Further structural studies of the lytic polysaccharide monooxygenase AoAA13 belonging to the starch-active AA13 family

Lytic polysaccharide monooxygenases (LPMOs) are recently discovered copper enzymes that cleave recalcitrant polysaccharides by oxidation. The structure of an Aspergillus oryzae LPMO from the starch degrading family AA13 (AoAA13) has previously been determined from an orthorhombic crystal grown in the presence of copper, which is photoreduced in the structure. Here we describe how crystals reliably grown in presence of Zn can be Cu-loaded post crystallization. A partly photoreduced structure was obtained by severely limiting the X-ray dose, showing that this LPMO is much more prone to photoreduction than others. A serial synchrotron crystallography structure was also obtained, showing that this technique may be promising for further studies, to reduce even further photoreduction. We additionally present a triclinic structure of AoAA13, which has less occluded ligand binding site than the orthorhombic one. The availability of the triclinic crystals prompted new ligand binding studies, which lead us to the conclusion that small starch analogues do not bind to AoAA13 to an appreciable extent. A number of disordered conformations of the metal binding histidine brace have been encountered in this and other studies, and we have previously hypothesized that this disorder may be a consequence of loss of copper. We performed molecular dynamics in the absence of active site metal, and showed that the dynamics in solution differ somewhat from the disorder observed in the crystal, though the extent is equally dramatic.