Mapping the Initial Stages of a Protective Pathway that Enhances Catalytic Turnover by a Lytic Polysaccharide Monooxygenase

Oxygenase and peroxygenase enzymes generate intermediates at their active sites which effect the controlled function-alization of inert C–H bonds in substrates, such as in the enzymatic conversion of methane to methanol. To be viable catalysts however, these enzymes must also prevent oxidative damage to essential active site residues, which can occur during both coupled and uncoupled turnover. Herein we use a combination of stopped-flow spectroscopy, targeted mu-tagenesis, TD-DFT calculations, high-energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS), and electron paramagnetic resonance spectroscopy (EPR) to study two transient intermediates that together form a protective pathway built into the active sites of copper-dependent lytic polysaccharide monooxygenas-es (LPMOs). First, a transient high-valent species is generated at the copper histidine brace active site following treat-ment of the LPMO with either hydrogen peroxide or peroxyacids in the absence of substrate. This intermediate, which we propose to be a CuII-(histidyl radical), then reacts with a nearby tyrosine residue in an intersystem-crossing reac-tion to give a ferromagnetically coupled (S = 1) CuII-tyrosyl radical pair, thereby restoring the histidine brace active site to its resting state and allowing it to re-enter the catalytic cycle through reduction. This process gives the enzyme the capacity to minimize damage to the active site histidine residues ‘on the fly’ to increase total turnover number prior to enzyme deactivation, highlighting how oxidative enzymes are evolved to protect themselves from deleterious side reac-tions during uncoupled turnover