Kinetic analysis of amino acid radicals formed in H2O2-driven CuILPMO reoxidation implicates dominant homolytic reactivity

Abstract

Lytic polysaccharide monooxygenases (LPMOs) have been proposed to react with bothO2andH2O2as cosubstrates. In this study, theH2O2reaction with reducedHypocrea jecorinaLPMO9A (CuI-HjLPMO9A) is demonstrated to be 1,000-fold faster than theO2reaction while producing the same oxidized oligosaccharide products. Analysis of the reactivity in the absence of polysaccharide substrate by stopped-flow absorption and rapid freeze–quench (RFQ) electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) yields two intermediates corresponding to neutral tyrosyl and tryptophanyl radicals that are formed along minor reaction pathways. The dominant reaction pathway is characterized by RFQ EPR and kinetic modeling to directly produce CuII-HjLPMO9A and indicates homolytic O–O cleavage. Both optical intermediates exhibit magnetic exchange coupling with the CuIIsites reflecting facile electron transfer (ET) pathways, which may be protective against uncoupled turnover or provide an ET pathway to the active site with substrate bound. The reactivities of nonnative organic peroxide cosubstrates effectively exclude the possibility of a ping-pong mechanism.