Molecular motors are natural (biological) or artificial 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.
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👉 Molecular motor in the context of Molecular machine
Molecular machines are a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli, mimicking macromolecular devices such as switches and motors. Naturally occurring or biological molecular machines are responsible for vital living processes such as DNA replication and ATP synthesis. Kinesins and ribosomes are examples of molecular machines, and they often take the form of multi-protein complexes. For the last several decades, scientists have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world.
The first example of an artificial molecular machine (AMM) was reported in 1994, featuring a rotaxane with a ring and two different possible binding sites. In 2016 the Nobel Prize in Chemistry was awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa for the design and synthesis of molecular machines. A major point is to exploit existing motion in proteins, such as rotation about single bonds or cis-trans isomerization. Different AMMs are produced by introducing various functionalities, such as the introduction of bistability to create switches. A broad range of AMMs has been designed, featuring different properties and applications; some of these include molecular motors, switches, and logic gates. A wide range of applications have been demonstrated for AMMs, including those integrated into polymeric, liquid crystal, and crystalline systems for varied functions (such as materials research, homogenous catalysis and surface chemistry).
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Molecular motor in the context of Archaellum
The archaellum (pl.: archaella; formerly archaeal flagellum) is a unique structure on the cell surface of many archaea that allows for swimming motility. The archaellum consists of a rigid helical filament that is attached to the cell membrane by a molecular motor. This molecular motor – composed of cytosolic, membrane, and pseudo-periplasmic proteins – is responsible for the assembly of the filament and, once assembled, for its rotation. The rotation of the filament propels archaeal cells in liquid medium, in a manner similar to the propeller of a boat. The bacterial analog of the archaellum is the flagellum, which is also responsible for their swimming motility and can also be compared to a rotating corkscrew. Although the movement of archaella and flagella is sometimes described as "whip-like", this is incorrect, as only cilia from Eukaryotes move in this manner. Indeed, even "flagellum" (word derived from Latin meaning "whip") is a misnomer, as bacterial flagella also work as propeller-like structures.
Early studies on "archaeal flagella" identified several differences between archaella and flagella, although those differences were dismissed as a possible adaptation of archaella to the extreme ecological environments where archaea were at the time known to inhabit. When the first genomes of archaeal organisms were sequenced, it became obvious that archaea do not code for any of the proteins that are part of the flagellum, thus establishing that the motility system of archaea is fundamentally different from that of bacteria. In order to highlight the difference between these two organelles, the name archaellum was proposed in 2012 following studies that showed it to be evolutionarily and structurally different from the bacterial flagella and eukaryotic cilia.
Molecular motor in the context of Intracellular transport
Intracellular transport is the movement of vesicles and substances within a cell. Intracellular transport is required for maintaining homeostasis within the cell by responding to physiological signals. Proteins synthesized in the cytosol are distributed to their respective organelles, according to their specific amino acid's sorting sequence. Eukaryotic cells transport packets of components to particular intracellular locations by attaching them to molecular motors that haul them along microtubules and actin filaments. Since intracellular transport heavily relies on microtubules for movement, the components of the cytoskeleton play a vital role in trafficking vesicles between organelles and the plasma membrane by providing mechanical support. Through this pathway, it is possible to facilitate the movement of essential molecules such as membrane‐bounded vesicles and organelles, mRNA, and chromosomes.
Intracellular transport is unique to eukaryotic cells because they possess organelles enclosed in membranes that need to be mediated for exchange of cargo to take place. Conversely, in prokaryotic cells, there is no need for this specialized transport mechanism because there are no membranous organelles and compartments to traffic between. Prokaryotes are able to subsist by allowing materials to enter the cell via simple diffusion. Intracellular transport is more specialized than diffusion; it is a multifaceted process which utilizes transport vesicles. Transport vesicles are small structures within the cell consisting of a fluid enclosed by a lipid bilayer that hold cargo. These vesicles will typically execute cargo loading and vesicle budding, vesicle transport, the binding of the vesicle to a target membrane and the fusion of the vesicle membranes to target membrane. To ensure that these vesicles embark in the right direction and to further organize the cell, special motor proteins attach to cargo-filled vesicles and carry them along the cytoskeleton. For example, they have to ensure that lysosomal enzymes are transferred specifically to the golgi apparatus and not to another part of the cell which could lead to deleterious effects.