Catalytic Mechanism of Fungal Lytic Polysaccharide Monooxygenases Investigated by First-Principles Calculations

Abstract

Lytic polysaccharide monooxygenases (LPMOs) are Cu-containing enzymes that facilitate the degradation of recalcitrant polysaccharides by the oxidative cleavage of glycosidic bonds. They are gaining rapidly increasing attention as key players in biomass conversion, especially for the production of second-generation biofuels. Elucidation of the detailed mechanism of the LPMO reaction is a major step toward the assessment and optimization of LPMO efficacy in industrial biotechnology, paving the way to utilization of sustainable fuel sources. Here, we used density functional theory calculations to study the reaction pathways suggested to date, exploiting a very large active-site model for a fungal AA9 LPMO and using a celloheptaose unit as a substrate mimic. We identify a copper oxyl intermediate as being responsible for H-atom abstraction from the substrate, followed by a rapid, water-assisted hydroxyl rebound, leading to substrate hydroxylation.

Reactivity of O 2 versus H 2 O 2 with polysaccharide monooxygenases

“…In addition, the PMO second coordination sphere residues are positioned to stabilize a cupric superoxide species (23,36,37). While extensive experimental work continually points to a cupric superoxide species for HAA by Cu enzymes (12,28,38), computational work has favored Cu-oxyl-based mechanisms for PMOs (39,40). Conversely, the peroxygenase mechanism appears to involve hydroxyl radical chemistry resulting from Fenton-like chemisty with H 2 O 2 and Cu(I).…”

Bracing copper for the catalytic oxidation of C–H bonds

“…A key reactive intermediate in LPMOs proposed primarily on the basis of DFT calculations (of varying merit, see reference 23 for detailed comment) is the Cu(II)-oxyl, [CuO] + . 63,67,76 Beyond LPMOs, such a species has long been proposed to be involved in monomeric copper oxidation catalysts/enzymes. 49,77 The proposal of [CuO] + as a reaction intermediate is particularly appealing in view of the results of computational work that support it being highly reactive with strong C H bonds 78,79,80 and experimental gas phase studies of the [CuO] + ion focused on its ability to hydroxylate methane.…”

Kinetic insights into the role of the reductant in H2O2-driven degradation of chitin by a bacterial lytic polysaccharide monooxygenase

“…Although the possibility of a (O 2 -using) monooxygenase reaction is still being debated, the fact is that the (H 2 O 2 -using) peroxygenase reaction is several orders of magnitude more efficient (23,25,26). Computational studies also support the plausibility of H 2 O 2 as co-substrate for LPMOs (27)(28)(29). The peroxygenase-like reaction also depends on an external electron donor; however, in principle, the reductant is only needed for initial activation (priming) of the Cu(II) resting state to the catalytically active Cu(I) state (23).…”