LAUSR

The thermophilic biomass-degrader Malbranchea cinnamomea exhibits poor growth on cellulose but excellent growth on hemicelluloses as the sole carbon source. This is surprising considering that its genome encodes eight lytic polysaccharide monooxygenases (LPMOs) from auxiliary activity family 9 (AA9), enzymes known for their high potential in accelerating cellulose depolymerisation. We characterized four of the eight McAA9s, namely McAA9A, McAA9B, McAA9F and McAA9H to gain a deeper understanding about their roles in the fungus. The characterised McAA9s were active on hemicelluloses, including xylan, glucomannan and xyloglucan, and furthermore, in accordance with transcriptomics data, differ in substrate specificity. Of the McAA9s, McAA9H is unique, as it preferentially cleaves residual xylan in phosphoric acid-swollen cellulose (PASC). Moreover, when exposed to cellulose-xylan blends, McAA9H shows a preference for xylan and for releasing (oxidized) xylo-oligosaccharides. The cellulose-dependence of the xylan-activity suggests that a flat conformation, with similar rigidity to that of cellulose microfibrils, is a prerequisite for productive interaction between xylan and the catalytic surface of the LPMO. McAA9H showed a similar trend on xyloglucan, underpinning the suggestion that LPMO activity on hemicelluloses may strongly depend on the polymersÕ physico-chemical context and conformation. Our results support the notion that LPMO multiplicity in fungal genomes relates to the large variety of co-polymeric polysaccharide arrangements occurring in the plant cell wall.Importance: The Malbranchea cinnamomea LPMOs (McAA9s) showed activity on a broad range of soluble and insoluble substrates, suggesting their involvement in various steps of biomass degradation besides cellulose decomposition. Our results indicate that the fungal AA9 family is more diverse than originally thought and able to degrade almost any kind of plant cell wall polysaccharide. The discovery of an AA9 that preferentially cleaves xylan enhances our understanding of the physiological roles of LPMOs and enables the use of xylan-specific LPMOs in future applications.