Progress and future prospects for pentose-specific biocatalysts in biorefining

Due to the fact that cellulose represents up to 60% of the dry weight of plant-derived biomass, so far R&D efforts have mainly focused on the extraction and conversion of cellulose into added value products. Consequently, abundant heteroxylans have been somewhat neglected in current biomass-to-fuel concepts. The “glucocentric” approach to biorefining means that extraction technologies are sub-optimal and non-specific with regard to pentose sugars, the development of hemicellulases as biorefining enzymes has been slow and pentose-specific conversion technologies for the production of value-added products are relatively scarce. Nevertheless, xylan-related biocatalysis has continued to make steady progress in many areas, including the discovery and characterization of a wide range of hemicellulases, which are important enzymes for biomass hydrolysis. Similarly, the development of high-performance ethanol producing yeast has focused for many years on the recruitment of pentose isomerases or, alternatively, pentose reductases and pentitol dehydrogenases. However, similar efforts are being made to develop microorganisms for alternative bioconversion processes that are widening the range of chemicals that can be derived from pentoses. Finally, progress is also being made in the area of glycosynthesis, which is opening new prospects for the use of pentose sugars as building blocks for engineered pentosides, which will have quite different applications, such as non-ionic surfactants or prebiotic food/feed ingredients. This review provides an overview of these different development areas and discusses future prospects for discovery and impact on biorefining.