A paradigm shift in biomass technology from complete to partial cellulose hydrolysis: lessons learned from nature

A key characteristic of current biomass technology is the requirement for complete hydrolysis of cellulose and hemicellulose, which stems from the inability of microbial strains to use partially hydrolyzed cellulose, or cellodextrin. The complete hydrolysis paradigm has been practiced over the past 4 decades with major enzyme companies perfecting their cellulase mix for maximal yield of monosaccharides, with corresponding efforts in strain development focus almost solely on the conversion of monosaccharides, not cellodextrin, to products. While still in its nascent infancy, a new paradigm requiring only partial hydrolysis has begun to take hold, promising a shift in the biomass technology at its fundamental core. The new paradigm has the potential to reduce the requirement for cellulase enzymes in the hydrolysis step and provides new strategies for metabolic engineers, synthetic biologists and alike in engineering fermenting organisms. Several recent publications reveal that microorganisms engineered to metabolize cellodextrins, rather than monomer glucose, can reap significant energy gains in both uptake and subsequent phosphorylation. These energetic benefits can in turn be directed for enhanced robustness and increased productivity of a bioprocess. Furthermore, the new cellodextrin metabolism endows the biocatalyst the ability to evade catabolite repression, a cellular regulatory mechanism that is hampering rapid conversion of biomass sugars to products. Together, the new paradigm offers significant advantages over the old and promises to overcome several critical barriers in biomass technology. More research, however, is needed to realize these promises, especially in discovery and engineering of cellodextrin transporters, in developing a cost-effective method for cellodextrin generation, and in better integration of cellodextrin metabolism to endogenous glycolysis.