Biomolecular Machines as Active Matter: Kinetic Asymmetry and Nonequilibrium Function

ABSTRACT Biological molecular machines operate far from thermodynamic equilibrium, yet function in a regime where inertia is negligible, and motion is overdamped, such that the system remains in mechanical equilibrium, while thermal fluctuations maintain equipartition among accessible degrees of freedom. In this Perspective, I argue that biomolecular machines are most naturally understood as elementary units of active matter: systems that continuously consume free energy from their environment and thereby sustain nonequilibrium probability currents. ATP hydrolysis does not act as a generalized mechanical force, nor does directional motion require a deterministic power stroke. ATP hydrolysis supplies energy but does not itself specify direction. Rather, directionality emerges through the ratchet principle: energy input selectively suppresses trajectories associated with backward or otherwise undesired motion. Net transport is therefore determined not by direct forcing of forward steps, but by kinetic asymmetry in the probabilities of competing microscopic pathways. Framing biomolecular machines as active matter unifies molecular biophysics with nonequilibrium statistical mechanics and clarifies how chemical free energy is transduced into motion, transport, and work in the absence of inertia or stored mechanical energy. This perspective shifts emphasis from forces and power strokes to probability fluxes, symmetry, and kinetic asymmetry, providing a coherent framework for understanding active behavior in living matter.