Mn²⁺ accelerates ligand-binding site activation of α_IIbβ₃ integrin: Insight from all-atom simulation

The activation of integrins by Mn²⁺ is a crucial area of research, yet the underlying mechanisms remain poorly understood. Previous studies have shown that substituting Mg²⁺ with Mn²⁺ at the metal ion-dependent adhesion site (MIDAS) enhances the affinities of high-affinity open and low-affinity closed integrins. However, the molecular effect of Mn²⁺ and how it compares to physiological activation mediated by Mg²⁺/Ca²⁺ remain unclear. This is partly due to the lack of experimental techniques capable of detecting these processes dynamically. In this study, we used equilibrium molecular dynamics simulations to examine the effects of Mn²⁺ on the binding site of platelet integrin α_IIbβ₃. Our findings show that Mn²⁺ accelerates conformational changes related to activation. Specifically, Mn²⁺ promotes an earlier displacement of M335 in the β6-α7 loop away from the ADMIDAS site (adjacent to the MIDAS site) and a rapid downward movement of the α7 helix in the βI domain. Additionally, Mn²⁺ leads to faster stabilization of the α1 helix, strengthening the interactions between the α_IIbβ₃ ligand-binding site and the RGD motif. These results suggest that Mn²⁺ accelerates high-affinity rearrangements at the ligand-binding site, resembling those seen in physiological activation, but occurring more rapidly than with Mg²⁺/Ca²⁺. Overall, our data suggest that Mn²⁺-induced affinity modulation proceeds through similar early activation steps, even without full integrin extension.