Lectins are carbohydrate-binding proteins that play important roles in cell recognition and host-pathogen interactions. Many pathogenic bacteria produce lectins that are specific for glycans present on the host surface and that participate in adhesion at the early stages of infection. Lectin-carbohydrate interactions are mostly formed by hydrogen bonds between the sugar hydroxyl groups and the amino acid residues of the lectin binding sites. Other types of interactions can also be involved including CH-π stacking, hydrophobic interactions, water-bridging or metal coordination.
Neutron macromolecular crystallography (NMX) offers unique insights into the hydrogen-bonding network as it directly locates and visualizes all hydrogen (or deuterium) atoms. Perdeuteration where all hydrogen atoms are replaced by deuterium atoms enhances their visibility in the neutron maps. While perdeuteration of recombinant proteins is almost routinely carried out in dedicated facilities, the production of perdeuterated sugars is still very challenging.
Using NMX, we have unravelled the details of protein-carbohydrate interactions in two fucose-specific lectins, with the unique feature of producing perdeuterated monosaccharide fucose using a glyco-engineered strain of E. coli bacteria for preparation of co-crystals [1]. PLL lectin from bacteria Photorhabdus luminescens was chosen as a model system for a detailed description of the H-bonding network involved in sugar recognition, including direct and water-bridged hydrogen bonds and CD-π stacking interactions between the apolar face of fucose and aromatic amino acids [2, 3].
LecB lectin from Pseudomonas aeruginosa, a human opportunistic pathogen that causes lethal infections in cystic fibrosis patients, is currently viewed as a potential drug target for glycomimetic compounds with antiadhesive properties. LecB displays an unusually high affinity towards fucose with two calcium ions involved in the binding. The neutron study enabled a complete description of the hydrogen-bonding network and the protonation states of charged amino acids involved in the sugar binding including the observation of a low-barrier hydrogen bond between fucose and the protein [3]. The new structural data may help in the design of new potent glycomimetic antiadhesive compounds for fighting antibiotic-resistant bacteria.