Figure 2.

Mechanochemical model of a single leg protein. (A) Mechanochemical cycle of the leg. The leg starts in the front conformation with the foot bound to the substrate. As ATP zippers into the catalytic site, the motor carries out a power stroke, pulling the cell forward. After the power stroke, the cell continues moving forward at constant velocity V, driven by the collective action of the other legs. The foot lags behind until the long segment is once again in tension. Acting on one end of the foot, the tension helps peel the foot off the substrate. The system must wait for the foot to release from the substrate so that the leg can reset to the front conformation, allowing the motor to bind ATP once again. The cycle repeats as the foot rebinds to the substrate. ATP binding and foot rebinding are assumed to happen very fast, thus not resolved in the analysis. During the cycle, the power stroke applies a positive force on the cell body, and the foot, tethered beyond its backward restressed position, applies a negative force (blue arrows). (B) Mechanism of foot peeling. The foot interacts with the sialic acids in the substrate through multiple binding sites. The bonds are shown by green projections in the zoom-in view on the right. When the stretched intermediate segment pulls on the foot from one end, most of the tension is exerted on the frontmost bond and thus significantly facilitates its unbinding, analogous to peeling-off a Velcro strip. In the idealized case, the bonds break off sequentially, forming a Markov process as shown in the sequence of events on the right. The Markov process gives an average peel-off rate of the foot as in Eq. 2.