Video: “Ion Channel Chemistry: The Electrical System of Life”
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Cliff Mead: Good evening and thank you for being here tonight for the Linus Pauling Award Legacy Lecture, sponsored by the Oregon State University Libraries. My name is Cliff Mead, and I am Head of Special Collections at Oregon State University, home of the Ava Helen and Linus Pauling Papers. The Pauling Legacy Award, established in 2001, is granted once every two years to an outstanding individual in an area of study in which Linus Pauling was also active. Past recipients of the award include Daisaku Ikeda, founder of Soka Gakkai International; Nobel laureate physicist Sir Joseph Rotblat; Harvard University biologist Matthew Meselson; and esteemed Caltech chemist John D. Roberts.
This year’s award winner, biophysicist and self-taught X-ray crystallographer Dr. Roderick MacKinnon of Rockefeller University, won the Nobel Prize in chemistry for his work in ion channel imaging. His research focuses on the physical and chemical processes that produce electricity in cells, and the passage of inorganic ions, such as potassium and chloride, across cell membranes. In 1998 MacKinnon became the first scientist ever to capture a three-dimensional image of a potassium ion channel, thus solving the mystery of its structure. His achievements have advanced the fields of both Biology and Medicine.
Dr. MacKinnon received his B.A. in Biochemistry from Brandeis University, and his M.D. from Tufts University School of Medicine. He completed medical residency at Beth Israel Hospital in Boston before returning to Brandeis for postdoctoral studies. He joined the faculty at Harvard Medical School in 1989 and in 1996 moved to Rockefeller University as a professor and head of the Laboratories of Molecular Neurobiology and Biophysics.
In addition to the 2003 Nobel Prize in Chemistry, Dr. MacKinnon is the recipient of numerous scientific awards, including the 2003 Louisa Gross Horwitz Prize, the 2001 Gairdner Foundation International Award, the 2001 Perl-UNC Neuroscience Prize, the 2000 Lewis S. Rosenstiel Award for Distinguished Work in Basic Medical Research and the 1999 Albert Lasker Basic Medical Research Award.
His Legacy Award Lecture this evening is entitled "Ion Channel Chemistry: The Electrical System of Life." It is my privilege to present to you Dr. Roderick MacKinnon. [3:50]
Roderick MacKinnon: Thank you very much for the nice introduction. It’s an honor to be here on this occasion to help celebrate the legacy of a great scientist, Linus Pauling. I should say that every time I give a scientific talk, I think of Linus Pauling, and the reason is that many years ago a friend of mine named Alex Rich, who worked with Linus Pauling, once just before a talk said to me "Linus used to say before you give a talk, you should always make sure you have a full mind and an empty bladder." So I figure I can definitely accomplish one of those.
Now, the title of my talk is "Ion Channel Chemistry: The Electrical System of Life." What I really want to do is tell you a story, a story about... well, first of all, what do I mean when I say the "electrical system of life?" What is this? I’m going to tell you a story that is historical. It begins back in the 1700s, and it’s a story about "How did we come to know about this?" I personally find that I can understand science better if I can understand not the condensed version you see in textbooks, but look back into why people looked into what they looked into. What were the questions, and why did people think of the experiments, and how did they come up with the ideas? And so this "electrical system of life" starts as long ago as in the 1700s.
Now, first of all, what do I mean by the "electrical system of life." It turns out that we have electrical signals in us, all living things do. If I want to wiggle my finger, a thought originates somewhere in my brain, and shortly thereafter the muscles in my fingers contract and I wiggle my finger, and you know that a message got from my brain to my finger in the form of an electrical impulse that travelled down the nerve. So it’s clear that yes, we do make electrical signals that are important for our motions and our movements and our thoughts. Nerve cells in our brain produce similar electrical signals, and communicate one nerve cell to the next, and that underlies our thinking. And in fact, when these lights are shining into my eyes so that I feel like a deer standing in the headlights, or when I see you, you see me, when we observe the world around us, when we hear sounds, when we taste, when we smell, when we experience tactile information, all of these senses come to our brain in the form of electrical impulses. So that is to say, we are completely wired. That is how we experience our world.
Before going further, I want to do an unusual little thing, not a back flip, I want to play a piece of music for you, for just two minutes and 40 seconds. It’s a suite... I’m not going to get a cello and do it, I’m going to play a sound. It’s a suite by Bach, and while you listen to this, close your eyes and imagine the cellist playing this music, and imagine how the electrical impulses have to be going from the cellist’s brain to give rise to the coordinated movement of muscles to make the beautiful music that you’ll hear.
The cellist on this soundtrack was Jacqueline du Pré, who at a later date developed a neurodegenerative disease called multiple sclerosis, or MS. She described her first onset of symptoms; she said just before a concert in 1972 that she lost all sensation in her hands and was unable to feel the fingers on the strings, or the bow of her cello. Her condition was subsequently diagnosed as the onset of multiple sclerosis. In fact, she went through the concert, and said she played the piece, I think it was a Hayden cello concerto, without feeling her bow or her fingers on the string. And she said she didn’t play it very well. This disease ended up taking away completely her ability to play the beautiful music that she made, and it eventually took away her life. Multiple sclerosis is a disease involving the architecture of the nerve cells, the axons that travel between the nerve body and the muscle. It’s a structural change that destroys the fidelity of the nerve impulse propagation. [11:20]
I think, actually, you all are well aware that we make electrical signals, because you’ve all heard of things like EKGs, and most of you have probably had an EKG, a device that measures the electrical rhythm of the heart; and you’ve heard of EEGs, or electroencephalograms, that measure brainwaves. So you’re well-aware that we produce electrical signals. But if you think about it for a minute, it seems a little funny that we produce electrical signals, because most electrical devices that we know about, like radios, television sets, computers, that are electrical, don’t have water in them. They’re dry, and we’re filled with water. And in fact, you’re well aware that if you pour water into your computer, it’s not good, because it short-circuits the electrical conducting elements, and ruins the solid-state electronics, and you’ll have to get a new computer. So even though we’re well aware that we make electrical signals, it’s also clear that somehow we make electrical signals in a very different way than electrical devices that we know about.
So where did this idea, where did the study of this process begin? The first systematic study of electricity in living things was in the late 1700s by a man named Luigi Galvani, an Italian physiologist. I’ll tell you in just a little while about his experiment, but first I want to tell you what did Galvani know about electricity before he interpreted his, what he called "animal electricity experiments." It turns out Galvani, what was known about electricity wasn’t very much. And in fact all he knew about, all people knew about before this time, much before this time, was the experience of static electricity. You know on a cold day, you walk across a rug, and you touch a metal doorknob, and you draw a spark. We’ve all experienced that, and in fact probably people have experienced that sort of thing for thousands or even tens of thousands of years. Undoubtedly it was a curiosity, or maybe even seemed like a mystical phenomenon, what we today call static electricity. But that’s really what people knew until the mid-1700s.
What happened in the mid-1700s is people started to get carried away on this phenomenon. They got really interested in it. In fact, they built - not because they understood it, but because it seemed like a curiosity - they built things called electrostatic generators. And here is a picture, an old picture of an electrostatic generator. Here is a wheel that is being spun, and it’s made of some material like a hardened resin, so it’s sort of like a plastic, and in fact this rope is a material, so actually this - and it’s spinning - and this rope material is rubbing against the resin, the hardened resin, and so in fact this is like the rug, and this is like your shoe on the carpet, this would be the carpet, and in fact what happens is a charge is built up and hops across to a metal rod, so a spark, so this would be the doorknob. And in fact, this is hanging on ropes that are nonconductive. They’re not wires, they’re just ropes. And so this is an electrostatic generator. Now, there’s somebody here is spinning the electrostatic generator, and there’s another guy over here who’s about to touch the metal rod with his left hand. If you look he’s doing something with his right hand. He’s got his right hand on some little object that’s hanging off the metal bar. I made a sort-of more interpretable picture of the same thing. Here’s my version of it. Here’s the electrostatic generator, the metal bar, the person about to put one hand on the metal bar.
Now, if you do this, spin this electrostatic generator, charge the bar, and just walk up and put your hand on it, you’ll draw a spark just as if you had rubbed your feet on a carpet and grabbed a metal doorknob. But what’s with this, this thing hanging here? It’s a glass jar. It’s got some water in it, and it’s got a metal hook stuck into the water and hooked up to the metal. That’s all it is. Ideally maybe it could have a little metal foil wrapped on the outside. But that’s it. It’s a glass jar, water in it, a metal hook in the water, going up to the thing. Now, if you do the following experiment: instead of touching the metal rod, you first put one hand on this for a while, while this is spinning, now you touch the metal rod, you’ll find maybe you’ll kill yourself, or you’ll get such a big shock that it will really jolt you.
This thing. This is an interesting little thing. It was invented independently by two people. A man named Van Musschenbroek, from the University of Leyden, and somebody named Von Kleist, from what was then called Pomerania. Now apparently Van Musschenbroek had more influence because this became known as the Leyden Jar, not the Pomerania Jar... I don’t know. But it became known as the Leyden Jar, and that’s what it is: a glass jar with water in it, with a metal wire into it, and maybe some foil wrapped on the outside. Very interesting. They didn’t know how it worked. But this thing had some interesting properties that although people didn’t understand how it worked – and I’ll tell about that, how it works in just a few minutes, two minutes – but what they knew is the following. If you did the experiment I just described, you would get a big jolt, which means the electrostatic generator was putting more electricity onto this thing than it does on the metal bar alone, first of all; and second of all, you could do the following. You could stand there and touch it without touching the metal bar for a while, while you spin this thing, and now take your hands off of it. And now you could take it, disconnect it from the metal rod, and it would hold the electricity in it, and you could shock someone later. [18:40]
And in fact, there was a kind-of a scientist, I guess you could call him, named Jean Nolet, in France, who was an abbot. And he is known for a famous experiment, a demonstration he did at the time, where he would charge up the Leyden Jar, and he would tell a bunch of his Carthusian monks to sit in a giant circle, each of them holding onto a little metal wire, one connected to the other, making a loop. Then he would come along, with an audience watching, and he would attach one end of the wire to the metal coming out of the inside of the Leyden Jar, and the other end of the wire to the metal foil on the outside of the Leyden Jar, sending a shock through the whole group and they’ll all jump spontaneously into the air while everybody’s watching, to their amusement. Apparently the occupational health and safety departments in the abbeys were different than what’s in our universities today, but this was a very nice demonstration of storage of something interesting in this Leyden Jar. Storage of this electrostatic energy, electrostatics from the electrostatic generator.
Now what’s this doing? So it turns out a man named Benjamin Franklin, an American, visiting in Europe came across the Leyden Jar and became very interested in it, and he brought one home to Philadelphia and he started studying it. And he did some very, very, interesting experiments with this thing. He wanted to know, "What’s this doing?." I mean other people wanted to know that to, but other people were just having fun with what it could do. So here’s sort of a blown up picture of the Leyden Jar, and in black here’s the glass. So, glass jar, kind of round shape, doesn’t matter, and here is this water inside, I call it conducting material and there’s a metal bar coming down to it and on the outside I have a conductor coated on the outside, so aluminum foil, say wrapped on the outside. That’s the Leyden Jar, its got a cork stopper on it. What Benjamin Franklin did, he had an electrostatic generator and he charged it up and wanted to know where this electricity is. And he did basically the following experiment: He charged it up and he asked, "if I take it all apart and put it back together is the electricity still there, and if I replace one part, one at a time, where is it?" And he found the amazing thing, that you could take it all apart, even pour the water out, put new water in, put it all back together, and it was still there.
And, the one thing you needed to hold it there was the glass jar. And so, Benjamin Franklin said that this stuff, electricity is actually, must be, matter, must be made out of charged particles, plus and minus, and what you can do is get them to separate some how, and that’s what happens in the Leyden Jar experiment. You’re actually separating charge across the glass. So, that’s what he said, electricity is a charge separation, a separation of oppositely charged particles. This was a very brilliant experiment and a very brilliant conclusion.
Now, Franklin apparently had a sense of humor because he concocted his own version of the Leyden Jar, which is called the Franklin Mirror. He took two pieces of metal, one is this thing on the outside, and the other is another piece and the two pieces would come together, one inserted into the other. But, the metal surfaces didn’t touch, they were separated by a little non-conducting material, like a resin, sort of like the glass in the Leyden Jar. So, you had two metal sheets pushed up against each other. He polished them very nicely so it was a mirror. Then, what he would do was charge up the two sides, put a charge separation across this. Of course, in order to discharge it, you’ve got to touch both metal sheets. So, he can hold his charged mirror by touching one, without touching the other, won’t hurt you, and hand it to his unsuspecting friend and say, "Take a look at my new mirror," who would look at it and of course get a good shock.
Now, that, except for the modern electrical capacitor, was Galvani’s understanding of electricity. So he was aware of the Leyden Jar, he was aware of the Franklin experiments, and he knew then that electricity could be, if you had something like a Leyden Jar that was charged, and then you took a conductor and touched the two halves of it - in the case of the jar the outer surface, and the wire on the inner surface - you would discharge it. He knew about that. So what was his experiment? His experiment was to freshly kill a frog, freshly prepare it, as shown in this picture, and then take a piece of metal and touch the nerve up here and the muscle down here and what he noticed was that if the frog was freshly killed, by doing that, the muscle would contract violently. A violent contraction. And so Galvani said, "Ah-hah! I’m taking a metal... basically the frog muscle is like a Leyden Jar, and when I touch the two ends of it, it contracts, so it’s discharging like a Leyden Jar." So he concluded that there must be electricity in this thing, and he called it Animal Electricity.
This fascinated people in intellectual circles in Europe at the time. They started to hear about taking a dead animal and getting it to move. And in fact it was the origins of this stuff that led to the Frankenstein story. This was getting something supposedly dead to move, and the idea of Animal Electricity. Other people repeated this experiment, and one person who became very interested in it and repeated it, his name was Alessandro Volta. Alessandro Volta was an Italian physicist who at first thought this was fantastic, this Animal Electricity, and started to do these experiments himself, and then began to doubt something. He decided that Galvani had made a mistake in his interpretation. Notice in this picture, Z and C here, this metallic arc is actually not one metal, it’s two. This half is zinc, and this half is copper. That’s what Galvani was using. And it turns out, Volta rightly pointed out, if this were like a Leyden Jar then why should you need two, why should you need a bimetallic arc, why should you need two metals? One metal should do it. And it doesn’t. And Volta, when he tried to do it with a single metal, couldn’t get it to work. And he, instead of focusing on the frog - the physiologist here focused on the frog - the physicist decided he should focus on the metal, because it seemed like something was wrong (and something was somewhat wrong in Galvani’s argument) that you needed two metals, and Volta said there was no Animal Electricity. [27:15]
And this started what is really one of the greatest controversies in science, called the Galvani-Volta controversy. It went on for a long, long time. And Volta was a very, very influential physicist. It’s an interesting way science goes, if you hear this story. It’s very human, and it gives you some sense of the way scientific ideas develop. Listen to what Volta did. He focused very much on the metallic arc, and he in fact said, "Look, there’s something very interesting. He is doing something interesting with the two metals." So Volta started asking what is he doing interesting with the two metals, and started looking at what happens when you put metals together.
And in a matter of a couple of year, he came up with something called the Voltaic Pile. Voltaic, Volta - you recognize volt from volts on a battery, the voltaic pile - which is a stack of zinc and copper metals, and you can do it with other metal pairs too, with some brine solution, salt solution, in gauze in between them. And what he found is that if you stack them up this way, zinc, copper, zinc, copper, zinc, copper, and then attached a wire to the ends, this thing actually produced a current. There was, what we today call a voltage across this stack because there is metallic electricity, that’s what Volta calls it, and it caused a current to flow through here. This was a huge advance. It was the invention of the battery. And this not only had great practical value, to invent a battery, but it had great value for the advancing of people’s understanding of things. And it kind-of went this way. It was the first time you could make a sustained current, controlled current. And then a man named Orsted happened to notice that if he put a compass close to the wire, hooked up to the battery, that the compass needle deflected. And Orsted said "Ah-hah! It must be making a magnetic field because magnetic objects are what usually make the compass needle deflect, and this thing is making it. So the moving current must make a magnetic field."
And then a man named Ampere - you recognize amperage - a man named Ampere followed up on Orsted’s observation, and he described a quantitative rule, an equation relating the current direction with the magnetic field. And shortly after that, two men independently, one in the United States, and one in England - in the United States, Henry, and in England a man named Michael Faraday - reasoned and then showed experimentally that if a current can produce a magnetic field, then it should be that a moving magnetic field should produce a current. And it was true. And that started, that was the birth of the idea of electromagnetic conduction. And that is the basis of today’s electrical motors, electrical transformers, very practical value. And finally, then it was James Clerk Maxwell, with all of this information, developed a theory of electricity and magnetism that combined electricity, magnetism, and light into a single unifying theory. All of this happened in a span of 50 years. And all of it started by the invention of Volta’s battery, which started because he was trying to prove that Galvani was wrong. Well, not just to prove he was wrong, but he was quite convinced he was wrong, there was something wrong here. And so it’s very interesting that all of this stuff ended up happening because of...it was at least catalyzed by Galvani’s observations.
Now obviously Volta was quite brilliant, and he was right to point out the necessity of the bimetallic arc. But in the end it turns out Galvani was not wrong. Galvani was right, if not completely for the right reasons. That in fact there is animal electricity. Because not too much longer, in 1831, a man named Carlo Matteucci measured electrical currents with a very sensitive detector close to injured muscle. And then about 10 years later, Du Bois Reymond discovered that you get an electrical impulse traveling along a stimulated nerve. Ten years after that, Von Helmholtz measured the velocity of the nerve conduction. And about 15 years later a man name Bernstein proposed that this was made by some change in the ionic conditions around a nerve. And so this was in the 1860s. So in fact there is animal electricity. Galvani was right. Not completely for the right reasons.
How does this work? What’s happening here? So you all know a cell is surrounded by a membrane, and if you looked closely at the cell membrane, you would discover that it’s... when people look closely they find that it’s what’s called a lipid bilayer. So this is a picture of a cell, just a hand drawing, a nucleus, a membrane around it, and if we blow up on the membrane around it, you see that it’s a bilayer made of lipid molecules. It’s very thin. There are called lipid head groups here, facing water outside the cell and water inside the cell, and the water outside and inside the cell has salt in it, has ions, charged atoms, floating around - sodium, chloride, potassium - and the inside of this membrane is oily. It’s non-conductive. Now anyone in the audience looking at this can tell me what this reminds you of. This thing is a lot like what I’ve been... this is like the Leyden Jar, it’s a lot like a capacitor, isn’t it? There is a conductor out here, that is the salt solution, a conductor in here, and a non-conductor in between. A capacitor.
Now the problem is, of course, electrical capacitors have batteries or electrostatic generators or something like that to put a charge across it. How could that happen in a living cell? What’s the equivalent of the battery there? It turns out in cell membranes there are proteins. Some proteins are called ion pumps, and some are called ion channels. What do the ion pumps do? The ion pumps use chemical energy, for example the hydrolysis of ATP and ADP, to, for example, pump sodium outside of a cell and potassium into a cell. So that surrounding our cells, all the cells in your body, you have a high concentration of potassium on the inside, depicted here as little green dots, and a higher concentration of sodium on the outside, depicted as little red dots. And that gradient, it’s called, the concentration difference, is set up by this pump. Now what happens, because the pump set this concentration difference up, if you have an ion channel - which is something that will let an ion across - and suppose that ion channel is a potassium channel, so it’s a kind of channel that when it opens potassium can run across the membrane. Well what happens when this opens, is potassium, because it’s at a high concentration inside, will tend to flow out to its lower concentration side. But these ions like potassium and sodium, carry a charge. They’re charged. They carry a plus charge. So what happens is when the potassium flows out, it makes a charge separation across the membrane, because the potassium pluses go out, leaving more negative inside and bringing some positive to the outside, so the membrane capacitor gets a charge across it. And all of the cells in your body have this. It has this charge separation across it.
So if I come along and I say "Give me your hand" and I stick an electrode into your cells, it will hurt a little, but if I can get one right inside your cell, and it’s hooked up to a volt meter, you’ll see that inside the cell, the needle on the volt meter will read something like minus one hundred millivolts, or -0.1 volts. And that’s because the membrane is a capacitor, there are ion differences across the membrane, set up by the pumps, and the channels open up, and the ions cross and charge the membrane. So all of our cells carry charge on their membranes in this way. [36:40]
Now you might say "That’s great, they’re all carrying a charge, but still how does that get the thought in my head down to my fingers?" And it turns out that this charge, because of some properties of the ion channels, that is, the proteins in the membrane that let the ions run down their concentration gradient and carry the charge across it, because they can open and close in a certain way to propagate an impulse. Now you might say "Now wait, I’ve understood everything he’s said until he just said that," and that’s okay. Because there’s a property of these channels called voltage dependence that gives rise to the following. Here I’ve drawn with my hand a cartoon depiction of an axon, an extension of a neuron, so this is a little cylinder running from left to right, and I made it green inside because to me potassium is green. I don’t know why, but it is. And sodium is red, so I put red on the outside. And if this nerve were at rest, and we could see the ions, this thing would be green on the inside from the high potassium, but some potassium would be trying to leak out, so it would charge up this membrane, negative inside and positive outside. This is a cylinder, so this is outside and this is outside, and all of this is inside. And it would sit there.
But actually when a nerve impulse goes down it, what’s happening is there’s a little region where sodium is rushing in. So it’s becoming red inside. That sodium rushing in makes it positive inside. That positive inside makes the neighboring channels open, sodium channels open, and they rush in. And this thing propagates along like dominoes, or like a wave. It’s a little flip, a transient flip from minus inside to plus inside and back. You still can’t put it together from everything I’ve told you, because I can’t now tell you everything because if I do, you’ll get very bored and run out. But I think you got the idea. The idea is the membrane is charged and the polarity of this charge can transiently flip, and if you stimulate a nerve cell, what happens is that stimulus is in the form of flipping the polarity and it will go like a wave right along the membrane. That’s what happens. That’s how it’s set up. So the impulse is propagated by a switch in this polarity.
But the essence is that the membrane is a capacitor and there are channels in the membrane that let the ions cross to charge the membrane. Now, how do we know the channels exist? This picture I just showed you, actually, I should say, this was an idea from a long time ago, from around 1950; 1952 to be exact. Famous papers published by Hodgkin and Huxley on their theory of the nerve impulse. And it was really essentially a very quantitative description of what I just described. They didn’t know about channels, but the idea was that the ions moved across the membrane and polarized the capacitance of the membrane, and this thing could propagate along the membrane. They didn’t know about channels. It’s only more recently that scientists following Hodgkin and Huxley wanted to say "Well, what is it about the membrane that lets the ions cross?" And people discovered ion channels, and one of the ways we know about ion channels is from a technique called the patch clamp technique, invented by Sakmann and Neher. And that’s a technique where you can, instead of looking at the electrical activity of an entire cell membrane, you can take a glass pipette, very small, come down on the surface of the cell, and suck in a little piece of the membrane that might only have one channel in it, and then if you measure the electrical activity of that little piece of membrane, under the right conditions you actually see this peculiar thing, where if you measure the current on the Y axis, on this axis, against time on this axis, you actually see it flip up and down this way.
In fact, what it’s doing is the channel is closed and then it opens for a little while, and it closes, and opens and closes, it’s jumping back and forth. And you might say "Well, why is it jumping back and forth, why is it not staying in one place?" And it turns out things as small as molecules never stay in one place. We stay in one place if we want, but little things don’t, they’re continuously getting jostled around. Now in a neuron, the conditions are such that usually this is either closed or opened, and I’ll say something about that in just a couple minutes, but if you hold it under the right conditions where it’s open sometimes and closed sometimes, that’s what you see. It jumps back and forth between opened and closed. So here you’re looking at the activity, the electrical activity due to a single potassium channel. And the patch clamp technique allowed that kind of view of the ion channels.
Now what do they look like? And this is where X-ray crystallography work comes in. And the way you find out what a molecule looks like, today, a molecule like a protein, like an ion channel, is essentially the same way that Linus Pauling found out what smaller molecules looked like. In fact, Linus Pauling, among other things, was an X-ray crystallographer. And in fact it’s from solving many structures of small molecules and studying them in great detail that he developed his deep, deep intuition into atomic interactions and the nature of chemical bonds. Today we use the same technique but for molecules that are much, much larger than the kind that he studied. Molecules such as ion channels. This is what is called a CryoLoop. This is 50 microns, so this whole thing is very small, and this is a crystal of an ion channel that you produce using a certain set of methods, and you put it in an X-ray beam, and you get these diffraction spots, and in fact that is due to the X-ray beam ricocheting off the atoms of the protein, and from this kind of information you actually don’t get... it’s a little like a completely defocused image, and I won’t go into why it is - some people here will know, others won’t, it’s not important for now - but why it shows up as separate spots. It’s because it’s a crystal. But the intensities of these spots have information on the structure. And with that information, plus some additional information called phase information, which is essentially information to focus this defocused image, you can actually get a picture of the molecule. [43:45]
So here is an example of a piece of a potassium channel protein, showing just a little part of it, with a blue mesh, it’s called electron density, you can see it has a particular shape, and in fact if you’re a protein chemist, you can tell what those shapes mean. You can tell they mean there are things like particular amino acids that you can then build, model into this electron density. So that’s sort-of giving you the general idea of how you get a picture of a molecule. It’s really the same general way that Linus Pauling did it, but with methods that have been developed that let us study much, much larger molecules.
Now, what I’m going to show you here is a little lesson on representations of a protein, it’s called, different representations of the same protein. What I’m showing you here is a part of a potassium channel called the pore. It has four sub units in a ring, I’ll show you in a minute, but I’m just showing you two of them here, and I’m showing you the same thing in different representations. This we call a stick representation, and this is called a ribbon representation, and this is called atom representation, and this is called CPK for Corey-Pauling-Koltun, our Pauling, who invented this kind of molecular model. The reason I’m showing you this is actually because when we often look at protein structures, or when you look at them in books, you see representations like this, or like this. But proteins don’t really look that way, they look maybe more like this. They’re all filled up, they’re filled with atoms, but this is very hard to look at, so we show a simpler representation where we follow the polypeptide chain with just a ribbon, or just a wire.
And so these are simpler representations of a protein. This is a potassium channel pore. There are four subunits shown in four different colors here. This would be outside the cell, this would be inside the cell, and the membrane would run from here to here. So of course this is a very enlarged potassium channel, this oversimplified representation of it, but it has a pore down the middle that crosses the membrane. And in fact if we again look at a simpler version of it, with just two subunits, we see that the pore has ions in it and in fact some water molecules. And there’s one interesting thing I’ll point out about this. So here would be the membrane, outside inside, and the ions that cross through the membrane this way. In the middle of this channel there’s a potassium ion with water around it, and what we often see in nature, what we usually see, is that form does follow function. And recall the membrane being a capacitor means it’s a nonconductor. By itself without channels, ions don’t cross the membrane. The ion wants to be in the water, hydrated, not in the oily part of the membrane. For the same reason, in the vinaigrette, when we mix vinegar and oil, and shake it up and let it separate, if you taste the oil, it’s not salty. If you taste the water, it’s salty. The salt, the ions, stay in the watery part. For the same reason, the ions don’t want to go through the membrane.
But if we look at this channel, in the middle of it, it’s pretty wide and it lets the ion have water around it. That’s a way of stabilizing it. And, in fact, here is the kind of vinaigrette experiment, but this is cobalt hexachloride - red color - in water, in mineral oil, and you shake it up and let it separate and all the ions, the red cobalt hexachloride, stays in the water. And that’s because the ions like to be surrounded by water. And you look at the channel, and in fact you discover that it has a little cavity in the middle of the membrane where the ion can be surrounded by water. So what we see at a molecular level is that form follows function.
Up here ions are not surrounded by water but are held by a special part of the protein called the selectivity filter. And this is what lets only potassium through because the little positions along this selectivity filter hold potassium ions, but not other kinds of ions, so they can slip across. So that’s what makes this kind of channel only let the potassium cross. And so when this kind of channel is open, the potassium can go out but the sodium can’t go in, and that’s why the membrane gets charged negative inside and positive outside.
This is, in fact, another CPK - P for Pauling - representation of that same region, and you can see the potassium ions that are normally out in the water - surrounded by water - are hugged by oxygen atoms for the protein, spaced for potassium to fit and slip across the membrane.
Now, of course, when I think "wiggle my finger," the impulse goes down, my finger wiggles, but when I think "don’t wiggle my finger," no impulse goes down and it doesn’t wiggle. Which means the channels have to be open sometimes, but closed at other times. We don’t understand everything about that but we know some things, and I’ll just give you an impression here. The channels actually have gates, and those gates open and close. And we can see, for example, for cases where you were inside the cell looking out through a closed potassium channel, it looks like this - again this is a simplified representation with the helices. By the way, I should point out, given the occasion, that these helices, called alpha-helices, this was a secondary protein structure that was discovered by Linus Pauling. In fact, we see it in proteins throughout nature, and here they are in a potassium channel.
But this is closed, and this is open. And you can see that protein can undergo a change like the aperture of a camera, to open and close the pore. And in fact different channels have protein attached to it that regulates that opening and closing. So here are three different kinds of potassium channels. A little potassium channel whose gating is probably regulated by the lipid membrane - it’s very small and it’s from a bacterium. Here’s a big one, a pore very much like here, but inside the cell there’s a structure attached that binds calcium, changes its shape and causes the pore to open. So calcium binding here makes that pore open. And so that’s called a calcium-gated channel.
And in this kind of channel there are many other parts, and up here there are components that actually let the membrane voltage open the channel...and in this kind of channel, these are called voltage sensors and they determine whether the pore opens. That’s the part of the nerve impulse that I told you I’m not telling you about. That I said I don’t have time. It’s these structures that actually allow that nervous impulse to propagate. The idea is, how does the channel that’s not open, that’s beside the channel that is open, know to open next? And it opens because the channel that does open lets ions go, changes the polarity on the membrane and then this guy beside it feels that polarity change and says "Ah, I feel the electrical environment changing." And it opens, and that’s what this does. And so that’s important for this nervous impulse propagation.
And so, it’s getting late, and I think I’ve given you more information than you ever want to know about electricity and biology. But, I actually I bet you all sort of understand it now. So, thank you very much. [52:40]
Questions? I think that’s what’s being said to me, questions?
[Audience question #1]
You mean per channel? I think the unit would be that if one elementary charge crosses 25 millivolts that would be one KT of energy, we could say. So, that means every time a channel opens and conducts at a very high rate, the ions are crossing a large voltage difference and, if it’s one KT per 25 millivolts, that you would calculate to be a huge energy actually.
I don’t know the details of how cochlear implants work. I see what you’re saying, your orders of magnitude larger energy to get a stimulus, and that doesn’t surprise me. If you look at something like the inner ear or any of our sensory organs, they are really incredible in their ability to amplify the stimulus. Light is an example, our eyes have the ability to see very few photons. Very few photons will actually be sensed by us. The reason is because we have an incredible amplification, that millions of years of evolution have developed these sensory systems that amplify very small signals, in work, in a setting with high noise. So, we come along and try to engineer a devise, and it doesn’t have near the sensitivity to work in a high noise environment. That’s the only general thing I could say.
Regarding energy in this process though, a large fraction of what we eat to say alive, goes into charging up all your cells. These batteries cost something to charge, so in fact a very large fraction of staying alive is to maintain the electrical charge difference across your cell membranes.
[Audience question #2]
I said that the channel beside the one that’s active, the one that’s active lets some ions cross, and it changes the charge across the capacitor in the channel right beside it I told you, because of this thing, feels it, and then it opens and lets it propagate. But, where does the directionality come from? The answer is, if you go to the middle of an axon and stimulate it, it will go in both directions. But, what is interesting is you could say, "But wait!," that means we have a problem because every time, what if you even start at one end and stimulate it as it’s propagating down, why doesn’t it keep spitting them off the back and then become a chaotic mess? That’s very interesting because it turns out, when these channels open and close, suppose they open, what happens is that they’re ready to open, then they open, but then once they’ve opened, and they close again, they’re not ready to open for a little while, they’re called refractory. They go into a state that they’re not prepared yet and it takes a little while for them to reset. It has to do with change of the molecular structure that mostly, we don’t yet understand. We understand in some cases some of it, but not most of it. It’s that sort of resetting time that prevents impulses from spitting out the back, so that when you start at one end it just propagates in one direction. If you think about what I said, that means if you stimulate one and then again very quickly it won’t go. So, there’s a little refractory period and the nervous system has these. In fact, different cells have channels with different refractory periods, such that different cells can fire at different rates, and it’s all related to the molecular structure and the conformational changes that take place. Great question.
[Audience question #3]
I can’t say too much about that, other than it’s not a quantum computer, because all the atoms I’m talking about are pretty big and they’re not, I mean quantum mechanics is running the show at a very fundamental level, but the movement of heavy ions like potassium, I mean they’re not tunneling. I think we can understand this in a pretty classical way, that’s what I think, how the brain computes. But, you know, fundamentally, I don’t know how the brain computes, all I’m talking about is the very thing, in a sense, about how the wire conducts itself. I’m giving the history of how the nerve conducts its impulse, how it responds. But then there are all these fundamental questions about where did the idea come from to begin with, and I can’t begin to address that. [59:55]
[Audience question #4]
I agree with Leonardo. [Audience laughter] But I think that it’s always something going in - you’re right about when you look into someone’s eyes, but I would interpret that to mean that there’s something going into the eye of the person looking. It’s what they see in the other person’s eyes. It’s not something coming out of the other person’s eyes - at least I would think.
[Audience question #5]
The pump there just keeps on working. So the channel there lets them run down the concentration gradient, but the pump is always working there to pump them back in. And you can get with one impulse, if you count how many ions it takes to charge the capacitor, it’s very very few. That’s a neat thing about the whole system. So even if you shut the pumps off, you could make many, many impulses, before you ran out of the gradient. So because of that situation, with the pumps chugging away in the background, they restore the battery. The energy is coming from somewhere - ATP hydrolysis - which is coming from your peanut butter sandwich.
[Audience question #6]
There have been a lot of experiments of speed based on the thickness of the axon, but also there are other structural properties of the axon the affect the speed. So it turns out, that where Hodgkin and Huxley did their work on squid axon, it’s a cylinder like I show. But in us, our axons, or at least most of them, actually have a wrapping around them called a myelin sheath, that changes the effective capacitance of that membrane. And actually the channels get bunched up at little edges of the myelin sheath where there are gaps in it. And in fact multiple sclerosis is actually a disease not of the nerve cell membrane but of the myelin membrane wrapped around the nerve cell. But your question - it’s a diffusive property and how can it go so fast, that’s very well understood. It’s very well modeled and models agree with experiment on the propagation rates. When I think to wiggle my finger, the potassium in my brain did not come down to my finger, it didn’t have to diffuse very far. It just crossed the membrane. But then that affected the situation of the membrane right beside it, the movement of the charges right across the membrane, the effect propagates across the surface. And in a sense, those cable properties are where it gets its speed.
[Audience question #7]
For the case of arrhythmia, its an abnormal electrical activity in the heart. And so, how does it relate? Well it’s the ion channels that are underlying the electrical activity. And so some drugs that treat arrhythmia work by binding to the ion channels and affecting their activity. And the hope in the future is that by understanding more about these ion channel molecules, they can someday develop drugs that will help with arrhythmias that don’t exist today. That’s sort of a short answer. There’s one kind of arrhythmia, called long QT syndrome, that is actually caused by drugs, and it causes sudden death. And the understanding of that happens to do with a particular potassium channel in the heart, and how certain drugs interact with it. So it’s related to arrhythmia because the ion channel is the little hardware unit that is making the electrical activity. And drugs work on proteins, such as ion channels, to regulate their function. So that’s the connection.
[Audience question #8]
Epilepsy is abnormal electrical activity in the brain and it, in many ways, is connected to ion channels. It doesn’t mean that every form of epilepsy has an abnormal ion channel - that’s not the case at all. In many forms of epilepsy there are abnormal ion channels. And even in forms where there are no abnormal ion channels, by modifying the function of certain ion channels, you can change the electrical activity to reduce the probability that there will be a seizure. [66:55]
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