Kβ X-ray Emission Spectroscopy of Cu(I)-Lytic Polysaccharide Monooxygenase: Direct Observation of the Frontier Molecular Orbital for H2O2 Activation

Lyticpolysaccharide monooxygenases (LPMOs) catalyze the degradationof recalcitrant carbohydrate polysaccharide substrates. These enzymesare characterized by a mononuclear Cu(I) active site with a three-coordinateT-shaped His-brace configuration including the N-terminalhistidine and its amine group as ligands. This study explicitly investigatesthe electronic structure of the d(10) Cu(I) active site ina LPMO using K & beta; X-ray emission spectroscopy (XES). The lackof inversion symmetry in the His-brace site enables the 3d/p mixingrequired for intensity in the K & beta; valence-to-core (VtC) XES spectrumof Cu(I)-LPMO. These K & beta; XES data are correlated to density functionaltheory (DFT) calculations to define the bonding, and in particular,the frontier molecular orbital (FMO) of the Cu(I) site. These experimentallyvalidated DFT calculations are used to evaluate the reaction coordinatefor homolytic cleavage of the H2O2 O-Obond and understand the contribution of this FMO to the low barrierof this reaction and how the geometric and electronic structure ofthe Cu(I)-LPMO site is activated for rapid reactivity with H2O2.

Automated summary

This study investigates the electronic structure of the d(10) Cu(I) active site in LPMO using Kβ X-ray emission spectroscopy (XES). The lack of inversion symmetry in the His-brace site enables the required 3d/p mixing for intensity in the Kβ valence-to-core (VtC) XES spectrum of Cu(I)-LPMO. These Kβ XES data are correlated to density functional theory (DFT) calculations to define the bonding, particularly the frontier molecular orbital (FMO) of the Cu(I) site. These experimentally validated DFT calculations are used to evaluate the reaction coordinate for homolytic cleavage of the H2O2 O-O bond and understand the activation of the geometric and electronic structure of the Cu(I)-LPMO site for rapid reactivity with H2O2.