Detailed studies of enzymatic hydrolysis of the abundant but recalcitrant biopolymers cellulose and chitin has led to a revised understanding of enzymatic decomposition of polysaccharides. Copper-enzymes now classified as lytic polysaccharide monooxygenases (LPMOs) were first annotated as either glycosyl hydrolases or as binding domains without catalytic activity. It was shown that enzymes at the time classified as family GH61 are copper-dependent enzymes that cleaves glucosidic bonds when provided with plant derived cofactors. The first crystal structures of Cu-LPMO, the observation of and evidence for a plausible role of N-methylation of the copper coordinating N-terminal histidine in fungal LPMOs, the first study of binding of dioxygen to reduced Cu-LPMO, the first atomic structures of a starch active LPMO and the first atomic structures of a LPMO-oligosaccharide complex are contributions that have led to significant advances in the scientific understanding.
In addition, use of LPMOs for industrial saccharification of lignocellulose was studied in detail and the importance of controlling the oxidative process was emphasised. A positive effect of adding catalase was attributed to prolonged enzyme half-life and thus the first evidence of oxidative deactivation of cellulolytic enzymes in this context. Furthermore, it was discovered that catalytic activity of LPMOs can be improved many fold when photo excited pigments, rather than any previously tested reducing agents, was used as the electron donating cofactor for the enzyme.
The discovery of LPMOs and the understanding that they constitute the “first wave of attack” by microbial organisms on the most recalcitrant natural polysaccharides, highlights the need to reflect on the specific roles of LPMOs in biology. Many good and relevant research questions are awaiting the scientific community. Based on current knowledge, the presence of LPMOs should be anticipated in all microbial systems where the degradation of recalcitrant polysaccharides is required to overcome a physical barrier, or when such polymers are the primary source of energy. It is therefore important to take the requirement of molecular oxygen and reducing agents into account when studying these systems and, moreover, to consider the impact of the powerful side reactions that may take place in such processes.