Lytic polysaccharide monoxygenase are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C−H bonds using O₂ and/or H₂O₂ as oxidants. In this article, we analyze the oxidation chemistry of an unusual mononuclear Cu complex bound by a podal ligand that can act as a H-bond/proton donor. Our results suggest that LPMOs react with O₂ to produce H₂O₂ (oxidase-like chemistry), which is used to promote C−H hydroxylation via a Fenton-like mechanism.

Abstract

Lytic polysaccharide monooxygenases (LPMOs) are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C−H bonds in polysaccharides using O₂ or H₂O₂ as oxidants (monooxygenase/peroxygenase). In the absence of C−H substrate, LPMOs reduce O₂ to H₂O₂ (oxidase) and H₂O₂ to H₂O (peroxidase) using proton/electron donors. This rich oxidative reactivity is promoted by a mononuclear Cu center in which some of the amino acid residues surrounding the metal might accept and donate protons and/or electrons during O₂ and H₂O₂ reduction. Herein, we utilize a podal ligand containing H-bond/proton donors (LH₂) to analyze the reactivity of mononuclear Cu species towards O₂ and H₂O₂. [(LH₂)Cu^I]¹⁺ (1), [(LH₂)Cu^II]²⁺ (2), [(LH⁻)Cu^II]¹⁺ (3), [(LH₂)Cu^II(OH)]¹⁺ (4), and [(LH₂)Cu^II(OOH)]¹⁺ (5) were synthesized and characterized by structural and spectroscopic means. Complex 1 reacts with O₂ to produce 5, which releases H₂O₂ to generate 3, suggesting that O₂ is used by LPMOs to generate H₂O₂. The reaction of 1 with H₂O₂ produces 4 and hydroxyl radical, which reacts with C−H substrates in a Fenton-like fashion. Complex 3, which can generate 1 via a reversible protonation/reduction, binds H₂O and H₂O₂ to produce 4 and 5, respectively, a mechanism that could be used by LPMOs to control oxidative reactivity.