Figure 4.

Epibatidine-induced conformational changes. (a) Backbone superposition between the Apo (gold) and Epi (blue) structures shows a clockwise rotation of the outer β-sheet (green box and arrow, bottom) and a counterclockwise rotation of the top part of the subunit structure (red box and arrow, top) when viewed down the pentamer axis. The stationary inner sheet is indicated by the black box, and the epibatidine molecule is shown in electron density. The side chain rotamer switch of Phe196 is also evident in the green box. (b) Backbone superposition of individual subunits show variable conformations of loop C in the Apo structure (ten subunits colored differently) but a single closed conformation in the Epi structure (black, only one structure shown). Epi indicates the epibatidine molecule. (c) The 2Fo − Fc electron density map (contoured at the 1.0-σ level) shows the distinct side chain conformations of Phe196 (arrows) in the Apo (left) and Epi (right) structures, demonstrating repacking of the protein core as a result of epibatidine-induced structural changes.