Changes in active-site geometry on X-ray photoreduction of a lytic polysaccharide monooxygenase active-site copper and saccharide binding.

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 Cu to Cu. Here we have used a systematic diffraction data collection method to monitor structural changes in two AA9 LPMOs, one from (AA9_A) and one from (AA9_A), as the active-site Cu is photoreduced in the X-ray beam. For AA9_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 Cu form use a total X-ray dose below 3 × 10 Gy, while the Cu form can be attained using 1 × 10 Gy. In all cases, we observe the transition from a hexa-coordinated Cu site with two solvent-facing ligands to a T-shaped geometry with no exogenous ligands, and a clear increase of the θ parameter and a decrease of the θ parameter by averages of 9.2° and 8.4°, respectively, but also a slight increase in θ. Thus, the θ and θ parameters are helpful diagnostics for the oxidation state of the metal in a His-brace protein. On binding of cello-oligosaccharides to AA9_A, regardless of the production source, the θ parameter increases, making the Cu site less planar, while the active-site Tyr-Cu distance decreases reproducibly for the Cu form. Thus, the θ increase found on copper reduction may bring AA9_A closer to an oligosaccharide-bound state and contribute to the observed higher affinity of reduced AA9_A for cellulosic substrates.