Mechanism of lignin inhibition of enzymatic deconstruction of pretreated biomass and how to overcome it

The conversion of plant biomass to ethanol via enzymatic cellulose hydrolysis offers a potentially sustainable route to biofuel production. However, the inhibition of enzymatic activity in pretreated biomass by lignin severely limits the efficiency of this process. We discuss integrated molecular dynamics (MD) computer simulations and small-angle neutron scattering (SANS) experiments aiming at understanding the mechanism of this inefficiency and suggest means to overcome it. Lignin binds preferentially both to the elements of cellulose to which the cellulases also preferentially bind (the hydrophobic faces) and also to the specific residues on the cellulose-binding module of the cellulase that are critical for cellulose binding of TrCel7A (Y466, Y492 and Y493) [1]. As predicted by MD and confirmed by SANS, the undesirable lignin aggregation on cellulose takes place during the heating phase of pretreatment [2,3]. A tetrahydrofuran (THF)-water pretreatment shifts the equilibrium structure of lignin from a globule state to an extended coil that does not aggregate on cellulose [4]. Finally, we find that the non-covalent association with hemicellulose of lignin containing aldehyde groups is reduced compared to the wild-type. This phase separation may increase the cell wall porosity in transgenic plants down-regulation of cinnamyl alcohol dehydrogenase, thus explaining their easier deconstruction to biofuels [5].

1. Vermaas et al.; Biotechnol. Biofuel 2015, accepted.

2. Petridis et al.; J. Am. Chem. Soc. 2011, 133, 20277-20287.

3. Pingali et al.; Cellulose 2014, 21, 873-878.

4. Carmona et al.; Phys. Chem. Chem. Phys. 2015, 17, 358-364.

5. Smith et al.;  Green Chem. 2015, DOI: 10.1039/C5GC01952D.