Molecular motors

At this very moment, inside every cell of your body, there are thousands, even millions of machines, so small that they can’t even be seen with a conventional microscope, performing tasks in mere fractions of a second without any direct supervision. There are tiny molecular robots working together inside us to accomplish goals that scientists are only beginning to understand, every moment, of every day, through out each of our lifetimes. And it’s not just in our bodies. In the bodies of every living thing these things are active. They are, in fact, required for the basic prerequisites of life, yet individually, they are nothing but dumb molecules, practically inert unless the molecules they interact with are present. These things are called molecular motors.

There are a bunch of examples of these and every time I think about them I’m amazed. I just have enough space to talk about maybe two of them, but really almost all molecular motors rely on three things: energy, tubes, and molecular components.

Chemical structure of ATP

The usual form of energy for molecular motors, the twenty dollar bill in the economics of proteins, is ATP. ATP stands for Adenosine triphosphate, and its called that because it is a molecule made up of adenosine attached to three phosphorus atoms (called phosphates while they’re still bonded to a certain number of oxygen atoms) arranged in a line. If proteins need energy for a reaction, they get the energy by lopping off one of the phosphates of an ATP. ATP then turns into ADP (adenosine diphosphate) and the energy that was stored in the bond between the phosphate and the rest of the molecule goes into whatever is needed by the molecular motor or by the reaction that’s going on. When a molecule snaps off a phosphate, the phosphate is bound to that molecule for a while and the molecule is said to be phosphorylated.

This might be a little boring in itself, but the cool thing is that so many different reactions inside a cell depend on the same ATP molecule. Almost every reaction between proteins involves a phosphorylation or a de-phosphorylation. When you eat food, what you’re really doing is supplying your body with ATP.

Adenosine is actually one example of a class of proteins called nucleotides. They can all carry phosphates, though adenesine and guanine are the ones typically used for energy. The reason nucleotides are called nucleotides though, is that they are all present in DNA, which is found in the nucleus of a cell. There are the four typical nucleotides you might remember from high school biology (Adenosine, Guanine, Thymine, and Cytosine) and then there is another that is only present in RNA, uracil. DNA forms a double helix, or a twisted ladder, with each nucleotide forming a rung in the ladder by binding with a partner nucleotide. As a general rule, adenosine binds with taurine, and cysteine binds with guanine. A goes to T and C goes to G. This is important because, by keeping to this rule, a cell can use single stranded DNA as a sort of photo negative, and use it to make the opposite strand over and over again as many times as it’s needed.

The other “tubes” come in two flavors: microtubules and actin fibers. Microtubules are the scaffolding and road system of an animal cell, keeping thing in place and providing a network of connections to every organelle. Actin fibers are similar to microtubules, except they are more temporary. They are used a lot in things like amoebas to form psuedopods and move around.

Finally there are the actual proteins that make up the molecular motor. Kinesins and myosins are perhaps the neatest looking proteins. They look like thin cartoon characters with really large feet. What they do is they carry or pull things along a tube (a microtubule for kinesins and actin fibers for myosins)by “walking” along it. You can see movies of this like the one below. The feet start out latched to a tube. Then one “foot” will release and latch on again a little ahead of where it was before. Then the other foot will to the same and so on so that the molecule is walking along the tube while carrying some cargo.

DNA polymerase is another molecular motor, though it’s often classified as an enzyme. It latches on to a single strand of DNA, takes phosphates of nearby nucleotides and then uses the energy from the phosphates to attach the nucleotide to its partner on the DNA. If the nucleotide is the wrong match, it gets thrown out. DNA polymerase is basically like a factory worker on an assembly line.

But it’s just a protein.

We’re talking atoms bound together, people. And yet they’re doing these sophisticated things. Perhaps its time we recognized our protein overlords.

Incidentally, if you are interested in this, you might want check out this blog as well:

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