Metabolic coupling of thermodynamics and mesoscopic stochastic fluctuations in an ATP chemostatted system

Many biological systems act as ATP chemostats, maintaining a constant ATP chemical potential across numerous and variable free energy sources and sinks. It remains unclear how much free energy cells allocate to active mechanical fluctuations of the cytoplasm among other processes. To characterize nonequilibrium cytoskeletal mechanics, we imaged shape fluctuations of single-walled carbon nanotubes (SWNTs) embedded in an actin-intact Xenopus cytoplasmic extract. Normal mode decomposition of SWNT shape fluctuations resolved the spatial structure of myosin-driven forces, which were not only stochastic, but also nonstationary due to dynamic remodeling of the actomyosin network. Based on the normal mode correlation functions, we defined metrics for nonequilibrium mechanical activity. To measure how active cytoskeletal mechanics depended on the ATP chemical potential and flux, we increased the ATP chemical potential via a phosphoenolpyruvate energy mix and decreased it via apyrase-catalyzed ATP hydrolysis. These passive measurements reveal how local nonequilibrium fluctuations of the cytoplasm respond to global thermodynamics and the ATP chemical potential.