Molecular motors are involved in many important biological tasks. They shuffle chromosomes around during cell division; they move organelles and neurotransmitters around inside braincells, they help microbes move and they cause the motion of muscles. Biological molecular motors are also involved in the development of organisms; they have been found to be important in the left-right development of asymmetric organisms (such as humans). There are several families of molecular motors: the myosins, the dyneins, the kinesins, the nucleic acid polymerases, and the ATPases. The first four of these all move along a cytoskeletal polymer track of some kind: Myosin moves along the actin polymer; the dyneins and the kinesins move along the polymer of tubulin known as microtubules. The third kind, the nucleic acid polymerases move along a track while they are creating another track: that of the complimentary nucleic acid chain. The last type of molecular motors – the ATPases – do not move along a track, rather, they sit still in one position and rotate.
The ubiquity of molecular motors in biological organisms results from the fact that complex processes taking place inside cells cannot occur by diffusion alone. No simple organism, even a bacterium, would survive without molecular motors. Human technology would also benefit greatly from being able to produce molecule-sized motors. Let’s take a journey and explore the great benefits that are reaped from molecular motors.
Motor Life Inside a Cell
We start with a tiny cell inside a developing fetus, and look inside to see what is happening. Inside, millions of molecules are diffusing this way and that, and enzymes are metabolizing nutrients that have diffused through the cell membrane. Other enzymes are building the cytoskeleton and making lipids for use in the cell membrane. Inside the nucleus, DNA polymerases are replicating the chromosomes, and in the mitochondria, the F1-ATPases are making ATP from ADP and phosphate, using the energy available from protons being channeled through the ATPase embedded in the mitochondrial membrane. Both the DNA polymerase and the ATPase are molecular motors that the cell could not survive without.
A few minutes later, the cell is about to divide. The chromosomes have been replicated, and two daughter cells will be created from the mother cell. Mitotic spindles are formed. These are made from long chains of tubulin known as microtubules that extend from the centromere and out into the two halves, which in the future will be two daughter cells. Chromokinesins transport the chromosomes from the center out to their destinations. Without these kinesins, the chromosomes would have been randomly distributed between the cells causing the next generation of cells to have problems, and probably unable to function normally. But that doesn’t happen in our cell. Its double set of paired chromosomes gets divided into the daughter cells and successful mitosis takes place. The cell divides into two equal halves, getting pinched off by the force produced by myosin molecules walking along assemblies of actin.
The new daughter cells continue growing, using nutrients that have been transported to the fetus through the umbilical cord and placenta that connect the fetus to its mother. The nutrients come from the mother’s blood. The blood is pumped by the mother’s heart. The muscle tissue in her heart is made in part of actin and myosin to produce motions that result in the pumping of the mother’s heart. This pumping in turn delivers nutrients to the fetus.
Outside the mother’s belly in the living room where she is sitting, the cat is eating some soft food that he finds delicious. Unfortunately, the food has been sitting out far too long, and little microbes have colonized the dish. Some of the microbes are paramecia that are moving through the food, also enjoying its nutritional value. The paramecia are able to move because they have cilia sticking out of their bodies. The cilia are powered by dynein motors embedded in their cell membranes. The paramecia consume nutrients from the environment by diffusion. But diffusion alone does not entirely feed the paramecia. They have to actively take in chunks of food by endocytosis. Some "unconventional" myosin molecules are involved in endocytosis, which basically involves pinching off a piece of the cell membrane, making a vesicle, and importing it into the cell body for further processing. When the paramecium has finished processing its meal, these unconventional myosins are also involved in the opposite function from endocytosis, namely exocytosis, where the paramecium rids itself of oxidized nutrients.
The previous description is a limited view of all the tasks that biological molecular motors perform, molecular motors are involved in a great number of important functions. And for that reason, many researchers have spent many years working to elucidate the way in which they produce force from the chemical energy of ATP. Learning about molecular motors is interesting not only from an academic point of view, it is also very stimulating to think of the possibilities that might arise from being able to create our own specially designed molecular motors, or nano-motors, that create force at a localized position. For example, the field of electronics would benefit if controllable molecular switches could be made. Several researchers across the world are making strides toward creating actual man-made molecular motors. Some of them are based on Mother Nature’s molecular motors, and some are new designs. We will discuss some of these later, but first let’s learn about an important biological molecular motor, kinesin, and how we believe it works. Then we’ll get into some of the preliminary motors that scientists have made.
How Molecular Motors Work
We do still not entirely understand how molecular motors work. The mechanochemical aspects (the internal motions during the chemical and kinetic process) of the motors have not yet been elucidated. But we have learned a great deal over the years about the general mechanisms by which they operate. In the following discussion, we will limit our discussion to the myosins, the dyneins, and the kinesins (the "true" molecular motors), and focus mainly on kinesin since it is the simplest of all the molecular motors. Kinesin is probably the progenitor of the other molecular motors. Excellent discussions about the ATPases and polymerases exist elsewhere (see the references at the end).
In general, the "true" molecular motors have a "head and stalk" structure (see figure 2). They very often occur as dimers. The heads are also known as the motor domains; they contain the surface that interacts with the actin or microtubule track. They also contain the nucleotide-binding site, where the ATP that supplies the chemical energy for the mechanochemical cycle is bound and hydrolyzed. Kinesin and myosin have homology to each other and to the G-proteins (proteins involved. Their homology to the G-proteins is in so-called "switch regions"— helices that move during nucleotide hydrolysis, causing a change of conformation of the protein. In the G-proteins, this switch event changes the affinity of the G-protein for its molecular partner subunits, allowing it to leave and activate adenylate cyclase elsewhere. In the molecular motors – for example kinesin – the molecular switching event alters the affinity of the motor domain for the microtubule and allows binding or release from the microtubule to take place.
In kinesin, the motor domain also contains a site that interacts with the second head. This site appears to change between a high-affinity and a low-affinity state depending on the stage of the mechanochemical cycle the dimer is in.
The Catalytic Cycle of Kinesin
Kinesin, the mother of the motor proteins, has a fairly well characterized catalytic cycle. A description of its properties will help to illustrate the catalytic cycles of other motor proteins, which differ in specifics, but are similar in general. Dr. Kenneth A. Johnson and coworkers determined the kinetic mechanism of kinesin during the 1990’s. There is an animation that follows the description of the mechanism.
In the initial state, the kinesin dimer approaches the microtubule, with an ADP bound to each nucleotide-binding pocket. One of the heads called the leading head, binds to the microtubule and ADP is released. The second head is referred to as the captive head because it is bound to the leading head in a captive state, unable to bind another microtubule binding site. This event makes the free head into the leading head, and the head that was previously the leading head now becomes the trailing head. The trailing head hydrolyzes the g-phosphate of the bound ATP, releases the phosphate and absorbs the energy. This creates a low- microtubule affinity state in the trailing head, which is released from the microtubule, and binds to the leading kinesin head, becoming the captive head. This is the first half cycle of the kinesin mechanism. In subsequent steps, a free ATP from solution then enters the vacated nucleotide pocket of the leading head. This in turn stimulates the release of the captive head that now is free to bind the microtubule and release its bound ADP, etc… See the accompanying animation for an illustration. Kinesin moves along the microtubule in single steps, 8 nanometers at a time. A single step produces 6 piconewtons of force.
Man-Made Molecular Motors: Recent Advances
In recent years, several research groups across the world have made molecule-sized contraptions that only begin to approach the biological motors. Dr. Ben L. Feringa of Groningen, Netherlands and coworkers have made a molecular motor that rotates in one direction in a four-step process, stimulated by ultraviolet light and heat. Dr. T. Ross Kelly and his colleagues at Boston College in Chestnut Hill, Mass. have also made a molecular motor made from triptycene and helicene. A series of chemical reactions causes the triptycene blades to brush past the helicene group, going in only one direction. Dr. Kelly explains: "There's probably a parallel between the way our [motor] works and the way nature's works." He also points out that "It does not turn very quickly. It takes several hours for the three-spindled wheel to make one revolution. Our next step is to speed it up." This work can be found in the Sept. 9 issue of Nature, 1999.
On this side of the Atlantic Ocean, Dr. Montemagno and coworkers at Cornell University recently fused the F1-ATPase to a zinc disc and successfully made it rotate for forty hours, thus achieving a great breakthrough in making a hybrid system composed of both metal and biological components, which is capable of producing force. They are planning to take this research farther by first making a motor that can be switched on and off at will. Next, they will make a self contained F1-ATPase motor system that uses light energy to produce the ATP that drives the F1-ATPase motor. This is a very exciting prospect. A light driven molecular machine that can be switched on and off at will.
Another organization interested in molecular motors is NASA. As early as 1997, they were working on theoretical models of molecular gears utilizing "buckytubes" as the "wheels" and attaching rigid rings to the surface to act as sprockets. These gears would be virtually indestructible, as wear and tear of the structures would not occur until the forces became great enough to destroy the covalent carbon-carbon bonds. In fact, these gears are predicted to work best at 100 billion turns per second. If successfully integrated into nanomachines, one might create a matter-compiler – a machine that if given raw materials and directed by a computer would assemble novel materials or parts of micro-machines.
Machining It All Together
The field of nanotechnology, predicted by Dr. Richard Feynman as early as 1959, is still in its infancy. We have yet to see a man-made molecular machine that is worth more than the laboratory equipment use to put it together. But it’s clear that we are making progress toward a stage where molecular machines might be used to create tracks in silicon wafers or to weave delicately intricate fabrics. As we have seen, Mother Nature already has the technology available and its really just a matter of us taking her work, evolved over billions of years, and applying it to our particular needs. The general physical principles behind the workings of molecular machines well conserved among the different biological motors.
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We start with a tiny cell inside a developing fetus, and look inside to see what is happening. Inside, millions of molecules are diffusing this way and that, and enzymes are metabolizing nutrients that have diffused through the cell membrane. Other enzymes are building the cytoskeleton and making lipids for use in the cell membrane. Inside the nucleus, DNA polymerases are replicating the chromosomes, and in the mitochondria, the F1-ATPases are making ATP from ADP and phosphate, using the energy available from protons being channeled through the ATPase embedded in the mitochondrial membrane. Both the DNA polymerase and the ATPase are molecular motors that the cell could not survive without.
We do still not entirely understand how molecular motors work. The mechanochemical aspects (the internal motions during the chemical and kinetic process) of the motors have not yet been elucidated. But we have learned a great deal over the years about the general mechanisms by which they operate. In the following discussion, we will limit our discussion to the myosins, the dyneins, and the kinesins (the "true" molecular motors), and focus mainly on kinesin since it is the simplest of all the molecular motors. Kinesin is probably the progenitor of the other molecular motors. Excellent discussions about the ATPases and polymerases exist elsewhere (see the references at the end).
In recent years, several research groups across the world have made molecule-sized contraptions that only begin to approach the biological motors. Dr. Ben L. Feringa of Groningen, Netherlands and coworkers have made a molecular motor that rotates in one direction in a four-step process, stimulated by ultraviolet light and heat. Dr. T. Ross Kelly and his colleagues at Boston College in Chestnut Hill, Mass. have also made a molecular motor made from triptycene and helicene. A series of chemical reactions causes the triptycene blades to brush past the helicene group, going in only one direction. Dr. Kelly explains: "There's probably a parallel between the way our [motor] works and the way nature's works." He also points out that "It does not turn very quickly. It takes several hours for the three-spindled wheel to make one revolution. Our next step is to speed it up." This work can be found in the Sept. 9 issue of Nature, 1999.