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Lytic polysaccharide monooxygenases (LPMOs) are copper metalloenzymes which cleave polysaccharides oxidatively and are important in pathogen biology, carbon cycling and biotechnology. The Lentinus similis family AA9 isoform A (LsAA9_A) has been extensively studied as a model system because its activity towards smaller soluble saccharide substrates has allowed detailed structural characterization of its interaction with a variety of substrates by X-ray crystallography at high resolution. Here, the joint X-ray/neutron room-temperature crystallographic structure of carbohydrate-free LsAA9_A in the copper(II) resting state refined against X-ray and neutron data at 2.1 and 2.8 Å resolution, respectively, is presented. The results provide an experimental determination of the protonation states of the copper(II)-coordinating residues and second-shell residues in LsAA9_A, paving the way for future neutron crystallographic studies of LPMO-carbohydrate complexes.

The recently discovered lytic polysaccharide monooxygenases (LPMOs) are Cu-containing enzymes capable of degrading polysaccharide substrates oxidatively. The generally accepted first step in the LPMO reaction is the reduction of the active-site metal ion from Cu2+ to Cu+. Here we have used a systematic diffraction data collection method to monitor structural changes in two AA9 LPMOs, one from Lentinus similis (LsAA9_A) and one from Thermoascus aurantiacus (TaAA9_A), as the active-site Cu is photoreduced in the X-ray beam. For LsAA9_A, the protein produced in two different recombinant systems was crystallized to probe the effect of post-translational modifications and different crystallization conditions on the active site and metal photoreduction. We can recommend that crystallographic studies of AA9 LPMOs wishing to address the Cu2+ form use a total X-ray dose below 3 x 10(4) Gy, while the Cu+ form can be attained using 1 x 10(6) Gy. In all cases, we observe the transition from a hexacoordinated Cu site with two solvent-facing ligands to a T-shaped geometry with no exogenous ligands, and a clear increase of the theta(2) parameter and a decrease of the theta(3) parameter by averages of 9.2 degrees and 8.4 degrees, respectively, but also a slight increase in theta(T). Thus, the theta(2) and theta(3) parameters are helpful diagnostics for the oxidation state of the metal in a His-brace protein. On binding of cello-oligosaccharides to LsAA9_A, regardless of the production source, the theta(T) parameter increases, making the Cu site less planar, while the active-site Tyr-Cu distance decreases reproducibly for the Cu2+ form. Thus, the theta(T) increase found on copper reduction may bring LsAA9_A closer to an oligosaccharide-bound state and contribute to the observed higher affinity of reduced LsAA9_A for cellulosic substrates.

Lytic Polysaccharide Monooxygenases (LPMOs) are copper-dependent enzymes discovered within the last ten years, which oxidatively degrade recalcitrant biomass, making chains available for further degradation by hydrolytic enzymes. LPMOs show great potential as a key component in formulation of efficient enzyme cocktails for biomass conversion, and as such, for the production of bioethanol. Very little experimental information on the interactions of LPMOs with their substrates was available until recently. A combined biochemical, crystallographic and spectroscopic study elucidated the interactions between a family AA9 LPMO of the filamentous fungus Lentinus similis (LsAA9_A) and cello-oligosaccharides. Similar interactions were observed through NMR studies for a family AA9 LPMO of Neurospora crassa and glucans. In this thesis, I present studies focused on comparing substrate interactions and activity of LsAA9_A and a related LPMO from Collariella virescens (CvAA9_A). I studied substrate interactions using X-ray crystallography, and was able to solve complex structures for the CvAA9_A LPMO. I have investigated the use of Differential Scanning Fluorimetry as a guide to make complex structures with LPMOs and their substrates, and investigate enzyme stability. Enzyme stability and plasticity was further explored using Molecular Dynamics simulations on LsAA9_A, CvAA9_A and the family AA13 LPMO from Aspergillus oryzae (AoAA13). A long-term goal is to elucidate the enzymatic mechanism of LPMOs. To that extent, I studied the first step in the LPMO mechanism – transition from Cu(II) to Cu(I). I did this using X-ray photoreduction, which revealed geometric changes around the Cu-site. A co-substrate of LPMOs has been discovered to be either O2 or H2O2. In this work, I have experimented with crystal oxygenation for chemically reduced and cello-oligosaccharide substrate soaked crystals. During my studies I also aimed to determine the first neutron structure of an LPMO in

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that cleave polysaccharide substrates oxidatively. First discovered because of their action on recalcitrant crystalline substrates (chitin and cellulose), a number of LPMOs are now reported to act on soluble substrates, including oligosaccharides. However, crystallographic complexes with oligosaccharides have been reported for only a single LPMO so far, an enzyme from the basidiomycete fungus Lentinus similis (LsAA9_A). Here we present a more detailed comparative study of LsAA9_A and an LPMO from the ascomycete fungus Collariella virescens (CvAA9_A) with which it shares 41.5% sequence identity. LsAA9_A is considerably more thermostable than CvAA9_A, and the structural basis for the difference has been investigated. We have compared the patterns of oligosaccharide cleavage and the patterns of binding in several new crystal structures explaining the basis for the product preferences of the two enzymes. Obtaining structural information about complexes of LPMOs with carbohydrates has proven to be very difficult in general judging from the structures reported in the literature thus far, and this can be attributed only partly to the low affinity for small substrates. We have thus evaluated the use of differential scanning fluorimetry as a guide to obtaining complex structures. Furthermore, an analysis of crystal packing of LPMOs and glycoside hydrolases corroborates the hypothesis that active site occlusion is a very significant problem for LPMO-substrate interaction analysis by crystallography, due to their relatively flat and extended substrate binding sites.

Lytic polysaccharide monooxygenases (LPMOs) have been found to be key components in microbial (bacterial and fungal) degradation of biomass. They are copper metalloenzymes that degrade polysaccharides oxidatively and act in synergy with glycoside hydrolases. Recently crystallographic studies carried out at pH 5.5 of the LPMO from Lentinus similis belonging to the fungal LPMO family AA9 have provided the first atomic resolution view of substrate-LPMO interactions. The LsAA9A structure presented here determined at pH 3.5 shows significant disorder of the active site in the absence of substrate ligand. Furthermore some differences are also observed in regards to substrate (cellohexaose) binding, although the major interaction with the N-terminal histidine remains unchanged, Lytic polysaccharide monooxygenases (LPMOs) have been found to be key components in microbial (bacterial and fungal) degradation of biomass. They are copper metalloenzymes that degrade polysaccharides oxidatively and act in synergy with glycoside hydrolases. Recently crystallographic studies carried out at pH 5.5 of the LPMO from Lentinus similis belonging to the fungal LPMO family AA9 have provided the first atomic resolution view of substrate-LPMO interactions. The LsAA9A structure presented here determined at pH 3.5 shows significant disorder of the active site in the absence of substrate ligand. Furthermore some differences are also observed in regards to substrate (cellohexaose) binding, although the major interaction with the N-terminal histidine remains unchanged.