In general, LPMO reactions with O2 and H2O2 have been intensively studied experimentally and computationally. Unlike the iron-heme proteins, computational studies focused mainly on events after binding of the co-substrate, 8, 9, 47, 48 whereas less attention has been paid to the priming reduction , substrate binding and the events before the co-substrate interacts with Cu(I). However, a recent combined experimental/theoretical study 51 by Lim et al. interrogated the Cu(I) species of an AA9 LPMO through core spectroscopy and density functional theory (DFT) calculations: They found the Cu(I) species to have a dx2-y2 frontier orbital that was proposed to be optimally oriented to react with an incoming co-substrate (H2O2 in ref. ). Lim et al. further argue that the dx2-y2-orbital is significantly higher in energy (1.1-1.4 eV) compared to the remaining occupied copper d-orbitals, bringing it closer to the LUMO (the H2O2 σ*-orbital). This facilitates the one-electron transfer from the copper to H2O2 in reaction 6b in Figure . Yet, this investigation did not explicitly include a substrate. Meanwhile, other studies have suggested that substrate binding has specific impacts on LPMO reactivity: The nature of the substrate has been proposed to take part in determining the mechanism for the generation of the oxidative species and substrate binding has been proposed to directly influence the orbitals of d-character. Further, several electron paramagnetic resonance (EPR) studies on AA9 and AA10 LPMOs report the appearance of super-hyperfine couplings and changes in the EPR spectra upon substrate binding. Different explanations for these observations were given. For example, conformational changes such as a distortion of the copper-coordinating amino acids and a reorganization of coordinating water molecules. It has also been proposed that substrate-induced electronic structure changes "predispose" the active site to form a stable Cu(II)-Cell-O2 intermediate. Some of the discussed electronic structure changes include, for example, a stronger interaction between the copper and the coordinating nitrogen atoms and an increase in energy of the dx2-y2 frontier orbital. To reveal further details of the first steps of the LPMO reaction and the effects of reduction and substrate binding, we use a frontier orbital approach. Analysing frontier molecular orbitals (MOs) has previously been used to understand the reactivity of both iron-heme enzymes and copper enzymes involving a Cu(II)-O2 moiety. The orbitals around the HOMO and LUMO are here taken as acceptors for incoming co-substrates. We analyse changes in the frontier MOs in the presence and absence of substrate and further how reduction and incoming O2 and H2O2 influence the electronic structure. We use the structures from a recent crystallographic study by Tandrup et al. to directly link the frontier MOs to accurate experimental structures: In the structures obtained in Ref. , a controlled photoreduction of the copper in the X-ray beam was exploited to obtain structures that can be assigned Cu(I) and Cu(II) oxidation states, both with and without bound cellooligosaccharide substrate (abbreviated as Cell). Thus, with outset in these crystal structures, we investigate how the observed changes in active-site geometry due to reduction and substrate binding influence the electronic structure through analysis of the frontier orbitals. Moreover, we consider also how the frontier orbitals change due to incoming oxidative species in the forms of O2 or H2O2.