Differential binding of biomass hydrolyzing enzymes to lignin: conversion yields suggest a paradigm shift

Lignin, a major plant cell wall component, has been implicated in the observed loss of cellulase activity during enzymatic conversions of biomass to sugars. Currently, there is much debate as to whether this “lignin effect” occurs due to steric obstruction of cellulose, non-productive interactions between lignin and cellulases, and/or lignin “poisoning” of enzymes. While the mechanism(s) are not fully understood, the lignin effect is often cited as a reason why enzyme recycling is not considered economically feasible. In order to gain understanding of the causes, and potential solutions to this problem, we investigated several commercial cellulase preparations’ interactions with purified lignin. Surprisingly, the results suggest that the enzymes having the highest affinity for lignin are those that lack carbohydrate-binding modules (CBMs).  The observed binding was shown to reduce process yields by 4%-15% depending on the lignin concentration. We suggest that the adsorption of cellulases to lignin follows a competitive affinity paradigm and may be predicted based on the physiochemical characteristics of the individual components within the cellulase complex including protein architecture, surface charge, and relative hydrophobicity. In our experiments, process parameters such as temperature, pH, and salt concentration influenced enzyme affinity for lignin, suggesting that hydrophobic and electrostatic interactions are likely the primary mechanisms responsible for this binding phenomenon.