Docking and SAR studies of calystegines: Binding orientation and influence on pharmacological chaperone effects for Gaucher’s disease

We report on the identification of the required configuration and binding orientation of nor-tropane alkaloid calystegines against β-glucocerebrosidase. Calystegine B₂ is a potent competitive inhibitor of human lysosomal β-glucocerebrosidase with K_i value of 3.3 μM. A molecular docking study revealed that calystegine B₂ had a favorable van der Waals interactions (Phe128, Trp179, and Phe246) and the hydrogen bonding (Glu235, Glu340, Asp127, Trp179, Asn234, Trp381 and Asn396) was similar to that of isofagomine. All calystegine isomers bound into the same active site as calystegine B₂ and the essential hydrogen bonds formed to Asp127, Glu235 and Glu340 were maintained. However, their binding orientations were obviously different. Calystegine A₃ bound to β-glucocerebrosidase with the same orientations as calystegine B₂ (Type 1), while calystegine B₃ and B₄ had different binding orientations (Type 2). It is noteworthy that Type 1 orientated calystegines B₂ and A₃ effectively stabilized β-glucocerebrosidase, and consequently increased intracellular β-glucocerebrosidase activities in N370S fibroblasts, while Type 2 orientated calystegines B₃ and B₄ could not keep the enzyme activity. These results clearly indicate that the binding orientations of calystegines are changed by the configuration of the hydroxyl groups on the nor-tropane ring and the suitable binding orientation is a requirement for achieving a strong affinity to β-glucocerebrosidase.