ABSTRACT
Our efforts to explain the mechanisms of motor proteins must take into account the fact that their properties are not constant but evolve in time and might fluctuate. Motor proteins can bind to their cellular tracks (nucleic acids or cytoskeleton filaments), or dissociate from them. They could also collide with other molecules in the cytosol and even stick for some time to other
cellular objects. During the chemical processes when motor proteins function as enzymes, they go through a variety of molecular conformations and biochemical states, and this behavior is repeated many times. This complex dynamics is critical for understanding biological molecular motors. However, equilibrium thermodynamics always assumes that any system is in equilibrium, i.e., no changes in properties of molecules are observed with time. Statistical mechanics allows us to calculate macroscopic features of systems from molecular properties, but it can be done only for equilibrium systems. Both of these fundamental approaches neglect the time-dependent features of motor proteins. At the same time, we know that motor proteins in cells operate at non-equilibrium conditions, i.e., their properties might change with time. For example, at one time the motor protein might be bound to the filaments transporting the cellular cargo, while at another time it became inactive after dissociating from the track. It is important to understand this temporal evolution at the single-molecule level.
