Molecular mechanism of lytic polysaccharide monooxygenases

TL;DR

This study uses advanced computational methods to elucidate the detailed molecular mechanism of LPMOs, revealing how reactive intermediates form and react with polysaccharides to enhance biofuel production.

Contribution

It provides the most comprehensive computational description of LPMO mechanisms, including new insights into reactive intermediate formation and hydrogen abstraction pathways.

Findings

Protonation of superoxo complex leads to oxyl and hydroxyl intermediates.

Oxyl and hydroxyl complexes efficiently abstract hydrogen from substrates.

Reaction barriers align with experimental rate constants.

Abstract

The lytic polysaccharide monooxygenases (LPMOs) are copper metalloenzymes that can enhance polysaccharide depolymerization through an oxidative mechanism and hence boost generation of biofuel from e.g. cellulose. By employing density functional theory in a combination of quantum mechanics and molecular mechanics (QM/MM), we report a complete description of the molecular mechanism of LPMOs. The QM/MM scheme allows us to describe all reaction steps with a detailed protein environment and we show that this is necessary. Several active species capable of abstracting a hydrogen from the substrate have been proposed previously. We investigate previously suggested paths as well as new ones. We describe the generation of the reactive intermediates, the abstraction of a hydrogen atom from the polysaccharide substrate, as well as the final recombination step in which OH is transferred back to the substrate. We show that a superoxo [CuO2]+ complex can be protonated by a nearby histidine residue (suggested by recent mutagenesis studies and crystallographic work) and, provided an electron source is available, leads to formation of an oxyl-complex after cleavage of the O-O bond and dissociation of water. The oxyl complex either reacts with the substrate or is further protonated to a hydroxyl complex. Both the oxyl and hydroxyl complexes are also readily generated from a reaction with H2O2. The C-H abstraction by the oxyl and hydroxy complexes is overall favorable with activation barriers of 69 and 94 kJ/mol, compared to the much higher barrier (156 kJ/mol) obtained for the copper-superoxo species. We obtain good structural agreement for intermediates for which structural data are available and the estimated reaction energies agree with experimental rate constants. Thus, our suggested mechanism is the most complete to date and concur with available experimental evidence.