Differential Ability of Lytic Polysaccharide Monooxygenases to Crack or Functionalize Cellulose Nano Crystals is Controlled by Substrate Ultrastructure

Lytic Polysaccharide MonoOxygenases display great variability towards cellulose ultrastructure while using these for carboxylation/functionalization of the polymers. Aiming at employing AA9 LPMOs enzymes for isolation of cellulose nano-crystals (CNCs), the functionalization/crystalline degradation ratio of these enzymes becomes crucial. Here we investigated the constrains posed by the substrate ultrastructure on the activity of seven different AA9 LPMOs representative of various regioselectivity and substrate affinity: TtAA9E, TaAA9A, PcAA9D, MtAA9A, MtAA9D, MtAA9I-CBM and MtAA9J. We found that the substrate crystallinity and dry matter loading greatly affects the seven AA9s in an enzyme-specific manner, impacting their efficiency for CNCs functionalization purposes. X-ray diffraction pattern analyses were used to assess the cracking efficacy of the enzymatic treatment to de-crystallize CNCs, revealing that those AA9 with minor efficiency in releasing oligosaccharides resulted instead the most disruptive towards the crystal lattice. These non-catalytic effects were found comparable with the one caused by the expansin BsEXLX1 enzyme.