PNNL

Abstract

Glycoside hydrolases (GHs) are key enzymes in the depolymerization of plant-derived cellulose, a process central to the global carbon cycle and the conversion of plant biomass to fuels and chemicals. A limited number of GH families hydrolyze crystalline cellulose, often by a processive mechanism along the cellulose chain. During cultivation of thermophilic cellulolytic microbial communities, substantial differences were observed in the crystalline cellulose saccharification activities of supernatants recovered from divergent lineages. Comparative community proteomics identified a set of hydrolases from a population closely related to actinobacterium Thermobispora bispora that were highly abundant in the most active consortium. Among the most abundant proteins, a glycoside hydrolase family 12 (GH12) protein was found to be correlated most closely with the changes in crystalline cellulose hydrolysis activity. Heterologous expression and biochemical characterization of the suite of T. bispora hydrolytic cellulases confirmed that the GH12 possessed the highest activity on multiple crystalline cellulose substrates and demonstrated that it hydrolyzes cellulose chains by a predominantly random mechanism. This work suggests that the role of GH12 in crystalline cellulose hydrolysis by cellulolytic microbes should be reconsidered.