Abstract
Lignocellulosic biomass is a renewable resource that significantly can substitute fossil resources for the production of fuels, chemicals, and materials. Efficient saccharification of this biomass to fermentable sugars will be a key technology in future biorefineries. Traditionally, saccharification was thought to be accomplished by mixtures of hydrolytic enzymes. However, recently it has been shown that lytic polysaccharide monooxygenases (LPMOs) contribute to this process by catalyzing oxidative cleavage of insoluble polysaccharides utilizing a mechanism involving molecular oxygen and an electron donor. These enzymes thus represent novel tools for the saccharification of plant biomass. Most characterized LPMOs, including all reported bacterial LPMOs, form aldonic acids, i.e., products oxidized in the C1 position of the terminal sugar. Oxidation at other positions has been observed, and there has been some debate concerning the nature of this position (C4 or C6). In this study, we have characterized an LPMO from Neurospora crassa (NcLPMO9C; also known as NCU02916 and NcGH61-3). Remarkably, and in contrast to all previously characterized LPMOs, which are active only on polysaccharides, NcLPMO9C is able to cleave soluble cello-oligosaccharides as short as a tetramer, a property that allowed detailed product analysis. Using mass spectrometry and NMR, we show that the cello-oligosaccharide products released by this enzyme contain a C4 gemdiol/keto group at the nonreducing end.
Highlights
Lytic polysaccharide monooxygenases (LPMOs) are recently discovered enzymes that cleave polysaccharides
In another study, using cellobiose dehydrogenase (CDH) as an electron donor, it was shown that three additional N. crassa AA9s, NcLPMO9C (NCU02916, NcGH61–3; attached to a CBM1), NcLPMO9F (NCU03328, NcGH61– 6), and NcLPMO9J (NCU01867, NcGH61–10; attached to a CBM1), degrade cellulose, but no attempts were made to unravel details of the reaction products of these enzymes (17)
The production of relatively short oligosaccharides by NcLPMO9C could be the result of a glucanase background activity in the enzyme sample or by NcLPMO9C having activity on soluble oligosaccharides
Summary
Lytic polysaccharide monooxygenases (LPMOs) are recently discovered enzymes that cleave polysaccharides. Recently it has been shown that lytic polysaccharide monooxygenases (LPMOs) contribute to this process by catalyzing oxidative cleavage of insoluble polysaccharides utilizing a mechanism involving molecular oxygen and an electron donor. These enzymes represent novel tools for the saccharification of plant biomass. Constituent of the plant cell wall, to fermentable sugars In nature this process is catalyzed by cellulases and the recently discovered lytic polysaccharide monooxygenases (LPMOs)[2] (1). Cellulose active LPMOs were found in both the AA10 (5) and AA9 (6 – 8) families These copper-dependent enzymes carry out oxidative cleavage of the -1,4-glycosidic bonds in polysaccharides, using molecular oxygen and an electron donor (1).
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