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Figure 1
Illustrated is a simplified lytic polysaccharide monooxygenase (LPMO) reaction scheme. (a) LPMOs can engage in a variety of reactions depending on the specific enzyme. After Cu reduction, reactions with O2 and/or H2O2 may occur, involving saccharides or a variety of small mol­ecules, in some cases in several steps (Brander, Tokin et al., 2021BB14; Brander, Lausten et al., 2021BB13). Saccharide substrates produce smaller oxidized and/or non-oxidized saccharides as products. The first step in the reaction can be triggered either chemically by a reducing agent or using X-ray photons. The transition from Cu2+ to Cu+ results in expulsion of the equatorial (eq) and axial (ax) ligand water mol­ecules in the type II Cu site. These ligands may be different based on the crystallization liquor com­ponents. Upon binding of the substrate, the axial water mol­ecule is displaced. The LsAA9_A His-brace is shown at low (left; PDB entry 7pxi) and high (right; PDB entry 7pxn) X-ray dose (see results). (b) The overall structure of LsAA9_A binding cello­tetra­ose (PDB entry 6ydg). (c) The overall structure of TaAA9_A (PDB entry 3zud). (d) The bond angles referenced throughout the article are defined in Vu & Ngo (2018BB87). Angles θ1, θ2 and θ3 are measured from N—Cu—N as indicated. θH–H is the angle between the two His imidazole ring planes. θT is the angle between the Nδ1—Cu—Nam plane and the Nɛ2—Cu line. θH1 (also denoted θHN) is the angle between one His imidazole ring plane and the line from Cu to the Cu-inter­acting N atom of the same imidazole ring.

Volume 9| Part 5| September 2022| Pages 666-681
ISSN: 2052-2525