Structural and Biochemical Basis for Mannan Utilization by Caldanaerobius polysaccharolyticus strain ATCC BAA-17 [Microbiology]

October 23rd, 2014 by Chekan, J. R., Kwon, I. H., Agarwal, V., Dodd, D., Revindran, V., Mackie, R. I., Cann, I., Nair, S. K.

Hemicelluloses, the polysaccharide component of plant cell walls, represent one of the most abundant biopolymers in nature. The most common hemicellulosic constituents of softwoods, such conifers and cycads, are mannans consisting of a 1,4 linked β-mannopyranosyl main chain with branch decorations. Efforts towards the utilization of hemicellulose for bioconversion into cellulosic biofuels have resulted in the identification of several families of glycoside hydrolases that can degrade mannan. However, effective biofermentation of manno-oligosaccharides is limited by a lack of appropriate uptake route in ethanologenic organisms. Here, we identify a mannan utilization gene cluster in the thermophilic bacterium Caldanaerobius polysaccharolyticus. The cluster contains several enzymes for the efficient mannan hydrolysis, as well as an unusual ABC transporter that shows an unexpected specificity for both linear and branched manno-oligosaccharides. Calorimetric analysis of the solute-binding component of the transporter (CpMnBP1) reveals high specificity for short mannose polymers, but with flexibility to accommodate polysaccharide decorations. Co-crystal structures of CpMnBP1 in complex with mannobiose (1.4 Å resolution) and mannotriose (2.2 Å resolution) reveal the molecular rationale for chain-length, oligosaccharide specificity. This work represents the first biochemical characterization of a mannose-specific solute binding protein, and provides a framework for engineering mannan utilization capabilities for microbial fermentation.