Crystallographic and Thermodynamic Comparison of Natural and Synthetic Ligands Bound to Streptavidin

Abstract

The thermodynamic parameters of binding biotin and the dye 2-(4′-hydroxyphenylazo)benzoic acid (HABA) to streptavidin have been determined and are compared, together with X-ray crystal structures of the protein-ligand complexes and apostreptavidin. The X-ray crystal structures of apostreptavidin (space group I222, a = 94.2 Å,b= 104.0 Å, c = 47.7 Å, crystallographic R-factor = 0.209 at 1.84 Å resolution), a streptavidin-biotin complex (I222, a = 95.6 Å, b = 105.5 Å, c = 47.2 Å, R-factor = 0.171 at 1.55 Å resolution), and a streptavidin-HABA complex (I222, a = 95.1 Å, b = 105.6 Å, c = 47.4 Å, R-factor = 0.185 at 1.78 Å resolution) show common aspects of solvent displacement from the protein binding site together with preservation of an important interaction where the biotin ureido and HABA carboxylate oxygens bind in an oxyanion pocket formed by oriented hydrogen-bond donors in the binding site. Nevertheless, titrating calorimetric measurements show that biotin binding is enthalpically favored (ΔG° = −18.3 kcal/mol, ΔH° = −32.0 kcal/mol), while entropy terms dominate HABA binding (ΔG° = −5.27 kcal/mol, ΔH° = 1.70 kcal/mol). © 1992, American Chemical Society. All rights reserved.