The X-Ray Structure of the Ligand-Free Antibiotic Ristocetin-A Reveals the Role of the Monomer/Dimer Equilibrium in Its Binding Mode

Figure 3

(a) The tetrasaccharide conformations and orientations in the ligand-free ristocetin-A. Superposition of the two subunits of the homodimer of ligand-free ristocetin-A using Cα of the two aglycon moieties as target by least squares (LSQAB program). The two aglycons are closely superposed but this is not the case for the tetrasaccharide connected to ring #4. Colors for carbon atoms are in yellow and cyan for the “extended” and “bent” conformations, respectively. (b) The cleft defined by the two tetrasaccharides on top of the dimer (colored in yellow and green). View along the local twofold axis building the homodimer and positioning the two saccharides head-to-tail (anti/anti) represented as Van der Waals surfaces. In this cleft sits the sulfate anion.

(a)

(b)