Characterization of two family AA9 LPMOs fromAspergillus tamariiwith distinct activities on xyloglucan reveals structural differences linked to cleavage specificity

Aspergillus tamariigrows abundantly in naturally composting waste fibers of the textile industry and has a great potential in biomass decomposition. Amongst the key (hemi)cellulose-active enzymes in the secretomes of biomass-degrading fungi are the lytic polysaccharide monooxygenases (LPMOs). By catalyzing oxidative cleavage of glycoside bonds, LPMOs promote the activity of other lignocellulose-degrading enzymes. Here, we analyzed the catalytic potential of two of the seven AA9-type LPMOs that were detected in recently published transcriptome data forA.tamarii, namelyAtAA9A andAtAA9B. Analysis of products generated from cellulose revealed thatAtAA9A is a C4-oxidizing enzyme, whereasAtAA9B yielded a mixture of C1- and C4-oxidized products.AtAA9A was also active on cellopentaose and cellohexaose. Both enzymes also cleaved the beta-(1 -> 4)-glucan backbone of tamarind xyloglucan, but with different cleavage patterns.AtAA9A cleaved the xyloglucan backbone only next to unsubstituted glucosyl units, whereasAtAA9B yielded product profiles indicating that it can cleave the xyloglucan backbone irrespective of substitutions. Building on these new results and on the expanding catalog of xyloglucan- and oligosaccharide-active AA9 LPMOs, we discuss possible structural properties that could underlie the observed functional differences. The results corroborate evidence that filamentous fungi have evolved AA9 LPMOs with distinct substrate specificities and regioselectivities, which likely have complementary functions during biomass degradation.