Fig. 3.
Ion binding onto Asp32 induces a kink and swivel conformational change (6). (A) Binding of a proton to Asp32 of MotB drives a rapid local reorganization of hydrogen bonds (including those of water). In particular, we focus on the creation of a hydrogen bond between the side chain of MotB’s Asp32 and the carbonyl group of MotA’s residue 169. Ion binding thus creates a local elastic strain in the MotA helix. The release of this strain leads to the proposed conformational change in MotA about the Pro173 residue. Adapted from Kim et al. (5). (B) Upon ion binding, MotA undergoes a rapid conformational change consisting of three motions: (i) a bending about Pro173 ϕ, (ii) a downward motion, , and (iii) a rotation about its central axis. Inspired by the work of Cordes et al. (6), we propose that this kink and swivel motion generates the power stroke. Importantly, we note that this figure is a 2D depiction of a 3D process, with the motion of the loop extending out of the plane of the page. (C) Our envisioned motion of the contact point between a FliG and a stator loop during the power stroke. The kink and swivel motion induces the contact point to follow a helical path on a cylinder of radius approximately equal to the radius of the stator. For simplicity, we assume that the vertical motion is a function of the angle ϕ subtended by the stator loop. Therefore, we explicitly model only the rotational motion ϕ of the stator loop.