Electron transfer in polysaccharide monooxygenase catalysis

Polysaccharide monooxygenase (PMO) catalysis involves the chemically difficult hydroxylation of unactivated C-H bonds in carbohydrates. The reaction requires reducing equivalents and will utilize either oxygen or hydrogen peroxide as a cosubstrate. Two key mechanistic questions are addressed here: 1) How does the enzyme regulate the timely and tightly controlled electron delivery to the mononuclear copper active site, especially when bound substrate occludes the active site? and 2) How does this electron delivery differ when utilizing oxygen or hydrogen peroxide as a cosubstrate? Using a computational approach, potential paths of electron transfer (ET) to the active site copper ion were identified in a representative AA9 family PMO from Myceliophthora thermophila (MtPMO9E). When Y62, a buried residue 12 Å from the active site, is mutated to F, lower activity is observed with O₂. However, a WT-level activity is observed with H₂O₂ as a cosubstrate indicating an important role in ET for O₂ activation. To better understand the structural effects of mutations to Y62 and axial copper ligand Y168, crystal structures were solved of the wild type MtPMO9E and the variants Y62W, Y62F, and Y168F. A bioinformatic analysis revealed that position 62 is conserved as either Y or W in the AA9 family. The MtPMO9E Y62W variant has restored activity with O₂. Overall, the use of redox-active residues to supply electrons for the reaction with O₂ appears to be widespread in the AA9 family. Furthermore, the results provide a molecular framework to understand catalysis with O₂ versus H₂O₂.