title: On the molecular basis of the high affinity binding of basic amino acids to LAOBP, a periplasmic binding protein from Salmonella typhimurium.
abstract: The rational designing of binding abilities in proteins requires an understanding of the relationship between structure and thermodynamics. However, our knowledge of the molecular origin of high-affinity binding of ligands to proteins is still limited; such is the case for l-lysine-l-arginine-l-ornithine periplasmic binding protein (LAOBP), a periplasmic binding protein from Salmonella typhimurium that binds to l-arginine, l-lysine, and l-ornithine with nanomolar affinity and to l-histidine with micromolar affinity. Structural studies indicate that ligand binding induces a large conformational change in LAOBP. In this work, we studied the thermodynamics of l-histidine and l-arginine binding to LAOBP by isothermal titration calorimetry. For both ligands, the affinity is enthalpically driven, with a binding ΔCp of ~-300 cal mol(-1)  K(-1) , most of which arises from the burial of protein nonpolar surfaces that accompanies the conformational change. Osmotic stress measurements revealed that several water molecules become sequestered upon complex formation. In addition, LAOBP prefers positively charged ligands in their side chain. An energetic analysis shows that the protein acquires a thermodynamically equivalent state with both ligands. The 1000-fold higher affinity of LAOBP for l-arginine as compared with l-histidine is mainly of enthalpic origin and can be ascribed to the formation of an extra pair of hydrogen bonds. Periplasmic binding proteins have evolved diverse energetic strategies for ligand recognition. STM4351, another arginine binding protein from Salmonella, shows an entropy-driven micromolar affinity toward l-arginine. In contrast, our data show that LAOBP achieves nanomolar affinity for the same ligand through enthalpy optimization.
authors.affiliation: Laboratorio de Fisicoquímica e Ingeniería de Proteínas, Departamento de Bioquímica, Facultad de Medicina, Universidad Nacional Autónoma de México, México, DF, Mexico.
authors.forename: Nancy O
authors.initials: NO
authors.lastname: Pulido
authors.forename: Daniel-Adriano
authors.initials: DA
authors.lastname: Silva
authors.forename: Luis A
authors.initials: LA
authors.lastname: Tellez
authors.forename: Gerardo
authors.initials: G
authors.lastname: Pérez-Hernández
authors.forename: Enrique
authors.initials: E
authors.lastname: García-Hernández
authors.forename: Alejandro
authors.initials: A
authors.lastname: Sosa-Peinado
authors.forename: D Alejandro
authors.initials: DA
authors.lastname: Fernández-Velasco
mesh_terms: D001120:Arginine / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002151:Calorimetry; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006639:Histidine / Q000378:metabolism*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D012486:Salmonella typhimurium / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; C033849:lysyl-arginyl-ornithine-binding protein, bacteria; D006639:Histidine; D001120:Arginine
keywords: Venus flytrap mechanism; enthalpy-driven binding; isothermal titration calorimetry; ligand recognition; structural thermodynamics