Structural and thermodynamic analyses of solute-binding protein from Bifidobacterium longum specific for core 1 disaccharide and lacto-N-biose I

Recently, a gene cluster involving a phosphorylase specific for lacto-N-biose I (LNB; Gal beta 1-3GlcNAc) and galacto-N-biose (GNB; Gal beta 1-3GalNAc) has been found in Bifidobacterium longum. We showed that the solute-binding protein of a putative ATP-binding cassette-type transporter encoded in the cluster crystallizes only in the presence of LNB or GNB, and therefore we named it GNB/LNB-binding protein (GL-BP). Isothermal titration calorimetry measurements revealed that GL-BP specifically binds LNB and GNB with K-d values of 0.087 and 0.010 mu M, respectively, and the binding process is enthalpy-driven. The crystal structures of GL-BP complexed with LNB, GNB, and lacto-N-tetraose ( Gal beta 1-3GlcNAc beta 1-3Gal beta 1-4Glc) were determined. The interactions between GL-BP and the disaccharide ligands mainly occurred through water-mediated hydrogen bonds. In comparison with the LNB complex, one additional hydrogen bond was found in the GNB complex. These structural characteristics of ligand binding are in agreement with the thermodynamic properties. The overall structure of GL-BP was similar to that of maltose-binding protein; however, the mode of ligand binding and the thermodynamic properties of these proteins were significantly different.