Cellulases have low affinity but improved activity on unnatural cellulose allomorphs

Crystalline cellulose can exist in various allomorphic states some of which are not found to occur naturally (e.g., cellulose III). With the advent of next-generation extractive ammonia based pretreatments it is now feasible to produce crystalline cellulose III allomorphs during pretreatment of lignocellulosic biomass for biofuel production. However, very little is known about how cellulases interact with such unnatural cellulosic allomorphs. In this study, we explore the interactions of various cellulolytic enzymes and their individual carbohydrate binding modules (CBM) with cellulose III to gain insight into the mechanism of cellulose deconstruction. We have found that both Type-A and Type-B CBMs (known to target crystalline and amorphous cellulose, respectively) have reduced affinity for cellulose III compared to native cellulose I. Coarse-grained and fully atomistic molecular simulations were carried out to gain further insights into the molecular-basis for interaction between CBMs and cellulose III. In summary, this work represents a first-step in the direction of engineering novel enzymes that are more efficient at deconstructing unnatural allomorphs of cellulose into fermentable sugars.

This work was funded by the DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science DE-FC02-07ER64494).