Crystal structures of human transthyretin complexed with glabridin

Transthyretin (TTR) is a plasma protein implicated in human amyloid diseases. Several small molecules that bind to the thyroxine-binding site of TTR have been shown to stabilize the TTR tetramer and to inhibit amyloid fibril formation of TTR. Herein, we demonstrated that glabridin (Glab), a prenylated isoflavan isolated from Glycyrrhiza glabra L., inhibited aggregation of TTR in a thioflavin assay. The TTR-Glab complex structure revealed a novel binding mode including a CH-π interaction with A108 and a hydrogen bond with K15. A structural comparison with the wild type-apo structure revealed that the CH-π interaction with A108 was strengthened by the induced-fit conformational change upon Glab binding. Furthermore, the binding of Glab induced a rotation of the T119 side chain, and the inclusion of a water molecule, leading to stabilization of the dimer-dimer interface. These results demonstrate that Glab is a novel inhibitor of TTR fibrillization and suggest the molecular mechanism by which Glab binding stabilizes the tetramer.