Multistep orthophosphate release tunes actomyosin energy transduction

Abstract

Muscle contraction and a range of critical cellular functions rely on force-producing interactions between myosin motors and actin filaments, powered by turnover of adenosine triphosphate (ATP). The relationship between release of the ATP hydrolysis product ortophosphate (Pi) from the myosin active site and the force-generating structural change, the power-stroke, remains enigmatic despite its central role in energy transduction. Here, we present a model with multistep Pi-release that unifies current conflicting views while also revealing additional complexities of potential functional importance. The model is based on our evidence from kinetics, molecular modelling and single molecule fluorescence studies of Pi binding outside the active site. It is also consistent with high-speed atomic force microscopy movies of single myosin II molecules without Pi at the active site, showing consecutive snapshots of pre- and post-power stroke conformations. In addition to revealing critical features of energy transduction by actomyosin, the results suggest enzymatic mechanisms of potentially general relevance.

Integrating comparative modeling and accelerated simulations reveals conformational and energetic basis of actomyosin force generation

“…A more recent kinetics study proposed a multistep Pi release mechanism in which Pi still leaves the active site before the powerstroke but pauses at a secondary Pi binding site in the tunnel. 50 Our results are consistent with this explanation and reveal how different ligand states shift the population of actomyosin conformations. The overlap in the populations of PPS and rigor states shown in the free energy profiles (Fig.…”

Insights into Muscle Contraction Derived from the Effects of Small-Molecular Actomyosin-Modulating Compounds

“…Moreover, in order to predict the typical effects of lowering [MgATP] from 1.2 to 0.5 mM, we had to assume a reduction from 1.2 to 0.1 mM in the simulations. This finding is actually not unexpected from several previous studies of the relationship between [MgATP] and V 0 predicting that the MgATP concentration ( K M ) for half-maximum unloaded velocity (i.e., V 0 ) is several-fold lower than in experiments [ 5 , 8 , 17 , 85 ]. These previous studies also suggested that a way to amend this problem is to leave the assumption of a linear cross-bridge elasticity and to instead assume a non-linear cross-bridge elasticity of the type observed in [ 43 ].…”

Validate the force-velocity relation of the Hill’s muscle model from a molecular perspective

“…Based on this, myosin-II can make a step toward the barbed ends of F-actins. To describe this process, mainly two scenarios have been proposed: the power stroke and the Brownian ratchet (Bustamante et al, 2001;Holmes et al, 2004;Moretto et al, 2022). The mechanism of power stroke illustrates how a conformational shift in myosin occurs, causing the myosin to take an irreversible step toward the barbed end.…”

“…In addition, a local thermodynamic equilibrium in each myosin ensemble can be assumed, since the state transitions occur in the time scale of nanoseconds while the muscle activities occurs in milliseconds (Jülicher et al, 1997;Moretto et al, 2022). Moreover, the length scale of myosin movements is only in nanometers which is extremely short compared to the cellular activities.…”