Shivegowda Thammannagowda and Pin-Ching Maness. Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401
Hydrogen is a clean and efficient energy carrier. H2 production from cellulosic biomass addresses both the energy security and environmental issues. Unlike the high cost of glucose feedstock, the cellulosic biomass is more abundant in nature. However, cellulosic biomass needs to be pretreated first to reduce its crystallinity. The thermophillic, anaerobic bacterium Clostridium thermocellum is a natural H2 producer and feeds on the pretreated cellulosic biomass such as corn stover or switch grass as the feedstock. Genome sequencing has revealed that C. thermocellum harbors four putative hydrogenase enzymes (three FeFe-H2ases and one NiFe-H2ase) involved in H2 catalysis. Under fermentative conditions it also produces byproducts including formate, ethanol, lactate, and acetate. Therefore, blocking those metabolic pathways leading to byproducts formation could potentially contribute to increased H2 production. Pyruvate: formate lyase and acetaldehyde dehydrogenase are involved in the conversion of pyruvate to formate and acetyl-CoA to acetaldehyde, respectively. The former enzyme can be inhibited by sodium hypophosphite and the latter by disulfiram. Our studies on blocking either the formate- or ethanol-production pathways resulted in increased H2 production, which correlated with an increase in hydrogenase activity, measured by the reduced methyl viologen assay. This finding implies an up-regulation of hydrogenase gene expression. Increased H2 production also altered lactate, acetate, ethanol, and formate synthesis. A study using the lactate dehydrogenase inhibitor is ongoing along with unraveling roles and contribution of the individual hydrogenase in H2 metabolism. Collectively, our results indicate that H2 production can be significantly enhanced via blocking competing pathways during cellulose metabolism.
