Cryo-Electron Microscopy Structure of the αIIbβ3-Abciximab Complex.

Objective: The alpha IIb beta 3 antagonist antiplatelet drug abciximab is the chimeric antigen-binding fragment comprising the variable regions of murine monoclonal antibody 7E3 and the constant domains of human IgG1 and light chain kappa. Previous mutagenesis studies suggested that abciximab binds to the beta 3 C177-C184 specificity-determining loop (SDL) and Trp129 on the adjacent beta 1-alpha 1 helix. These studies could not, however, assess whether 7E3 or abciximab prevents fibrinogen binding by steric interference, disruption of either the alpha IIb beta 3-binding pocket for fibrinogen or the beta 3 SDL (which is not part of the binding pocket but affects fibrinogen binding), or some combination of these effects. To address this gap, we used cryo-electron microscopy to determine the structure of the alpha IIb beta 3-abciximab complex at 2.8 angstrom resolution. Approach and Results: The interacting surface of abciximab is comprised of residues from all 3 complementarity-determining regions of both the light and heavy chains, with high representation of aromatic residues. Binding is primarily to the beta 3 SDL and neighboring residues, the beta 1-alpha 1 helix, and beta 3 residues Ser211, Val212 and Met335. Unexpectedly, the structure also indicated several interactions with alpha IIb. As judged by the cryo-electron microscopy model, molecular-dynamics simulations, and mutagenesis, the binding of abciximab does not appear to rely on the interaction with the alpha IIb residues and does not result in disruption of the fibrinogen-binding pocket; it does, however, compress and reduce the flexibility of the SDL. Conclusions: We deduce that abciximab prevents ligand binding by steric interference, with a potential contribution via displacement of the SDL and limitation of the flexibility of the SDL residues.