Molecular motors are biological molecular machines that are the essential agents of movement in living organisms. In general terms, a motor is 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. In terms of energetic efficiency, this type of motor 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 in which the fluctuations due to thermal noise are significant.
Some examples of biologically important molecular motors:
Cytoskeletal motors
Myosins are responsible for muscle contraction, intracellular cargo transport, and producing cellular tension.
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.
Polymerisation motors
Actin polymerization generates forces and can be used for propulsion. ATP is used.
Microtubule polymerization using GTP.
Dynamin is responsible for the separation of clathrin buds from the plasma membrane. GTP is used.
Rotary motors:
FoF1-ATP synthase family of proteins convert the chemical energy in ATP to the electrochemical potential energy of a proton gradient across a membrane or the other way around. The catalysis of the chemical reaction and the movement of protons are coupled to each other via the mechanical rotation of parts of the complex. This is involved in ATP synthesis in the mitochondria and chloroplasts as well as in pumping of protons across the vacuolar membrane.
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 protons across a membrane, possibly using a similar mechanism to that found in the Fo motor in ATP synthase.
Nucleic acid motors:
RNA polymerase transcribes RNA from a DNA template.
DNA polymerase turns single-stranded DNA into double-stranded DNA.
Helicases separate double strands of nucleic acids prior to transcription or replication. ATP is used.
Topoisomerases reduce supercoiling of DNA in the cell. ATP is used.
RSC and SWI/SNF complexes remodel chromatin in eukaryotic cells. ATP is used.
SMC proteins responsible for chromosome condensation in eukaryotic cells.
Viral DNA packaging motors inject viral genomic DNA into capsids as part of their replication cycle, packing it very tightly. Several models have been put forward to explain how the protein generates the force required to drive the DNA into the capsid; for a review, see [1]. An alternative proposal is that, in contrast with all other biological motors, the force is not generated directly by the protein, but by the DNA itself. In this model, ATP hydrolysis is used to drive protein conformational changes that alternatively dehydrate and rehydrate the DNA, cyclically driving it from B-DNA to A-DNA and back again. A-DNA is 23% shorter than B-DNA, and the DNA shrink/expand cycle is coupled to a protein-DNA grip/release cycle to generate the forward motion that propels DNA into the capsid.
Synthetic molecular motors have been created by chemists that yield rotation, possibly generating torque.
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.
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), total internal reflection fluorescence (TIRF).
Magnetic tweezers can also be useful for analysis of motors that operate on long pieces of DNA.
Neutron spin echo spectroscopy can be used to observe motion on nanosecond timescales.
Optical tweezers (not to be confused with molecular tweezers in context) are well-suited for studying molecular motors because of their low spring constants.
Scattering techniques: single particle tracking based on dark field microscopy or interferometric scattering microscopy (iSCAT)
Single-molecule electrophysiology can be used to measure the dynamics of individual ion channels.
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 nanoscale motors.
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. Systems like the nanocars, while not technically motors, are illustrative of recent efforts towards synthetic nanoscale motors.
