Structural and thermodynamic insights into -1,2-glucooligosaccharide capture by a solute-binding protein in Listeria innocua

Abstract

-1,2-Glucans are bacterial carbohydrates that exist in cyclic or linear forms and play an important role in infections and symbioses involving Gram-negative bacteria. Although several -1,2-glucan–associated enzymes have been characterized, little is known about how -1,2-glucan and its shorter oligosaccharides (Sopns) are captured and imported into the bacterial cell. Here, we report the biochemical and structural characteristics of the Sopn-binding protein (SO-BP, Lin1841) associated with the ATP-binding cassette (ABC) transporter from the Gram-positive bacterium Listeria innocua. Calorimetric analysis revealed that SO-BP specifically binds to Sopns with a degree of polymerization of 3 or more, with Kd values in the micromolar range. The crystal structures of SO-BP in an unliganded open form and in closed complexes with tri-, tetra-, and pentaoligosac-charides (Sop3–5) were determined to a maximum resolution of 1.6 Å. The binding site displayed shape complementarity to Sopn, which adopted a zigzag conformation. We noted that water-mediated hydrogen bonds and stacking interactions play a pivotal role in the recognition of Sop3–5 by SO-BP, consistent with its binding thermodynamics. Computational free-energy calculations and a mutational analysis confirmed that interactions with the third glucose moiety of Sopns are significantly responsible for ligand binding. A reduction in unfavorable changes in binding entropy that were in proportion to the lengths of the Sopns was explained by conformational entropy changes. Phylogenetic and sequence analyses indicated that SO-BP ABC transporter homologs, glycoside hydrolases, and other related proteins are co-localized in the genomes of several bacteria. This study may improve our understanding of bacterial -1,2-glucan metabolism and promote the discovery of unidentified -1,2-glucan–associated proteins.