Abstract
Lytic polysaccharide monooxygenases (LPMOs) are copper dependent enzymes that carry out oxidative cleavage of cellulose and other polysaccharides. Aspergillus nidulans, an ascomycete fungus that contains multiple AA9 LPMOs in the genome, offers an excellent model system to study their activity during the oxidative degradation of biomass. AN1602, a dual domain AA9-LPMO in A. nidulans appended with a carbohydrate-binding module, CBM1, was expressed in Pichia pastoris for analyzing oxidative cleavage on cellulosic substrates. The mass spectral and HPAEC analyses showed that the enzyme cleaves phosphoric acid swollen cellulose (PASC) in the presence of a reducing agent, yielding a range of cello-oligosaccharides. In addition to the polymeric substrate cellulose, AN1602 is also active on soluble cellohexaose, a property that is restricted to only a few characterized LPMOs. Product analysis of AN1602 cleaved cellohexaose revealed that C4 was the sole site of oxidation. The sequence and predicted structure of the catalytic domain of AN1602 matched very closely to known C4 cellohexaose active enzymes.
Highlights
Plant biomass degrading fungi secrete a diverse set of enzymes and significantly contribute to the global carbon cycle
Our studies show that AN1602 is capable of conducting oxidative cleavage on soluble cello-oligosaccharides in addition to cellulose
AN1602, an AA9-lytic polysaccharide monooxygenases (LPMOs) abundantly induced by cellulose, contains a carbohydrate binding module 1 (CBM1) in addition to the catalytic domain (Jagadeeswaran et al 2016)
Summary
Plant biomass degrading fungi secrete a diverse set of enzymes and significantly contribute to the global carbon cycle. Relatively recently lytic polysaccharide monooxygenases (LPMOs), a class of novel copper-dependent redox enzymes were discovered These enzymes break glycosidic linkages by oxidation rather than hydrolysis. They make up a significant portion of plant cell wall degrading fungal secretomes (Floudas et al 2012; Glass et al 2013). Later studies demonstrated fungal LPMOs act synergistically when combined with other glycoside hydrolases during the decomposition of crystalline cellulose to glucose monomers (Forsberg et al 2011; Kim et al 2014; Langston et al 2011) To harness this potential in the conversion of lignocellulosic biomass, most modern commercial enzymatic cocktails, which are employed in biorefineries, contain LPMOs (Johansen 2016; Muller et al 2015)
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