Molecular Motors: Power Strokes Outperform Brownian Ratchets

Abstract

Molecular motors convert chemical energy (typically from ATP hydrolysis) to directed motion and mechanical work. Their actions are often described in terms of "Power Stroke" (PS) and "Brownian Ratchet" (BR) mechanisms. Here, we use a transition-state model and stochastic thermodynamics to describe a range of mechanisms ranging from PS to BR. We incorporate this model into Hill's diagrammatic method to develop a comprehensive model of motor processivity that is simple but sufficiently general to capture the full range of behavior observed for molecular motors. We demonstrate that, under all conditions, PS motors are faster, more powerful, and more efficient at constant velocity than BR motors. We show that these differences are very large for simple motors but become inconsequential for complex motors with additional kinetic barrier steps.

Allocating dissipation across a molecular machine cycle to maximize flux

“…For given bare rates k 0 ij and total dissipation ω tot , an FL cycle will always produce more flux than the corresponding RL cycle, similar to previous results [29]. FL and RL schemes represent extremes of a more general mechanism, whereby some dissipation is spent speeding up the forward transitions (as for FL), and the remaining fraction slows down the reverse transitions (as for RL):…”

“…'Labile' here denotes the direction in which rate constants change with dissipation. This dependence of forward or reverse rate constants on dissipation for the FL and RL schemes, respectively, is analogous to their dependence on the work performed by a motor in a 'power stroke' or 'Brownian ratchet' [29].…”

“…This is similar to splitting force-dependence among reaction rates in previous studies [28,29]. (ω + ij +ω − ij ) = ω tot is fixed, leaving 2n − 1 free parameters to optimize over in an n-state cycle.…”

“…Flux is maximized when the 'barriers' are at or below the states, thus no longer acting as barriers. This is an intuitive result, that all else equal, faster transitions (due to lowered barriers) produce a higher flux [29]. Increasing E 2 is equivalent to decreasing the dissipation ω 12 and increasing ω 21 , and vice versa.…”

“…The ratio between the forward and reverse rate constants of a given transition path is fixed by its free energy dissipation ω ij [29,30],…”