Tangled up in fibers: How a lytic polysaccharide monooxygenase binds its chitin substrate

Lytic polysaccharide monooxygenases (LPMOs) are redox-enzymes that bind to and oxidize insoluble carbohydrate substrates, such as chitin or cellulose. This class of enzymes has attracted considerable attention due to their ability to convert biomaterials of high abundance into oligosaccharides that can be useful for producing biofuels and bioplastics. However, processes at the interface between solution and insoluble substrates represent a major challenge to biochemical and structural characterization. This study used the four-domain LPMO from Vibrio cholerae, N- acetyl glucosamine binding protein A (GbpA), to elucidate how it docks onto its insoluble substrate with its two terminal domains. First, we developed a protocol that allowed GbpA and chitin to form a stable complex in suspension, overcoming incompatibilities of the two binding partners with respect to pH. After determining the neutron scattering contrast match point for chitin, we characterized the structure of GbpA in complex with chitin by Small-Angle Neutron Scattering (SANS). We found that GbpA binds rapidly to chitin, where it spreads out on the chitin fibers unevenly. These findings are supported by electron microscopy. Placing our findings into a biological context, we discussed the potential advantages of GbpA secretion and how chitin binding may prepare the ground for microcolony formation of the bacteria. ### Competing Interest Statement The authors have declared no competing interest. * CBM : carbohydrate-binding module GbpA : N- acetylglucosamine binding protein A D-GbpA : deuterated GbpA (EM) : Electron Microscopy H-GbpA : hydrogenated (= non-deuterated) GbpA HAc : acetic acid ILL : Institut Laue Langevin LPMO : lytic polysaccharide monooxygenase NIST : National Institute of Standards and Technology PDB : Protein Data Bank SANS : Small-Angle Neutron Scattering SAXS : Small-Angle X-ray Scattering USANS : Transmission Electrom Microscopy (TEM) : Ultra-Small-Angle Neutron Scattering