Abstract
The discovery of oxidative cleavage of recalcitrant polysaccharides by lytic polysaccharide monooxygenases (LPMOs) has affected the study and industrial application of enzymatic biomass processing. Despite being widespread in fungi, LPMOs belonging to the auxiliary activity (AA) family AA11 have been understudied. While these LPMOs are considered chitin active, some family members have little or no activity toward chitin, and the only available crystal structure of an AA11 LPMO lacks features found in bacterial chitin-active AA10 LPMOs. Here, we report structural and functional characteristics of a single-domain AA11 LPMO from Aspergillus fumigatus, AfAA11A. The crystal structure shows a substrate-binding surface with features resembling those of known chitin-active LPMOs. Indeed, despite the absence of a carbohydrate-binding module, AfAA11A has considerable affinity for α-chitin and, more so, β-chitin. AfAA11A is active toward both these chitin allomorphs and enhances chitin degradation by an endoacting chitinase, in particular for α-chitin. The catalytic activity of AfAA11A on chitin increases when supplying reactions with hydrogen peroxide, showing that, like LPMOs from other families, AfAA11A has peroxygenase activity. These results show that, in stark contrast to the previously characterized AfAA11B from the same organism, AfAA11A likely plays a role in fungal chitin turnover. Thus, members of the hitherto rather enigmatic family of AA11 LPMOs show considerable structural and functional differences and may have multiple roles in fungal physiology.
Highlights
Many microorganisms produce an arsenal of carbohydrateactive enzymes that concertedly operate to degrade carbohydrate biomass
While chitin-active lytic polysaccharide monooxygenase (LPMO) are commonly found among members of the AA10 family, which lacks fungal enzymes, and while fungi are known for their ability to produce multiple chitinases and degrade chitin [29], little is known about fungal chitin-active LPMOs
A recent in-depth functional study of AfAA11B [32], which is very similar to a twodomain AA11 from Aspergillus oryzae (AoAA11) (73% sequence identity for the catalytic domains), showed that this enzyme has low activity on chitin while being efficient when acting on soluble substrates
Summary
Many microorganisms produce an arsenal of carbohydrateactive enzymes that concertedly operate to degrade carbohydrate biomass. A recent in-depth functional study of AfAA11B [32], which is very similar to AoAA11 (73% sequence identity for the catalytic domains), showed that this enzyme has low activity on chitin (i.e., much lower than chitin-active bacterial AA10 LPMOs) while being efficient when acting on soluble substrates.
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