Molecular motors
Encyclopedia
Molecular motors are biological molecular machine
s 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 . 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 where the fluctuations
due to thermal noise
are significant.
, molecular motors are often modeled with the Fokker-Planck equation
or with Monte Carlo method
s. These theoretical models are especially useful when treating the molecular motor as a Brownian motor
.
, the activity of molecular motors is observed with many different experimental approaches, among them:
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.
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 nanocar
s, while not technically motors, are illustrative of recent efforts towards synthetic nanoscale motors.
Molecular machine
A molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements in response to specific stimuli . The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level...
s that are the essential agents of movement in living organisms. Generally speaking, a motor
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
may be defined as a device that consumes energy in one form and converts it into motion or mechanical work
Mechanical work
In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...
; for example, many protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
-based molecular motors harness the chemical free energy
Gibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
released by the hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
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
Thermal bath (thermodynamics)
In thermodynamics and statistical mechanics, a thermal bath is a reservoir of particles at finite temperature undergoing Brownian motion, and that in thermal contact with an object of interest. Because of its nonzero temperature, this system contains thermal energy. This yields an environment where...
, an environment where the fluctuations
Fluctuations
Fluctuations may refer to:* Quantum fluctuations arising from the uncertainty principle* Primordial fluctuations, density variations in the early universe* Statistical fluctuations, very important in statistics, statistical mechanics, and thermodynamics...
due to thermal noise
Fluctuation dissipation theorem
The fluctuation-dissipation theorem is a powerful tool in statistical physics for predicting the behavior of non-equilibrium thermodynamical systems. These systems involve the irreversible dissipation of energy into heat from their reversible thermal fluctuations at thermodynamic equilibrium...
are significant.
Examples
Some examples of biologically important molecular motors:- Cytoskeletal motors
- MyosinMyosinMyosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
is responsible for muscle contraction - KinesinKinesinA kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...
moves cargo inside cells away from the nucleus along microtubuleMicrotubuleMicrotubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
s - DyneinDyneinDynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...
produces the axonemalAxonemeNumerous eukaryotic cells carry whip-like appendages whose inner core consists of a cytoskeletal structure called the axoneme....
beating of cilia and flagella and also transports cargo along microtubules towards the cell nucleus
- Myosin
- Polymerisation motors
- ActinActinActin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
polymerization generates forces and can be used for propulsion. ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
is used. - MicrotubuleMicrotubuleMicrotubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
polymerization using GTPGuanosine triphosphateGuanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
. - DynaminDynaminDynamin is a GTPase responsible for endocytosis in the eukaryotic cell. Dynamins are principally involved in the scission of newly formed vesicles from the membrane of one cellular compartment and their targeting to, and fusion with, another compartment, both at the cell surface as well as at the...
is responsible for the separation of clathrinClathrinClathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1975. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskelia interact they form a polyhedral lattice...
buds from the plasma membrane. GTPGuanosine triphosphateGuanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
is used.
- Actin
- Rotary motors:
- FoF1-ATP synthaseATP synthaseright|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...
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 membraneV-ATPaseVacuolar-type H+-ATPase is a highly conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons across the plasma membranes of numerous cell types... - The bacterial flagellumFlagellumA flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
responsible for the swimming and tumbling of E. coliEscherichia coliEscherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
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.
- FoF1-ATP synthase
- Nucleic acid motors:
- RNA polymeraseRNA polymeraseRNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
transcribes RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
from a DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
template - DNA polymeraseDNA polymeraseA DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
turns single-stranded DNA into double-stranded DNA. - HelicaseHelicaseHelicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands using energy derived from ATP hydrolysis.-Function:Many cellular processes Helicases are a...
s separate double strands of nucleic acids prior to transcription or replication. ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
is used. - TopoisomeraseTopoisomeraseTopoisomerases are enzymes that regulate the overwinding or underwinding of DNA. The winding problem of DNA arises due to the intertwined nature of its double helical structure. For example, during DNA replication, DNA becomes overwound ahead of a replication fork...
s reduce supercoiling of DNA in the cell. ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
is used. - RSCChromatin Structure Remodeling (RSC) ComplexRSC is a 17-subunit complex with the capacity to remodel the structure of chromatin. It exhibits a DNA-dependent ATPase activity stimulated by both free and nucleosomal DNA and a capacity to perturb nucleosome structures...
and SWI/SNFSWI/SNFSWI/SNF is a yeast nucleosome remodeling complex composed of several proteins – products of the SWI and SNF genes as well as several other polypeptides...
complexes remodel chromatin in eukaryotic cells. ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
is used. - SMC proteinSMC proteinSMC proteins represent a large family of ATPases that participate in many aspects of higher-order chromosome organization and dynamics. SMC stands for Structural Maintenance of Chromosomes.-Prokaryotic:...
responsible for chromosome condensation in eukaryotic cells. - Viral DNA packaging motors inject viral genomic DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
into capsids as part of their replication cycle, packing it very tightly.
- RNA polymerase
- Synthetic molecular motorsSynthetic molecular motorsSynthetic molecular motors are molecular machines capable of rotation under energy input. Although the term "molecular motor" has traditionally referred to a naturally occurring protein that induces motion , some groups also use the term when referring to non-biological, non-peptide synthetic...
have been created by chemists that yield rotation, possibly generating torque.
Theoretical Considerations
Because the motor events are stochasticStochastic
Stochastic refers to systems whose behaviour is intrinsically non-deterministic. A stochastic process is one whose behavior is non-deterministic, in that a system's subsequent state is determined both by the process's predictable actions and by a random element. However, according to M. Kac and E...
, molecular motors are often modeled with the Fokker-Planck equation
Fokker-Planck equation
The Fokker–Planck equation describes the time evolution of the probability density function of the velocity of a particle, and can be generalized to other observables as well.It is named after Adriaan Fokkerand Max Planck...
or with Monte Carlo method
Monte Carlo method
Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...
s. These theoretical models are especially useful when treating the molecular motor as a Brownian motor
Brownian motor
Brownian motors are nano-scale or molecular devices by which thermally activated processes are controlled and used to generate directed motion in space and to do mechanical or electrical work...
.
Experimental Observation
In experimental biophysicsBiophysics
Biophysics is an interdisciplinary science that uses the methods of physical science to study biological systems. Studies included under the branches of biophysics span all levels of biological organization, from the molecular scale to whole organisms and ecosystems...
, the activity of molecular motors is observed with many different experimental approaches, among them:
- Fluorescent methods: fluorescence resonance energy transfer (FRETFluorescence resonance energy transferFörster resonance energy transfer , also known as fluorescence resonance energy transfer, resonance energy transfer or electronic energy transfer , is a mechanism describing energy transfer between two chromophores.A donor chromophore, initially in its electronic excited state, may transfer energy...
), fluorescence correlation spectroscopy (FCSFluorescence correlation spectroscopyFluorescence correlation spectroscopy is a correlation analysis of fluctuation of the fluorescence intensity. The analysis provides parameters of the physics under the fluctuations. One of the interesting applications of this is an analysis of the concentration fluctuations of fluorescent...
) - Magnetic tweezers can also be useful for analysis of motors that operate on long pieces of DNA
- Neutron spin echoNeutron spin echoNeutron spin echo spectroscopy is an inelastic neutron scattering technique invented by Ferenc Mezei in the 1970's, and developed in collaboration with John Hayter.In recognition of his work and in other areas, Mezei was awarded the first in 1999....
spectroscopy can be used to observe motion on nanosecond timescales - Optical tweezersOptical tweezersOptical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force , depending on the refractive index mismatch to physically hold and move microscopic dielectric objects...
are well-suited for studying molecular motors because of their low spring constants - Single-molecule electrophysiologyElectrophysiologyElectrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
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.
Non-biological
Recently, chemists and those involved in nanotechnologyNanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
have begun to explore the possibility of creating molecular motors de novo. These synthetic molecular motors
Synthetic molecular motors
Synthetic molecular motors are molecular machines capable of rotation under energy input. Although the term "molecular motor" has traditionally referred to a naturally occurring protein that induces motion , some groups also use the term when referring to non-biological, non-peptide synthetic...
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 nanocar
Nanocar
The nanocar is a molecule designed in 2005 at Rice University by a group headed by Professor James Tour. Despite the name, the original nanocar does not contain a molecular motor, hence, it is not really a car...
s, while not technically motors, are illustrative of recent efforts towards synthetic nanoscale motors.
See also
- Brownian motorBrownian motorBrownian motors are nano-scale or molecular devices by which thermally activated processes are controlled and used to generate directed motion in space and to do mechanical or electrical work...
- Brownian ratchetBrownian ratchetIn the philosophy of thermal and statistical physics, the Brownian ratchet, or Feynman-Smoluchowski ratchet is a thought experiment about an apparent perpetual motion machine first analysed in 1912 by Polish physicist Marian Smoluchowski and popularised by American Nobel laureate physicist Richard...
- CytoskeletonCytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
- Molecular machineMolecular machineA molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements in response to specific stimuli . The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level...
s - Molecular mechanicsMolecular mechanicsMolecular mechanics uses Newtonian mechanics to model molecular systems. The potential energy of all systems in molecular mechanics is calculated using force fields...
- Molecular propellerMolecular propellerMolecular propeller is a molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers : it has several molecular-scale blades attached at a certain pitch angle around the circumference of a shaft, aligned along the rotational...
- Motor proteins
- NanomotorNanomotorA nanomotor is a molecular device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.A proposed branch of research is the integration of molecular motor proteins found in living cells into molecular motors implanted in artificial devices...
- Protein dynamics
- Synthetic molecular motorsSynthetic molecular motorsSynthetic molecular motors are molecular machines capable of rotation under energy input. Although the term "molecular motor" has traditionally referred to a naturally occurring protein that induces motion , some groups also use the term when referring to non-biological, non-peptide synthetic...
External links
- Cymobase - A database for cytoskeletal and motor protein sequence information