Molecular motors

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Molecular motors are biological "nanomachines" that are the essential agents of movement in living organisms. Generally speaking, a motor may be defined as a device that consumes energy in one form and converts it into motion or mechanical work; for example, many protein-based molecular motors harness the chemical free energy released by the hydrolysis of ATP in order to perform mechanical work ^[1]. In terms of energetic efficiency, these types of motors can be superior to currently available man-made motors. One important difference between molecular motors and macroscopic motors is that molecular motors operate in the thermal bath, an environment where the fluctuations due to thermal noise are significant.

Recently, chemists and those involved in nanotechnology have begun to explore the possibility of creating molecular motors de novo. These synthetic molecular motors currently suffer many limitations that confine their use to the research laboratory. However, many of these limitations may be overcome as our understanding of chemistry and physics at the nanoscale increases. In particular, there are already examples of molecular machines that operate in solution or when attached to solid surfaces, such as the nanocar. Such systems are paving the way towards synthetic motors.

Examples

Some examples of biologically important molecular motors are:

Motor proteins
- Myosin is responsible for muscle contraction
- Kinesin moves cargo inside cells away from the nucleus along microtubules
- Dynein produces the axonemal beating of cilia and flagella and also transports cargo along microtubules towards the cell nucleus
F₀F₁ ATP synthase generates ATP using the transmembrane electrochemical proton gradient inside mitochondria^[2]
RNA polymerase transcribes RNA from a DNA template^[3]
Actin polymerization generates forces and can be used for propulsion
Topoisomerases reduce supercoiling of DNA in the cell
The bacterial flagellum responsible for the swimming and tumbling of E. coli and other bacteria acts as a rigid propeller that is powered by a rotary motor. This motor is driven by the flow of ions across a membrane, possibly using a similar mechanism to that found in the F₀ motor in ATP synthase.
Synthetic molecular motors have been created by chemists that yield rotation, possibly generating torque

Theoretical Considerations

Because the motor events are stochastic, molecular motors are often modeled with the Fokker-Planck equation or with Monte Carlo methods. These theoretical models are especially useful when treating the molecular motor as a Brownian motor.

Experimental Observation

In experimental biophysics, the activity of molecular motors is observed with many different experimental approaches, among them:

Fluorescent methods: fluorescence resonance energy transfer (FRET), fluorescence correlation spectroscopy (FCS)
Single-molecule electrophysiology can be used to measure the dynamics of individual ion channels
Optical tweezers are well-suited for studying molecular motors because of their low spring constants
Magnetic tweezers can also be useful for analysis of motors that operate on long pieces of DNA

Many more techniques are also used. As new technologies and methods are developed, it is expected that knowledge of naturally occurring molecular motors will be helpful in constructing synthetic nano-scale motors.

References