The role of lytic polysaccharide monooxygenases in commercial cellulase mixtures

The recently discovered lytic polysaccharide monooxygenases (LPMOs) are capable of oxidatively cleaving polysaccharides, including chitin, cellulose, hemicellulose and starch. New commercial cellulase preparations contain LPMOs which increase the rate of cellulose conversion through synergistic action with hydrolytic cellulases. LPMOs catalyse oxidative cleavage of the β-1,4 glycosidic bonds of cellulose using molecular oxygen, leading to oxidation of either the C1 or the C4 carbon. LPMO activity further requires an external electron donor such as ascorbic acid. We report the effect of LPMO activity in commercial enzyme cocktails on the overall saccharification of Avicel and steam exploded birch wood. Saccharification experiments were performed at 10% and 20% (w/w) solid loadings with and without addition of ascorbic acid under both anaerobic and aerobic conditions. Under aerobic conditions the amount of glucose released from Avicel and birch was 20% and 60% higher than under anaerobic conditions, respectively. These higher yields were associated with the detection of oxidized products in the hydrolysates. The positive effect of oxygen in Avicel degradation was only observed if an external electron donor had been added. For degradation of steam exploded birch an external electron donor was not needed, indicating that lignin may serve as electron donor. Furthermore, the effect of LPMOs was investigated by spiking cellulase cocktails with purified LPMOs at varying concentrations. Our results show that LPMOs represent a significant contribution for more efficient cellulose degradation and that regulation of process conditions is essential to take advantage of the full potential of these oxidative enzymes.