feedstocks such as grass and wood. This xylan impedes enzyme access to the cellulose, in part through links with the lignin. One of the main problems is that it is a branched polymer that is difficult to break down with enzymes. Acid treatments to break up the hemicellulose can generate inhibitors that prevent effective microbial fermentation and reduce the yield of sugars. This programme aims to achieve a better understanding of the genetic control of hemicellulose synthesis, especially the branched xylan component of biomass, and the impact of xylan branching on enzyme accessibility. It will develop a comprehensive characterisation of plant polysaccharide synthesis machinery, and how the synthesis enzymes work together in protein complexes. The programme will also discover and characterise effective enzymes that break down this component to monosaccharides. The programme will deliver enabling technologies for high throughput, detailed, quantitative analysis of biomass hemicelluloses and the activity of the enzymes that break them down. Based on this knowledge, strategies of plant breeding or modification, and also of hydrolytic enzyme selection, will be proposed in order to reduce the costs of use of the branched xylan component of biomass, and to release the cellulose for saccharification. The programme in Cambridge to study cell wall synthesis and to develop the polysaccharide and hydrolase profiling technologies is supported by enzyme discovery in the University of Newcastle, with Dr David Bolam and collaboration with Professor Harry Gilbert. Shell Global Solutions are collaborators in the programme, providing an important industrial perspective and bioinformatic support. Additional enzymes for method development and for analysis of cell wall polysaccharides will be provided and studied in collaboration with Novozymes, the world leader in enzyme production.
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