The high-resolution structure of (+)-epi-biotin bound to streptavidin

(+)-Epi-biotin differs from (+)-biotin in the configuration of the chiral center at atom C2. This could lead to a difference in the mode of binding of (+)-epi-biotin to streptavidin, a natural protein receptor for (+)-biotin. Diffraction data were collected to a maximum of 0.85 (A) over circle resolution for structural analysis of the complex of streptavidin with a sample of (+)-epi-biotin and refinement was carried out at both 1.0 and 0.85 (A) over circle resolution. The structure determination shows a superposition of two ligands in the binding site, (+)-biotin and (+)-epi-biotin. The molecules overlap in the model for the complex except for the position of S1 in the tetrahydrothiophene ring. Differences in the conformation of the ring permits binding of each molecule to streptavidin with little observable difference in the protein structures at this high resolution.