Engineering Efficient Xylose Metabolism Using Synthetic Biology

Volatile crude oil prices coupled with global warming concerns emphasise the urgent need for alternative technologies that can replace fossil fuels and reduce environmental impact. Plant biomass is the most abundant renewable feedstock in the world with a low production cost, making it ideal for the biological production of liquid fuels (e.g. biobutanol) and bulk chemicals. However, recalcitrance of lignocellulosic feedstocks to degradation is a pressing challenge. We are using a synthetic biology approach to engineer industrially important bacterial strains for the efficient breakdown and uptake of hemicellulose-derived xylooligomers (XOS). During enzymatic degradation, xylan is hydrolysed by endoxylanases resulting in the formation of XOS, primarily xylobiose (X₂) but also xylotriose (X₃) and xylotetraose (X₄), of which small amounts enter the cell and are then cleaved by beta-xylosidases to release monomeric xylose. Compared with xylose, butanol-producing bacteria growing on xylan are energy limited with slow growth rates resulting in low fermentation productivity. Engineering such microbes to use this material more efficiently would increase their growth rates as oligo-xylans are the primary carbon source available when growing on hemicellulose and hence these reactions have high flux. Central to our design therefore are xylooligomer transporters. By using transporters for X₂-X₄, which take up one of these molecules for one unit of energy, bacteria are much more efficient than those which need 2-4 units of energy for uptake of equivalent amounts of sugar present in the monomeric form. Once inside the cell, there are no energetic costs for the breakdown of X₂-X₄ to X₁. At the bioprocess level, the use of XOS would allow for milder pretreatment procedures in contrast to chemical methods typically required for the complete hydrolysis of xylose which can release inhibitors and retard fermentation.