1 00:00:00,000 --> 00:00:04,000 I've emphasized in the first lecture, you know, that there's a lot of 2 00:00:04,000 --> 00:00:08,000 stuff that happens just in your ordinary life. 3 00:00:08,000 --> 00:00:12,000 I saw two examples of this. Yesterday's Boston Globe, just on 4 00:00:12,000 --> 00:00:16,000 the front page there was a discovery about ìHeart Cell Discovery Raises 5 00:00:16,000 --> 00:00:20,000 Treatment Hopesî. Scientists announced yesterday the 6 00:00:20,000 --> 00:00:24,000 discovery of cells in the heart that can create new muscle cells raising 7 00:00:24,000 --> 00:00:29,000 hopes that doctors may find dramatic new ways to treat heart disease. 8 00:00:29,000 --> 00:00:32,000 The team showed that the cells, which are similar to stem cells, 9 00:00:32,000 --> 00:00:35,000 can be expanded from just a few hundred in the laboratory dish up to 10 00:00:35,000 --> 00:00:38,000 more than a million. And these can be guiding into 11 00:00:38,000 --> 00:00:41,000 becoming the pulsing muscles that power the heart. 12 00:00:41,000 --> 00:00:44,000 So when we were talking about those yeast dividing and saying how one 13 00:00:44,000 --> 00:00:47,000 cell becomes two, this is a general principle 14 00:00:47,000 --> 00:00:51,000 throughout life that cells come from other cells and they divide. 15 00:00:51,000 --> 00:00:54,000 And we'll see the relationship to that with DNA replication as we go 16 00:00:54,000 --> 00:00:57,000 along. In the case of yeast, as I said, they're just always the 17 00:00:57,000 --> 00:01:00,000 same. Your progeny are always the same. 18 00:01:00,000 --> 00:01:04,000 But in something like our own cells we start out as a single fertilized 19 00:01:04,000 --> 00:01:08,000 cell but somewhere along the way the cells have to become specialized. 20 00:01:08,000 --> 00:01:12,000 So the very early ones are the embryonic stem cells. 21 00:01:12,000 --> 00:01:15,000 They have the potential to become any cell in the body. 22 00:01:15,000 --> 00:01:19,000 But at some point, at one of these cell divisions the cells are going 23 00:01:19,000 --> 00:01:23,000 to have to start to become more specialized. And, 24 00:01:23,000 --> 00:01:27,000 for example, this one might be a lineage that would lead to heart 25 00:01:27,000 --> 00:01:31,000 muscle or to becoming a nerve or something. 26 00:01:31,000 --> 00:01:35,000 And at that point it loses its ability to become any cell in the 27 00:01:35,000 --> 00:01:39,000 body. And in many cases by the time you get out ultimately to the final 28 00:01:39,000 --> 00:01:43,000 cell that's making up the muscle or the nerve or something it has no 29 00:01:43,000 --> 00:01:47,000 capacity to regenerate. So that's why, for example, 30 00:01:47,000 --> 00:01:51,000 spinal cord injuries are so damaging because nerves at this point cannot 31 00:01:51,000 --> 00:01:55,000 be regenerated. Or heart disease, 32 00:01:55,000 --> 00:02:00,000 you get a damaged heart we're stuck. This is why this result is exciting. 33 00:02:00,000 --> 00:02:04,000 Because there seem to be at least a few cells in the heart that have the 34 00:02:04,000 --> 00:02:08,000 capacity to regenerate more heart muscle. Now, this is early on. 35 00:02:08,000 --> 00:02:13,000 It hasn't been rigorously shown to be a stem cell. 36 00:02:13,000 --> 00:02:17,000 But there's an example from the front of yesterday's paper about 37 00:02:17,000 --> 00:02:21,000 something we were virtually alluding to in class. There was also an 38 00:02:21,000 --> 00:02:26,000 article about AIDS testing. Again, you know, we're talk more 39 00:02:26,000 --> 00:02:30,000 about the HIV-1 virus. And then today on the front page of 40 00:02:30,000 --> 00:02:35,000 the Boston Globe yet again is ìRomney Draws Fire on Stem Cellsî. 41 00:02:35,000 --> 00:02:39,000 And you can look at this. But, you know, he's sort of trying 42 00:02:39,000 --> 00:02:44,000 to straddle, I guess, between being supportive of research 43 00:02:44,000 --> 00:02:49,000 on the one hand and the concerns of the conservatives and the religious 44 00:02:49,000 --> 00:02:54,000 right on the other hand, and he's drawing fire from both 45 00:02:54,000 --> 00:02:59,000 sides. But it's an issue that is in our society today. 46 00:02:59,000 --> 00:03:03,000 You're going to be expected to make decisions on it, 47 00:03:03,000 --> 00:03:08,000 to know about it and understand. I'm just trying to drive home that 48 00:03:08,000 --> 00:03:12,000 what we're talking about isn't taking place in a vacuum. 49 00:03:12,000 --> 00:03:17,000 Nobody emailed me an idea as to what happened here. 50 00:03:17,000 --> 00:03:22,000 I showed you this little movie. This is water that is cooled below 51 00:03:22,000 --> 00:03:26,000 the freezing point but hasn't formed ice crystals, but if we put a little 52 00:03:26,000 --> 00:03:31,000 bit of this pseudomonas syringae in it then somehow that super-cooled 53 00:03:31,000 --> 00:03:35,000 water turned into ice. And I told you it was a protein on 54 00:03:35,000 --> 00:03:39,000 the surface. Nobody had any ideas. So why don't you turn to whoever is 55 00:03:39,000 --> 00:03:43,000 close to you and you can talk about it for 30 seconds and see if anybody 56 00:03:43,000 --> 00:03:46,000 can come up with an idea as to why. All right? I won't look. You know, 57 00:03:46,000 --> 00:03:50,000 just go ahead. Talk to somebody and come up with 58 00:03:50,000 --> 00:04:32,000 an idea. 59 00:04:32,000 --> 00:04:36,000 OK. Well, let's see. Did we manage to get any ideas? 60 00:04:36,000 --> 00:04:41,000 Anybody got the courage to try and guess what that protein might be 61 00:04:41,000 --> 00:04:45,000 doing? Pardon? It's a nonpolar molecule. 62 00:04:45,000 --> 00:04:50,000 It's not disturbing the bonds. It's an interesting idea. Do you 63 00:04:50,000 --> 00:04:55,000 have an idea then, are you able to extend that as to 64 00:04:55,000 --> 00:05:00,000 why then the ice would start to form? 65 00:05:00,000 --> 00:05:04,000 I mean it's certainly true that nonpolar bonds sort of interfere 66 00:05:04,000 --> 00:05:08,000 with the water. That's something we've talked about. 67 00:05:08,000 --> 00:05:12,000 Let's see. Any other ideas? Yeah? That's a version of the same 68 00:05:12,000 --> 00:05:16,000 idea, I think, hydrophobic because you think it 69 00:05:16,000 --> 00:05:20,000 wants to repel the water and push it together. That's interesting. 70 00:05:20,000 --> 00:05:24,000 You're sort of getting closer on these. Yeah? 71 00:05:24,000 --> 00:05:40,000 There it is. If you were to design 72 00:05:40,000 --> 00:05:45,000 a protein that basically could bind water molecules in a lattice that 73 00:05:45,000 --> 00:05:49,000 mimicked what you found in ice then the water molecules coming up and 74 00:05:49,000 --> 00:05:54,000 binding to these little pockets in the protein would present then a 75 00:05:54,000 --> 00:05:59,000 little field of stable water molecules that looked to the next 76 00:05:59,000 --> 00:06:04,000 water molecule like it was part of an ice crystal. 77 00:06:04,000 --> 00:06:08,000 And that's indeed how that bacterium does that trick. 78 00:06:08,000 --> 00:06:13,000 It's called the ice nucleation protein. And they do things like 79 00:06:13,000 --> 00:06:17,000 take this bacterium and they put it into things like when you're doing 80 00:06:17,000 --> 00:06:22,000 snowmaking, you put this in and then you spray the super-cooled water, 81 00:06:22,000 --> 00:06:26,000 and this makes it go into ice crystals and then it helps you get 82 00:06:26,000 --> 00:06:31,000 nice snow for ski resorts and things. 83 00:06:31,000 --> 00:06:37,000 That's at least one of the areas where it's used. 84 00:06:37,000 --> 00:06:43,000 OK. So I'm just going to show you this movie again. 85 00:06:43,000 --> 00:06:49,000 These are just baker's yeast, saccharomyces cerevisiae, a kind of 86 00:06:49,000 --> 00:06:55,000 single-celled yeast that's used in baking bread or making beer. 87 00:06:55,000 --> 00:07:00,000 And here we're seeing cells divide. And this particular kind of yeast 88 00:07:00,000 --> 00:07:04,000 has a way of doing, it kind of buds the daughter off 89 00:07:04,000 --> 00:07:07,000 from the side. Some double and then split down the 90 00:07:07,000 --> 00:07:11,000 middle. But you can see what's going on. There's a lot of cell 91 00:07:11,000 --> 00:07:14,000 growth going on. And the issue that we're going to 92 00:07:14,000 --> 00:07:18,000 address now is where does the energy come that's needed to do that? 93 00:07:18,000 --> 00:07:22,000 You know from your own experience that to build things, 94 00:07:22,000 --> 00:07:25,000 to make things takes energy. You cannot put up a bridge, you 95 00:07:25,000 --> 00:07:29,000 cannot put up a building, you cannot build a computer chip 96 00:07:29,000 --> 00:07:32,000 without somehow putting energy in. You're taking a bunch of matter in 97 00:07:32,000 --> 00:07:36,000 the universe and ordering it in a very specific way making new 98 00:07:36,000 --> 00:07:40,000 contacts that didn't be there. It's an energy-requiring process. 99 00:07:40,000 --> 00:07:43,000 And I'm going to talk today about where that energy comes from. 100 00:07:43,000 --> 00:07:47,000 And then I want to tell you a little bit, just a very brief 101 00:07:47,000 --> 00:07:50,000 historical thing along the way, because a point I've emphasized here 102 00:07:50,000 --> 00:07:54,000 is biology is an experimental science. And many of the greatest 103 00:07:54,000 --> 00:07:58,000 discoveries weren't because somebody had the idea and then went 104 00:07:58,000 --> 00:08:02,000 out to prove it. Very often we didn't even understand 105 00:08:02,000 --> 00:08:07,000 how it worked. And somebody was investigating a 106 00:08:07,000 --> 00:08:11,000 phenomenon, found some peculiar things, and then began to get 107 00:08:11,000 --> 00:08:16,000 insights. And the insights were what then led to a fundamental 108 00:08:16,000 --> 00:08:21,000 increase in our understanding. And this little bit of history 109 00:08:21,000 --> 00:08:26,000 involves some names that you see on the MIT buildings around here. 110 00:08:26,000 --> 00:08:31,000 One is Lavoisier who is a French scientist. 111 00:08:31,000 --> 00:08:39,000 And he was studying what happened when grapes were converted into wine, 112 00:08:39,000 --> 00:08:47,000 a good topic for a French scientist to be studying. 113 00:08:47,000 --> 00:08:55,000 So, in essence, what he was studying was glucose 114 00:08:55,000 --> 00:09:03,000 being converted to two molecules, excuse me, of -- 115 00:09:03,000 --> 00:09:12,000 -- ethanol and two molecules of 116 00:09:12,000 --> 00:09:20,000 carbon dioxide. This transformation, 117 00:09:20,000 --> 00:09:28,000 there's C6H12O6. Remember, carbohydrates have that composition. 118 00:09:28,000 --> 00:09:34,000 And so he was studying that. 119 00:09:34,000 --> 00:09:39,000 He managed to figure out that's what happened to the sugar when you 120 00:09:39,000 --> 00:09:44,000 were making the wine. And at that point he got beheaded. 121 00:09:44,000 --> 00:09:49,000 That terminated that part of his investigation. 122 00:09:49,000 --> 00:09:54,000 But this problem was then picked up by Lois Pasteur who, 123 00:09:54,000 --> 00:09:59,000 again, his name is on one of the MIT buildings. 124 00:09:59,000 --> 00:10:02,000 He worked in France as well. There's a Pasteur Institute in 125 00:10:02,000 --> 00:10:06,000 Paris. There's a nice museum in Lille in Northern France that has a 126 00:10:06,000 --> 00:10:10,000 lot of this. But he grew up in Arbois which is a town in sort of 127 00:10:10,000 --> 00:10:14,000 Eastern France that, as you can see from the little 128 00:10:14,000 --> 00:10:18,000 picture of the village, winemaking was a major industry. 129 00:10:18,000 --> 00:10:22,000 So he was interested in that probably from when he was a small, 130 00:10:22,000 --> 00:10:26,000 small kid, although probably not dressed like that. But anyway. 131 00:10:26,000 --> 00:10:31,000 So one of the issues that he took on, which was a real problem for the 132 00:10:31,000 --> 00:10:37,000 wine growers in his little town and in France in general was sometimes 133 00:10:37,000 --> 00:10:42,000 wines would go bad. They'd come out sour and couldn't 134 00:10:42,000 --> 00:10:48,000 be drunk and then you'd lose all the profit that would have come from 135 00:10:48,000 --> 00:10:53,000 that wine. So there was a lot of interest in trying to figure out how 136 00:10:53,000 --> 00:10:59,000 to prevent wines from going bad. And so Lois Pasteur started to 137 00:10:59,000 --> 00:11:04,000 study this. And he discovered that there was this conversion that had 138 00:11:04,000 --> 00:11:10,000 been figured out now of two ethanol and two carbon dioxide. 139 00:11:10,000 --> 00:11:15,000 So this was a conversion. And we now refer to it generally as 140 00:11:15,000 --> 00:11:21,000 ìa fermentationî. But what he discovered with this 141 00:11:21,000 --> 00:11:33,000 conversion occurred -- 142 00:11:33,000 --> 00:11:39,000 -- if yeast were present. That the rate of this conversion 143 00:11:39,000 --> 00:11:45,000 varied as the number of yeast, so it went faster if there were more 144 00:11:45,000 --> 00:11:52,000 yeast. And the yeast stopped growing -- 145 00:11:52,000 --> 00:12:04,000 -- when the sugar ran out. 146 00:12:04,000 --> 00:12:08,000 So what he discovered here was a correlation. He hadn't proven 147 00:12:08,000 --> 00:12:12,000 anything. He just saw that if you watch sugar go to ethanol there were 148 00:12:12,000 --> 00:12:16,000 yeast around, if you had more yeast it went faster, 149 00:12:16,000 --> 00:12:21,000 and when you ran out of sugar the yeast stopped growing. 150 00:12:21,000 --> 00:12:25,000 There was something connected here. So he came up with the idea that 151 00:12:25,000 --> 00:12:29,000 the yeast were responsible for this conversion that was happening 152 00:12:29,000 --> 00:12:33,000 when you made wine. And it was further helped out in 153 00:12:33,000 --> 00:12:37,000 this because he discovered an alternative -- 154 00:12:37,000 --> 00:12:48,000 -- conversion in which C6H12O6 went 155 00:12:48,000 --> 00:12:56,000 instead to give two molecules of CH3CHOH. This molecule which you 156 00:12:56,000 --> 00:13:05,000 know, galactic acid, it too has C6H12O6 on both sides of 157 00:13:05,000 --> 00:13:14,000 the equation but it's a different molecule. 158 00:13:14,000 --> 00:13:18,000 And what he found was that this is the lactic acid you know as what's 159 00:13:18,000 --> 00:13:22,000 in yogurt. It makes yogurt sour. Or if you exercise really hard and 160 00:13:22,000 --> 00:13:26,000 your muscles are sore that's because you accumulate lactic acid in your 161 00:13:26,000 --> 00:13:31,000 muscles, and I'll tell you why that is in the next lecture. 162 00:13:31,000 --> 00:13:35,000 But what the other thing that Pasteur realized was when you got 163 00:13:35,000 --> 00:13:39,000 this alternative conversion you didn't have yeast present, 164 00:13:39,000 --> 00:13:43,000 you had some other organism. And so that was a huge advance just 165 00:13:43,000 --> 00:13:47,000 of practical value to the winemakers because they knew they had to have 166 00:13:47,000 --> 00:13:51,000 yeast in there to get wine and there problems were coming when some other 167 00:13:51,000 --> 00:13:55,000 organism that wasn't yeast got in there and it did something different 168 00:13:55,000 --> 00:13:59,000 with the sugar and made it into lactic acid instead of making it 169 00:13:59,000 --> 00:14:04,000 into ethanol and carbon dioxide. So there was Pasteur working away on 170 00:14:04,000 --> 00:14:10,000 a practical problem and it was, you know, a really major advance to 171 00:14:10,000 --> 00:14:16,000 the winemaking industry for him to do this, but it also then sort of 172 00:14:16,000 --> 00:14:22,000 unexpectedly led to another issue. And that was why were the yeast 173 00:14:22,000 --> 00:14:28,000 doing this? Because one of the things that Lavoisier had noticed 174 00:14:28,000 --> 00:14:34,000 and Pasteur noticed was that you did this conversion. 175 00:14:34,000 --> 00:14:40,000 The two ethanol plus two carbon 176 00:14:40,000 --> 00:14:45,000 dioxide. But you could account for virtually all of the carbon and 177 00:14:45,000 --> 00:14:51,000 hydrogens and oxygens that started out as sugar and seemed like 178 00:14:51,000 --> 00:14:56,000 virtually of them showed up in the ethanol and the carbon dioxide. 179 00:14:56,000 --> 00:15:01,000 So why was the yeast doing this? And the idea began to develop out of 180 00:15:01,000 --> 00:15:05,000 that was that rather than being used to make biomass, 181 00:15:05,000 --> 00:15:10,000 in which case you would have expected to see a whole lot of mass 182 00:15:10,000 --> 00:15:14,000 in the yeast cells and no so much up here, that instead most of this 183 00:15:14,000 --> 00:15:19,000 sugar was being used to make energy and that somehow the cell was 184 00:15:19,000 --> 00:15:23,000 getting the energy necessary to all that synthetic work involved in cell 185 00:15:23,000 --> 00:15:28,000 division by carrying out this conversion. 186 00:15:28,000 --> 00:15:34,000 And there's a fundamental relationship then between chemical 187 00:15:34,000 --> 00:15:40,000 energy and whether a reaction can proceed. And I'll just take it 188 00:15:40,000 --> 00:15:47,000 through in sort of your typical introductory chemistry reaction, 189 00:15:47,000 --> 00:15:53,000 A plus B going to C plus D. You know, there are certain classes of 190 00:15:53,000 --> 00:16:00,000 reactions that will go almost to completion. 191 00:16:00,000 --> 00:16:04,000 Probably an overstatement to say it's to go to completion, 192 00:16:04,000 --> 00:16:08,000 but it's effectively over here. Those are termed irreversible 193 00:16:08,000 --> 00:16:12,000 reactions, and there are certainly some of them. If I have hydrogen 194 00:16:12,000 --> 00:16:17,000 and oxygen and I light a little match, you pretty much go all the 195 00:16:17,000 --> 00:16:21,000 way to making water with a great big boom and no hydrogen or not much 196 00:16:21,000 --> 00:16:25,000 hydrogen and oxygen left on the other side. However, 197 00:16:25,000 --> 00:16:30,000 most reactions that one finds in nature don't have that quality. 198 00:16:30,000 --> 00:16:36,000 Instead they are going forward at some rate and back at another. 199 00:16:36,000 --> 00:16:42,000 And they reach eventually an equilibrium that's characterized by 200 00:16:42,000 --> 00:16:48,000 what's known as an equilibrium constant which is the product of the 201 00:16:48,000 --> 00:16:54,000 concentrations of the products over the product of the concentration of 202 00:16:54,000 --> 00:16:59,000 the reactants. And that's a characteristic of every 203 00:16:59,000 --> 00:17:03,000 particular chemical reaction. And we really have to worry about 204 00:17:03,000 --> 00:17:07,000 this in biology because if everything was irreversible that 205 00:17:07,000 --> 00:17:11,000 would be fine, but in order to do all this 206 00:17:11,000 --> 00:17:16,000 synthetic work you have to deal with a lot of reactions that aren't going 207 00:17:16,000 --> 00:17:20,000 to go to completion. And nature has had to figure out a 208 00:17:20,000 --> 00:17:24,000 way of doing that, just the same way that bridges and 209 00:17:24,000 --> 00:17:28,000 buildings don't spontaneously assemble and engineers and others 210 00:17:28,000 --> 00:17:33,000 have had to work out ways of putting all of those things together. 211 00:17:33,000 --> 00:17:39,000 So at some level you see the same kind of problem. 212 00:17:39,000 --> 00:17:45,000 Now, there's a way of expressing this energy associated with a 213 00:17:45,000 --> 00:17:51,000 chemical reaction that can be used to directly calculate whether a 214 00:17:51,000 --> 00:17:57,000 reaction is going to go and how far it will go. And a person who did 215 00:17:57,000 --> 00:18:03,000 this work is another person who's on one of the MIT buildings. 216 00:18:03,000 --> 00:18:09,000 It was [Willard? Gibbs who was a faculty member 217 00:18:09,000 --> 00:18:15,000 chemist who worked at Yale in the 1980s, excuse me, 218 00:18:15,000 --> 00:18:21,000 1800s, and he came up with an expression that's now known as 219 00:18:21,000 --> 00:18:27,000 ìGibbs free energyî. And what's important about this way 220 00:18:27,000 --> 00:18:33,000 of talking about the energy change associated with the chemical 221 00:18:33,000 --> 00:18:39,000 reaction is it considers not only the internal energy of the system 222 00:18:39,000 --> 00:18:44,000 but also the change in disorder. Or another way of saying that, 223 00:18:44,000 --> 00:18:48,000 for those of you who've run into the laws of thermodynamics, 224 00:18:48,000 --> 00:18:52,000 it combines the first and second laws of thermodynamics. 225 00:18:52,000 --> 00:18:56,000 And you have to consider both of those if you're going to consider 226 00:18:56,000 --> 00:19:01,000 whether a reaction will go. And you cannot measure an absolute 227 00:19:01,000 --> 00:19:07,000 free energy but you can measure a change. And this is the equation. 228 00:19:07,000 --> 00:19:13,000 It's the change associated with a chemical reaction is equal to the 229 00:19:13,000 --> 00:19:19,000 change associated with the chemical reaction under some set of standard 230 00:19:19,000 --> 00:19:25,000 conditions times RT times the log of the concentration of the products 231 00:19:25,000 --> 00:19:32,000 multiplied together over the concentration of the reactants. 232 00:19:32,000 --> 00:19:38,000 So if we could just go to the same example we were just thinking about, 233 00:19:38,000 --> 00:19:45,000 the energy change with that reaction that we were considering 234 00:19:45,000 --> 00:19:54,000 would have been this. 235 00:19:54,000 --> 00:20:07,000 So this is the energy change -- 236 00:20:07,000 --> 00:20:10,000 -- associated with the concentrations -- 237 00:20:10,000 --> 00:20:19,000 -- the reactants and products that 238 00:20:19,000 --> 00:20:27,000 we're considering. 239 00:20:27,000 --> 00:20:32,000 This is the energy change under standard, or the term standard 240 00:20:32,000 --> 00:20:38,000 conditions where everything, each reactant, each product is 241 00:20:38,000 --> 00:20:44,000 present under one molar concentrations. 242 00:20:44,000 --> 00:20:50,000 So not something you'd ever find in most cases, but it's a frame of 243 00:20:50,000 --> 00:20:56,000 reference. And then this is the universal gas constant -- 244 00:20:56,000 --> 00:21:04,000 -- which is two times ten to the 245 00:21:04,000 --> 00:21:11,000 minus third kilocalories per mole per degree Calvin, 246 00:21:11,000 --> 00:21:17,000 the temperature in absolute. This is the temperature in degrees 247 00:21:17,000 --> 00:21:24,000 Calvin. And the temperature for most biology, most life is around 25 248 00:21:24,000 --> 00:21:31,000 degrees Centigrade, so that's equal to 298 degrees 249 00:21:31,000 --> 00:21:38,000 Calvin, which is about equal to 300 degrees Calvin. So for most -- 250 00:21:38,000 --> 00:21:43,000 And since the range in which life can occur on an absolute temperature 251 00:21:43,000 --> 00:21:48,000 scale is really pretty small, it sort of fluctuates in only very 252 00:21:48,000 --> 00:21:53,000 minor ways around 25 degrees Centigrade, then for most of the 253 00:21:53,000 --> 00:21:59,000 biological reactions we'll be thinking about this RT number is 254 00:21:59,000 --> 00:22:08,000 about 0.6 kilocalories per mole. 255 00:22:08,000 --> 00:22:15,000 Now, biochemists actually have a special form of free energy they use, 256 00:22:15,000 --> 00:22:23,000 which we put a delta G prime. And in this case the delta G prime 257 00:22:23,000 --> 00:22:31,000 is equal to delta G prime under a set of standard conditions plus RT 258 00:22:31,000 --> 00:22:39,000 natural log of C products over the reactants. 259 00:22:39,000 --> 00:22:46,000 But the assumption is made that the reaction is in water which, 260 00:22:46,000 --> 00:22:54,000 I mentioned the other day, is 55 molar. Yeah? 261 00:22:54,000 --> 00:23:03,000 This is the degree Celsius. 262 00:23:03,000 --> 00:23:08,000 I've just expressed it in degrees Calvin. 263 00:23:08,000 --> 00:23:21,000 Sorry. My mistake. 264 00:23:21,000 --> 00:23:27,000 Excuse me. Because I was wrong is why. OK. Thanks for catching 265 00:23:27,000 --> 00:23:32,000 that. All right. So water is very concentrated. 266 00:23:32,000 --> 00:23:36,000 And so under these conditions the other convention is then you can set 267 00:23:36,000 --> 00:23:41,000 the hydrogen ions and water molecules to one. 268 00:23:41,000 --> 00:23:46,000 And you don't have to think about them when we're doing this. 269 00:23:46,000 --> 00:23:50,000 This is a convention that biochemists do. 270 00:23:50,000 --> 00:23:55,000 Now, this free energy, the delta G that gives free energy 271 00:23:55,000 --> 00:24:06,000 is a thermodynamic -- 272 00:24:06,000 --> 00:24:12,000 -- property. And I'll just share with you the same visual image I've 273 00:24:12,000 --> 00:24:18,000 had since I was an undergrad, which I think is not a bad way of 274 00:24:18,000 --> 00:24:24,000 thinking about it trying to understand what happens, 275 00:24:24,000 --> 00:24:30,000 that if we have a plot of the free energy as a function of what happens 276 00:24:30,000 --> 00:24:36,000 as the reaction goes along so that we have A plus B here and 277 00:24:36,000 --> 00:24:42,000 C plus D down here. When you go from reaction to 278 00:24:42,000 --> 00:24:46,000 products, the way I've drawn it, some kind of energy is given off in 279 00:24:46,000 --> 00:24:50,000 this kind of reaction. And if you know that you will know 280 00:24:50,000 --> 00:24:54,000 then that the reaction will be able to go forward because it's able to 281 00:24:54,000 --> 00:24:58,000 give off energy just the same way hydrogen and oxygen give off a lot 282 00:24:58,000 --> 00:25:02,000 of heat and stuff, and you know that reaction really 283 00:25:02,000 --> 00:25:06,000 goes a long way to completion. So it's kind of as if you were out 284 00:25:06,000 --> 00:25:11,000 here on your spring break on your skis already to go down the black 285 00:25:11,000 --> 00:25:16,000 diamond hill, you know, you can sort of see what would 286 00:25:16,000 --> 00:25:20,000 happen. Now, because it's a thermodynamic property it doesn't 287 00:25:20,000 --> 00:25:25,000 matter what route you take to get from the reactions to the products. 288 00:25:25,000 --> 00:25:30,000 So if you go down the double diamond slope or you go down the 289 00:25:30,000 --> 00:25:35,000 bunny slope you still end up with the same amount of energy coming out 290 00:25:35,000 --> 00:25:40,000 of the reaction. And that's important because if that 291 00:25:40,000 --> 00:25:46,000 wasn't true you could make a perpetual motion machine and you'd 292 00:25:46,000 --> 00:25:51,000 be very rich. The second thing that's important is that the free 293 00:25:51,000 --> 00:25:57,000 energy will tell you what would happen if the reaction went but it 294 00:25:57,000 --> 00:26:03,000 will not tell you whether it can go. 295 00:26:03,000 --> 00:26:07,000 If I did a demo here and I brought some hydrogen and some oxygen and I 296 00:26:07,000 --> 00:26:11,000 mixed them together in a vessel in the front of the class we could all 297 00:26:11,000 --> 00:26:15,000 sit here waiting for it to explode. But the likelihood is we would sit 298 00:26:15,000 --> 00:26:19,000 here for a very, very long time and not see an 299 00:26:19,000 --> 00:26:23,000 explosion, right? And the reason is that in order to 300 00:26:23,000 --> 00:26:27,000 get that hydrogen and oxygen close enough together we had to give them 301 00:26:27,000 --> 00:26:32,000 some extra energy and push them so they overcome repulsion and stuff. 302 00:26:32,000 --> 00:26:36,000 So if you were out here on your skis again getting already to go, 303 00:26:36,000 --> 00:26:41,000 but in fact you got off at the wrong stop on the ski lift and you were 304 00:26:41,000 --> 00:26:46,000 there, even though there would be energy getting down from here it's 305 00:26:46,000 --> 00:26:51,000 not going to happen at any discernable rate given the sort of 306 00:26:51,000 --> 00:26:56,000 little bounce in energy you have in your normal lives. 307 00:26:56,000 --> 00:26:59,000 So what we're doing when we do hydrogen and oxygen is by putting a 308 00:26:59,000 --> 00:27:03,000 match into it or something we're giving it enough energy that 309 00:27:03,000 --> 00:27:06,000 actually a few of the molecules get up here, they drop down, 310 00:27:06,000 --> 00:27:10,000 then they give up so much energy and heat that all the rest of them get 311 00:27:10,000 --> 00:27:14,000 pushed up and the thing goes. But that's sort of not a bad way of 312 00:27:14,000 --> 00:27:17,000 thinking about it. And we're going to talk in a minute 313 00:27:17,000 --> 00:27:21,000 about what determines how fast reactions go, not whether they go or 314 00:27:21,000 --> 00:27:25,000 not. And then, of course, at that point we're going 315 00:27:25,000 --> 00:27:29,000 to have to worry about this issue. But before that what I want to show 316 00:27:29,000 --> 00:27:35,000 you is that there's a direct relationship between this Gibbs free 317 00:27:35,000 --> 00:27:41,000 energy and the equilibrium constant. So we have this, well, what we 318 00:27:41,000 --> 00:27:46,000 could do is you have the reaction over there. So let's consider that 319 00:27:46,000 --> 00:27:52,000 reaction has come to equilibrium. And that means there'll be no 320 00:27:52,000 --> 00:27:58,000 further energy change. So we'll just set the delta G to 321 00:27:58,000 --> 00:28:04,000 zero. And that would mean then that delta 322 00:28:04,000 --> 00:28:10,000 G prime zero is equal to minus RT concentration C over D over 323 00:28:10,000 --> 00:28:17,000 concentration of A over B. You'll recognize this. That's the 324 00:28:17,000 --> 00:28:23,000 equilibrium constant, right? I'm sorry. There's a 325 00:28:23,000 --> 00:28:30,000 natural log in here. I didn't get it in. OK? 326 00:28:30,000 --> 00:28:37,000 So which is equal to minus RT the natural log of the equilibrium 327 00:28:37,000 --> 00:28:44,000 constant or the natural log of the equilibrium constant is equal to 328 00:28:44,000 --> 00:28:51,000 minus delta G prime zero over RT. Or another way of saying that is 329 00:28:51,000 --> 00:28:58,000 the K equilibrium is equal E to the minus delta G prime zero over RT. 330 00:28:58,000 --> 00:29:02,000 So if you think back to consequences of an equilibrium constant, 331 00:29:02,000 --> 00:29:07,000 if the reaction is going to go almost all the way then there are 332 00:29:07,000 --> 00:29:12,000 going to be mostly products, very few reactions, so the K 333 00:29:12,000 --> 00:29:17,000 equilibrium will be large. So if a reaction is going to go a 334 00:29:17,000 --> 00:29:22,000 long way then the equilibrium constant will be large. 335 00:29:22,000 --> 00:29:27,000 And in order for an equilibrium constant to be large then this delta 336 00:29:27,000 --> 00:29:32,000 G is going to have to have a large negative sign. So if 337 00:29:32,000 --> 00:29:44,000 the reaction -- 338 00:29:44,000 --> 00:29:52,000 -- is favorable then K equilibrium will be large and the delta G prime 339 00:29:52,000 --> 00:30:00,000 zero will have, at least within the scale of an 340 00:30:00,000 --> 00:30:08,000 activation energy, a large negative value. 341 00:30:08,000 --> 00:30:12,000 And let me give you a couple of examples. When we talked about 342 00:30:12,000 --> 00:30:16,000 carbohydrates, I briefly told you sucrose was what 343 00:30:16,000 --> 00:30:20,000 we call a disaccharide, two sugars joined together. 344 00:30:20,000 --> 00:30:24,000 What do we do when we join two things together pretty much usually 345 00:30:24,000 --> 00:30:28,000 in nature? You split out a molecule of water. 346 00:30:28,000 --> 00:30:33,000 So we take a molecule of glucose, a molecule of fructose, both 347 00:30:33,000 --> 00:30:38,000 carbohydrates, stick them together and we get table 348 00:30:38,000 --> 00:30:43,000 sugar. If we want to reverse that reaction we have to put in a 349 00:30:43,000 --> 00:30:48,000 molecule of water and we can run it the other way. 350 00:30:48,000 --> 00:30:53,000 We get glucose plus fructose. The K equilibrium for that reaction 351 00:30:53,000 --> 00:30:59,000 is 140,000. The delta G prime zero is minus 352 00:30:59,000 --> 00:31:05,000 seven kilocalories per mole. So that's an example of what I was 353 00:31:05,000 --> 00:31:12,000 just telling you, a fairly large negative value. 354 00:31:12,000 --> 00:31:19,000 If we think about a reaction that's not favorable, 355 00:31:19,000 --> 00:31:26,000 here's acidic acid. That's what makes vinegar sour. 356 00:31:26,000 --> 00:31:33,000 And the hydrogen ion can come off here to give you a hydrogen ion and 357 00:31:33,000 --> 00:31:41,000 the negative ion of acidic acid or acetate ion. The equilibrium 358 00:31:41,000 --> 00:31:48,000 constant for that one is, what is it, I think two times ten to 359 00:31:48,000 --> 00:31:56,000 the minus five. So only a little tiny bit of the 360 00:31:56,000 --> 00:32:03,000 acidic acid actually ionizes. And the K equilibrium constant then, 361 00:32:03,000 --> 00:32:09,000 excuse me, the delta G prime zero is plus 6.3 kilocalories per mole. 362 00:32:09,000 --> 00:32:16,000 So buried in this example is not showing you that a reaction that's 363 00:32:16,000 --> 00:32:23,000 unfavorable will have a positive free energy associated with it, 364 00:32:23,000 --> 00:32:30,000 whereas one that's favorable will have a negative free energy. 365 00:32:30,000 --> 00:32:35,000 This is also sort of telling you why you don't die when you put salad 366 00:32:35,000 --> 00:32:40,000 dressing on your salad, because if acidic acid ionized as 367 00:32:40,000 --> 00:32:45,000 thoroughly as sulfuric acid and you put an equivalent amount of sulfuric 368 00:32:45,000 --> 00:32:50,000 acid on our salads none of us would be here. It's only a little tiny 369 00:32:50,000 --> 00:32:56,000 bit that's going, and so that's what's happening. 370 00:32:56,000 --> 00:33:00,000 So what this really sets us up for is this fundamental problem in 371 00:33:00,000 --> 00:33:04,000 biology, and that is that this reaction here, 372 00:33:04,000 --> 00:33:09,000 you can see what it would go, this one doesn't go, but most of the 373 00:33:09,000 --> 00:33:13,000 reactions that you have to carry out in biology demand an energy input 374 00:33:13,000 --> 00:33:17,000 because they just won't go. We could sort of force this a 375 00:33:17,000 --> 00:33:22,000 little bit. We could raise the concentration of the reactions and 376 00:33:22,000 --> 00:33:26,000 it would give us a little bit more product, but that's not a useful 377 00:33:26,000 --> 00:33:32,000 solution to all the things. So this was a really fundamental 378 00:33:32,000 --> 00:33:38,000 problem that had to be solved in evolution in order for life to ever 379 00:33:38,000 --> 00:33:45,000 exist. And I'll give you just an example. If we consider taking a 380 00:33:45,000 --> 00:33:51,000 couple of molecules of glutamate, which is one of the amino acids we 381 00:33:51,000 --> 00:33:57,000 talked about, a couple of molecules of amino and making it into a couple 382 00:33:57,000 --> 00:34:02,000 of molecules of glutamine. Now, this is an amino acid needed 383 00:34:02,000 --> 00:34:05,000 for making proteins. This is an amino acid needed for 384 00:34:05,000 --> 00:34:09,000 making proteins. The cell has to have both of them. 385 00:34:09,000 --> 00:34:19,000 Glutamate has two methylene groups 386 00:34:19,000 --> 00:34:25,000 and then are carboxyl group that's one of the acid amino acids. 387 00:34:25,000 --> 00:34:34,000 And glutamine the side chain -- 388 00:34:34,000 --> 00:34:39,000 -- is now amid. The delta G from zero associated 389 00:34:39,000 --> 00:34:44,000 with this reaction is plus seven kilocalories per mole, 390 00:34:44,000 --> 00:34:49,000 so it's as unfavorable almost as that one we're looking at. 391 00:34:49,000 --> 00:34:54,000 In fact, it's worse than the one we're looking at over there. 392 00:34:54,000 --> 00:34:59,000 The reason that this is sort of pushing the thing uphill 393 00:34:59,000 --> 00:35:04,000 energetically is that the electrons here actually distribute themselves 394 00:35:04,000 --> 00:35:09,000 back and forth. So you can kind of think of the 395 00:35:09,000 --> 00:35:13,000 molecule as going back and forth between these two forms. 396 00:35:13,000 --> 00:35:17,000 And that makes it more stable. And when you stick on the amine 397 00:35:17,000 --> 00:35:22,000 group to make the amid it cannot do that, and so you're actually pushing 398 00:35:22,000 --> 00:35:26,000 everything energetically uphill. So how does a cell accomplish this? 399 00:35:26,000 --> 00:35:32,000 There's energy available. If we consider what happens with 400 00:35:32,000 --> 00:35:40,000 C6H12O6 going to two lactate the delta G prime zero associated with 401 00:35:40,000 --> 00:35:48,000 that is minus 50 kilocalories per mole. So the cell has got a lot of 402 00:35:48,000 --> 00:35:56,000 energy out of making even that simple conversation of a sugar 403 00:35:56,000 --> 00:36:03,000 molecule into two lactate. But it somehow has to figure out how 404 00:36:03,000 --> 00:36:09,000 to use that energy in order to drive these unfavorable reactions. 405 00:36:09,000 --> 00:36:16,000 And the solution, which is really one of the secrets to life, 406 00:36:16,000 --> 00:36:27,000 is to use coupled reactions -- 407 00:36:27,000 --> 00:36:34,000 -- with a common intermediate. 408 00:36:34,000 --> 00:36:38,000 And if you look outside a cell, as Lavoisier did or Pasteur did, 409 00:36:38,000 --> 00:36:43,000 this is what you'd see. But if you could look inside the cell and see 410 00:36:43,000 --> 00:36:48,000 what's happening when that conversion is being made you'd 411 00:36:48,000 --> 00:36:53,000 discover that the full reaction looks like this. 412 00:36:53,000 --> 00:36:58,000 It's the sugar molecule plus two molecules of ADP plus two molecules 413 00:36:58,000 --> 00:37:03,000 of inorganic phosphate are going to give two molecules of lactate plus 414 00:37:03,000 --> 00:37:10,000 two molecules of ATP. What's ATP? It's a ribonucleotide. 415 00:37:10,000 --> 00:37:29,000 That's ADP. And what happens when 416 00:37:29,000 --> 00:37:34,000 you make ATP is an extra phosphate gets added onto that end 417 00:37:34,000 --> 00:37:39,000 of the molecule. So by having yet another phosphate 418 00:37:39,000 --> 00:37:43,000 on here you've got a whole role of negative charges. 419 00:37:43,000 --> 00:37:47,000 This is a molecule in which the various parts are not happy to be 420 00:37:47,000 --> 00:37:51,000 together because all these negative charges would like to push apart so 421 00:37:51,000 --> 00:37:55,000 when you break the bond of ATP then energy is released. 422 00:37:55,000 --> 00:37:59,000 So using ATP is a way of sort of storing chemical energy so you can 423 00:37:59,000 --> 00:38:03,000 use it in some other kind of context. 424 00:38:03,000 --> 00:38:09,000 And so by burning it, by carrying out the reaction in this 425 00:38:09,000 --> 00:38:16,000 way a cell is able to not only make a molecule of sugar, 426 00:38:16,000 --> 00:38:22,000 glucose into two lactate, it's able to generate ATP along the 427 00:38:22,000 --> 00:38:29,000 way. And the delta G prime zero for this reaction is minus 34 428 00:38:29,000 --> 00:38:35,000 kilocalories per mole. So even though it's taking out some 429 00:38:35,000 --> 00:38:41,000 of that energy and putting it in ATP, this is a reaction that goes very, 430 00:38:41,000 --> 00:38:47,000 very efficiently. Then instead of trying to carry out just that 431 00:38:47,000 --> 00:38:53,000 reaction, what the cell is actually doing is taking the two glutamate 432 00:38:53,000 --> 00:38:59,000 plus the two molecules of ammonia plus two ATP. 433 00:38:59,000 --> 00:39:05,000 And then this is converting it to two glutamine plus two water. 434 00:39:05,000 --> 00:39:12,000 I think I failed to put that in here so you can correct it back 435 00:39:12,000 --> 00:39:19,000 there. Plus two ADP plus two molecules of inorganic phosphate. 436 00:39:19,000 --> 00:39:26,000 And so the Pi very commonly used in biochemistry to denote just 437 00:39:26,000 --> 00:39:33,000 inorganic phosphate ion. So what's happen here then are these 438 00:39:33,000 --> 00:39:40,000 two reactions going on. This reaction now, because ATP is 439 00:39:40,000 --> 00:39:47,000 involved, is now favorable, and the delta G for this reaction is 440 00:39:47,000 --> 00:39:54,000 minus nine kilocalories per mole. So by having an ATP hydrolyzed as 441 00:39:54,000 --> 00:40:01,000 part of the reaction mechanism, this reaction that used to be 442 00:40:01,000 --> 00:40:09,000 unfavorable is now favorable. And then the kind of cute thing then 443 00:40:09,000 --> 00:40:17,000 is if you sum this all up, the ATPs and the ADPs are on both 444 00:40:17,000 --> 00:40:25,000 sides of the equation so they just drop out. And what you're left with 445 00:40:25,000 --> 00:40:33,000 is C6H12O6 plus the two glutamines plus two ammonias going to give two 446 00:40:33,000 --> 00:40:41,000 glutamines, excuse me, two lactate plus two glutamines plus 447 00:40:41,000 --> 00:40:49,000 the two waters. And the delta G prime zero for this 448 00:40:49,000 --> 00:40:57,000 is minus 43 kilocalories per mole. So this is not, you can think of it 449 00:40:57,000 --> 00:41:05,000 as using energy in the form of ATP like this a little the way we use 450 00:41:05,000 --> 00:41:11,000 money in our society. I do some work at MIT. 451 00:41:11,000 --> 00:41:15,000 I don't get given food to eat or TV to watch the Super Bowl. 452 00:41:15,000 --> 00:41:20,000 Instead I get given money, then I go to the store, I give them 453 00:41:20,000 --> 00:41:25,000 the money, I end up with the food or the stuff. And if you're watching 454 00:41:25,000 --> 00:41:29,000 it from the outside you see me do work at school and then food, 455 00:41:29,000 --> 00:41:34,000 TV or whatever shows up at home. But what's happening is the money is 456 00:41:34,000 --> 00:41:39,000 serving as a common intermediate in those transactions. 457 00:41:39,000 --> 00:41:44,000 And that's what basically ATP is in the cell. It's energy money. 458 00:41:44,000 --> 00:41:49,000 And in making ATP the cell has to take this ribose with an adenine on 459 00:41:49,000 --> 00:41:54,000 it, I think I didn't put the adenine on here I realize. 460 00:41:54,000 --> 00:42:00,000 The adenine is sitting on the ribose now. 461 00:42:00,000 --> 00:42:03,000 There are two phosphates, both of which have a negative charge 462 00:42:03,000 --> 00:42:07,000 on them. And to create that third bond it has to push it together. 463 00:42:07,000 --> 00:42:11,000 It's a very sort of an intrinsically unstable molecule. 464 00:42:11,000 --> 00:42:14,000 When you break the bond it will give you energy back. 465 00:42:14,000 --> 00:42:18,000 And that's one of the really amazing secretes to life, 466 00:42:18,000 --> 00:42:22,000 and that's the underlying principal of why it is that life can go 467 00:42:22,000 --> 00:42:26,000 forward. Now, the second issue that we need to 468 00:42:26,000 --> 00:42:37,000 quickly address here is -- 469 00:42:37,000 --> 00:42:41,000 -- not only can a reaction go, which is what thermodynamics tells 470 00:42:41,000 --> 00:42:46,000 us, but how can fast can it go. And this epitomizes the problem 471 00:42:46,000 --> 00:42:50,000 that all chemical reactions face because literally every chemical 472 00:42:50,000 --> 00:42:55,000 reaction that you carry out involves bringing a couple of 473 00:42:55,000 --> 00:43:00,000 entities together. And as they get closer and closer 474 00:43:00,000 --> 00:43:05,000 and closer they don't want to be there so you have to sort of push 475 00:43:05,000 --> 00:43:11,000 them together in some kind of way or make sure they have enough energy to 476 00:43:11,000 --> 00:43:16,000 get together. And that's what we see represented here. 477 00:43:16,000 --> 00:43:21,000 And that's a special term called the activation energy. 478 00:43:21,000 --> 00:43:27,000 It's given the term delta G with a double-dagger. And that is what -- 479 00:43:27,000 --> 00:43:31,000 It's the size of that activation energy that limits how fast chemical 480 00:43:31,000 --> 00:43:35,000 reactions can go. So the solution you use in 481 00:43:35,000 --> 00:43:39,000 chemistry, most of you, is you use a catalyst. And the 482 00:43:39,000 --> 00:43:43,000 catalyst doesn't change the outcome of the reaction. 483 00:43:43,000 --> 00:43:47,000 It just changes how fast you get there. So there are many reactions 484 00:43:47,000 --> 00:43:51,000 you've heard about in chemistry. Just stick the thing at 500 degrees 485 00:43:51,000 --> 00:43:55,000 centigrade, put in a piece of platinum, and now the reaction will 486 00:43:55,000 --> 00:43:58,000 go a whole lot faster. By heating it up molecules have more 487 00:43:58,000 --> 00:44:02,000 energy. So if they have more energy they can get closer together just 488 00:44:02,000 --> 00:44:06,000 from that. And then what the platinum surface would do is allow 489 00:44:06,000 --> 00:44:10,000 the molecules to both stick and that would bring them in proximately and 490 00:44:10,000 --> 00:44:14,000 also help them come together. Well, you cannot raise the 491 00:44:14,000 --> 00:44:18,000 temperature in a biological system, but still you have to overcome this. 492 00:44:18,000 --> 00:44:22,000 But the principal then, what you have to do when you carry out a 493 00:44:22,000 --> 00:44:26,000 catalyst, what any catalyst would do is that it lowers this 494 00:44:26,000 --> 00:44:41,000 activation energy. 495 00:44:41,000 --> 00:44:45,000 And if you lower the activation energy then enough of the molecules, 496 00:44:45,000 --> 00:44:49,000 just at whatever condition they're in will have enough energy to be 497 00:44:49,000 --> 00:44:53,000 able to go. It won't change the size of the drop. 498 00:44:53,000 --> 00:44:57,000 It just changes how fast you reach that final equilibrium. 499 00:44:57,000 --> 00:45:01,000 And there are two forms of biological -- 500 00:45:01,000 --> 00:45:06,000 Two molecules that are biological catalysts. 501 00:45:06,000 --> 00:45:16,000 One of the molecules you know is 502 00:45:16,000 --> 00:45:23,000 enzymes. Enzymes are made of a protein. We spent a bunch of time 503 00:45:23,000 --> 00:45:29,000 working at that. One of the things I showed you the 504 00:45:29,000 --> 00:45:33,000 very first day, this is a thing made by the anthrax 505 00:45:33,000 --> 00:45:37,000 bacterium, anthrax lethal factor. What it actually is, it's a protein 506 00:45:37,000 --> 00:45:42,000 and it's an enzyme that's able to catalyze the cleavage of certain 507 00:45:42,000 --> 00:45:46,000 peptide bonds in proteins in our body. And in particular it goes 508 00:45:46,000 --> 00:45:51,000 after molecules that are involved in signaling processes inside of cells. 509 00:45:51,000 --> 00:45:55,000 And if we don't have those then we die. More recently it was 510 00:45:55,000 --> 00:46:00,000 discovered that RNA can be a catalyst. 511 00:46:00,000 --> 00:46:04,000 And these are called, if you have an RNA that's a catalyst 512 00:46:04,000 --> 00:46:09,000 it's called a ribozyme. And these seemed pretty exotic for 513 00:46:09,000 --> 00:46:13,000 a little while they first discovered the idea that a piece of RNA could 514 00:46:13,000 --> 00:46:18,000 serve as a catalyst in a biological system, but it eventually turned out 515 00:46:18,000 --> 00:46:22,000 that the ribosome, which we'll talk about in some 516 00:46:22,000 --> 00:46:27,000 detail which is the protein synthesizing machinery that creates 517 00:46:27,000 --> 00:46:31,000 those peptide bonds between each of the amino acids to make 518 00:46:31,000 --> 00:46:36,000 the proteins. It's a big conglomeration of RNA 519 00:46:36,000 --> 00:46:40,000 shown in gray and a bunch of different proteins that are shown in 520 00:46:40,000 --> 00:46:45,000 yellow, but the actual formation of the peptide bond, 521 00:46:45,000 --> 00:46:49,000 the thing that makes all proteins is actually catalyzed by a piece of RNA. 522 00:46:49,000 --> 00:46:54,000 And so the ribosome is actually a ribozyme. And it's ironic that that 523 00:46:54,000 --> 00:46:58,000 sense that a piece of RNA is catalyzing the bond that makes 524 00:46:58,000 --> 00:47:03,000 proteins possible. So we'll finish this up and get in 525 00:47:03,000 --> 00:47:07,000 then to glycolysis which is the most evolutionary ancient of these 526 00:47:07,000 --> 00:47:10,000 energy-producing systems on Monday. OK?