Fig. 5. Brownian-ratchet mechanism for fast substrate translocation by ClpB.

(A) In a Brownian ratchet, an effective pawl periodically switches molecular dynamics between a flat free-energy surface and a structured surface, promoting unidirectional motion. (B) Model for a potential Brownian-ratchet action of pore loops. As a substrate is engaged, pore loops gradually change their average conformation even while continuing to fluctuate on the microsecond time scale between two conformational states. The change in the population ratio of the two states of PL2 and PL3, which likely takes place only upon hydrolysis of ATP, is equivalent to a shift between two free-energy surfaces, as in (A), and turns them into effective pawls. At the same time, PL1 and PL3 function in pulling the substrate through the central channel.