Understanding mechanisms responsible for strong cellulase inhibition by mannans

Hemicellulose mannans occur in many softwoods, palm kernel residues, and coffee residues as (galacto)glucomannan and in small amounts in hardwoods and some grasses as glucomannan. However, mannan’s inhibition of cellulase was unknown until a recent study by Kumar and Wyman (Biotechnol. Bioeng., 2014) showed that heteromannans and their enzymatically prepared oligomers strongly inhibit cellulase and reduce cellulose conversion by more than 45% even at a loading as low as 0.1 g/L. Although the mechanism was not clear, the virtual disappearance of inhibition with increasing cellulose to mannan weight ratios led us to hypothesize that mannan polysaccharide adsorption and strong association with the cellulose surface were partly responsible for cellulase inhibition. Furthermore, it was shown that the highly substituted mannan polysaccharides were stronger inhibitors than previously known inhibitors, such as cellobiose and xylooligomers, for pure cellulose hydrolysis performed with commercial cellulase cocktails. However, the presence of β-glucosidase and heteroxylan specific enzymes can reduce the amounts of inhibitory cellobiose and xylooligomers, respectively, during the course of hydrolysis, and, consequently, make it difficult to conclude whether mannan polysaccharides are the strongest inhibitors of cellulase. Therefore, to identify controlling factors and understand the role of mannan backbone substitution including galactose and/or acetyl groups on cellulase inhibition, cellulose hydrolysis was performed with purified cellulase components. Furthermore, this study investigated potential causes to establish mechanisms for ultra-strong inhibition of cellulase by mannan polysaccharides.  In addition, the inhibition type was determined for complete cellulase as well as purified components.