The Study On Dynamic Instability And Efficiency Of Collective Motors

Molecular motors,or motor proteins are widely exist in cells,which can convert chemical energy into mechanical work efficiently,thereby generating directional movement.Various processes happened in biology,such as cargo transport,gene expression,cell motility,muscle contraction and so on,are related to the movement of molecular motors.In recent years,the dynamics of molecular motor has been one of the central issue in biophysics research.Especially the collective motor system attracts much attention,because of its complex dynamic behavior.The molecular motors can be divided into processive motors and nonprocessive motors.Nonprocessive motors often play a collective role because of its lower “duty ratio”(?2%),but its cooperative mechanism is still not very clear.In this paper,we investigated the dynamic instability and efficiency of the nonprocessive myosin ? molecular motor.The main contents and results are as follows:First,some kinds of muscle can oscillate spontaneously,which is related to the dynamic instability of collective motors.Based on the two-state ratchet model and with consideration of motor stiffness(model ?),the dynamics of collective myosin ? motors are studied.It has been shown that,when the motor stiffness is small,the velocity of collective motors decreases monotonically with load increasing.When the motor stiffness becomes large,dynamics instability appears in the force-velocity relationship of collective-motor transport.For enough large motor stiffness,the zero-velocity point lies in the unstable range of the force-velocity curve,and the motor system becomes unstable before the motion is stopped,so spontaneous oscillations can be generated if the system is elastically coupled to its enviroment via a spring.The oscillations frequency is related to motor stiffness,motor binding rate,spring stiffness and the width of ATP excitation interval.For medium motor stiffness,the zero-velocity point lies outside the unstable range of the force-velocity curve,and the motion will be stopped before the instability occurs.Second,mechanical efficiency is an important indicator of collective motor transport.Results show that the efficiency of collective myosin ? motors in model ? is far smaller than that of experiment observations.If the following two characters of myosin ? motor are considered(call it as model ?),the efficiency can be increased greatly and consistent well with experiments:(1)motor transitions from detached state to bound state are localized in “binding sites”in the reaction coordinate;(2)the potential wells for bound states are independent with each other,i.e.,one binding motor have to detach from the filament before it binds to another binding site.In both models,the mechanical efficiency increases with motor stiffness.In model ?,if the ATP excitation rate is large,this motor system can work with high efficiency and velocity simultaneously.This feature is good for the function of muscle.These studies can help us to understand the cooperative mechanism of molecular motors and the relation between structure difference and functional difference of muscle.It can also provide us some theoretic guidance in designing efficient artificial molecular motor system.