Investigating the substrate oxidation mechanism in lytic polysaccharide monooxygenase: H2O2- versus O2-activation

Lytic polysaccharide monooxygenases (LPMOs) form a copper-dependent family of enzymes classified under the auxiliary activity (AA) superfamily. The LPMOs are known for their boosting of polysaccharide degradation through oxidation of the glycosidic bonds that link the monosaccharide subunits. This oxidation has been proposed to be dependent on either O2 or H2O2 as co-substrate. Theoretical investigations have previously supported both mechanisms, although this contrasts with recent experiments. A possible explanation is that the theoretical results critically depend on how the Cu active site is modeled, i.e., which second-sphere residues were included in the QM region. This has also led to different results even when employing only H2O2 as co-substrate. In this paper, we investigate both the O2- and H2O2-driven pathways, employing LsAA9 as the underlying LPMO and a theoretical model based on a quantum mechanics/molecular mechanics (QM/MM) framework. We ensure to consistently include all residues known to be important by using extensive QM regions. We also investigate several conformers that can partly explain the differences seen in previous studies. We find that the O2-driven reaction is unfeasible, in contrast to our previous QM/MM calculations with smaller QM regions. Meanwhile, the H2O2-driven pathway is feasible showing that for LsAA9, only H2O2 is a viable co-substrate as proposed experimentally.