A C4-oxidizing lytic polysaccharide monooxygenase cleaving both cellulose and cello-oligosaccharides [Enzymology]

December 9th, 2013 by Isaksen, T., Westereng, B., Aachmann, F. L., Agger, J. W., Kracher, D., Kittl, R., Ludwig, R., Haltrich, D., Eijsink, V. G. H., Horn, S. J.

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 a 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 which 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 non-reducing end.