Specific Xylan Activity Revealed for AA9 Lytic Polysaccharide Monooxygenases of the Thermophilic Fungus Malbranchea cinnamomea by Functional Characterization

• Published in: Applied and environmental microbiology Vol. 85; no. 23; p. 1
• Main Authors: Hüttner, Silvia, Várnai, Anikó, Petrović, Dejan M, Bach, Cao Xuan, Kim Anh, Dang Thi, Thanh, Vu Nguyen, Eijsink, Vincent G H, Larsbrink, Johan, Olsson, Lisbeth
• Format: Journal Article
• Language: English
• Published: United States American Society for Microbiology 01.12.2019
• Subjects: Biodegradation; biomass degradation; Carbon sources; Cell walls; Cellulose; Conformation; Depolymerization; Fungi; Genomes; Hemicellulose; Malbranchea cinnamomea; Microfibrils; monooxygenases; Phosphoric acid; Polymers; Polysaccharides; Rigidity; Substrate specificity; Substrates; thermophilic; Thermophilic fungi; Xylan; Xyloglucan; thermophilic; biomass degradation; hemicellulose; monooxygenases
• ISSN: 0099-2240; 1098-5336; 1098-5336
• DOI: 10.1128/AEM.01408-19

The thermophilic biomass-degrader 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 depolymerization. We characterized four of the eight ( AA9s) AA9s, namely, AA9A, AA9B, AA9F, and AA9H, to gain a deeper understanding about their roles in the fungus. The characterized AA9s were active on hemicelluloses, including xylan, glucomannan, and xyloglucan, and furthermore, in accordance with transcriptomics data, differed in substrate specificity. Of the AA9s, AA9H is unique, as it preferentially cleaves residual xylan in phosphoric acid-swollen cellulose (PASC). Moreover, when exposed to cellulose-xylan blends, AA9H shows a preference for xylan and for releasing (oxidized) xylooligosaccharides. The cellulose dependence of the xylan activity suggests that a flat conformation, with rigidity similar to that of cellulose microfibrils, is a prerequisite for productive interaction between xylan and the catalytic surface of the LPMO. AA9H showed a similar trend on xyloglucan, underpinning the suggestion that LPMO activity on hemicelluloses strongly depends on the polymers' physicochemical context and conformation. Our results support the notion that LPMO multiplicity in fungal genomes relates to the large variety of copolymeric polysaccharide arrangements occurring in the plant cell wall. The LPMOs ( AA9s) 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.