K 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.