Fig. 9.

Mechanism of action of hydroxychloroquine (CLQ-OH). The N-terminal domain (NTD) bound to GM1 was superposed onto GM1 in interaction with two CLQ-OH molecules. The models only show the NTD and both CLQ-OH molecules (not GM1, to improve clarity). (a,b) The aromatic ring of F-135 (in red), which stacks onto the glucose cycle of GM1, overlaps the aromatic CLQ-OH rings (in green) which also bind to GM1 via a CH-π stacking mechanism. The N-137 residue of the NTD interacts with the N-acetylgalactosamine residue of GM1, but this interaction cannot occur in the presence of CLQ-OH as this part of GM1 is totally masked by the drug. (c,d) Superposition of the NTD surface (in purple) with the two CLQ-OH molecules bound to GM1, illustrating the steric impossibility that prevents NTD binding to GM1 when both CLQ-OH molecules are already interacting with the ganglioside.