The wide velocity variations observed are rather surprising and cannot be explained by simple kinetics models. A simple class of models applied to many motors assumes that discrete stepping is tightly coupled to the ATP hydrolysis cycle, occurring at a well defined average rate. In such a model, velocity variations occur because Brownian fluctuations play an important role in driving the motor transitions. In the most random possible case, referred to as a ‘‘Poisson stepper,’’ one expects an exponential distribution of waiting times between steps. …
We emphasize that two types of variability were observed: (i) variation in the average velocities of different complexes, referred to in the literature as ‘‘static disorder,’’ and (ii) variations in the velocities of single complexes in time, referred to as ‘‘dynamic disorder’’ . Although it is difficult to rule out protein degradation or instability as a contributing factor to static disorder, such effects cannot fully explain dynamic disorder because the velocity was observed to vary both up and down in time. Degradation would be expected to only cause decreases.
…, Similar levels of static and dynamic disorder in enzyme kinetics have been reported in several previous single-molecule experiments… Our data show that such behavior can also occur in a more complex, multicomponent motor. Other researchers have provided evidence that such variability can be attributed to the existence of multiple active conformational states of the enzyme complexes and slow interconversion between them. …, suggest multiple … conformers and assembly states exhibiting different levels of ATPase activity.
Fuller, D.N., et al., Single phage T4 DNA packaging motors exhibit large force generation, high velocity, and dynamic variability. Proc Natl Acad Sci U S A, 2007. 104(43): p. 16868-73
