1 00:00:00,000 --> 00:00:01,924 [SQUEAKING] 2 00:00:01,924 --> 00:00:04,329 [RUSTLING] 3 00:00:04,329 --> 00:00:06,253 [CLICKING] 4 00:00:10,590 --> 00:00:11,280 PROFESSOR: OK. 5 00:00:11,280 --> 00:00:11,780 Great. 6 00:00:11,780 --> 00:00:13,570 Now let's dive into the material today. 7 00:00:13,570 --> 00:00:16,770 So today's topic is carbohydrates as well as 8 00:00:16,770 --> 00:00:19,680 an introduction to membrane structure. 9 00:00:19,680 --> 00:00:21,810 And from the very first class, I believe 10 00:00:21,810 --> 00:00:25,770 Professor Yaffe talked to you about the four main classes 11 00:00:25,770 --> 00:00:27,940 of biological molecules. 12 00:00:27,940 --> 00:00:30,000 So proteins and amino acids, you've 13 00:00:30,000 --> 00:00:32,070 spent a lot of time talking about that. 14 00:00:32,070 --> 00:00:33,750 Nucleic acids is coming up. 15 00:00:33,750 --> 00:00:35,310 And the other two are carbohydrates 16 00:00:35,310 --> 00:00:37,920 or sugars and lipids. 17 00:00:37,920 --> 00:00:40,710 Today, we're really going to focus most of the lecture 18 00:00:40,710 --> 00:00:44,940 on carbohydrates, what they are, structure, 19 00:00:44,940 --> 00:00:46,692 something about nomenclature. 20 00:00:46,692 --> 00:00:48,900 And then at the end, we're going to talk a little bit 21 00:00:48,900 --> 00:00:52,140 about lipids and basic membrane structure. 22 00:00:52,140 --> 00:00:57,330 Now carbohydrates and lipids are really critical energy storage 23 00:00:57,330 --> 00:00:58,840 molecules for cells. 24 00:00:58,840 --> 00:01:01,650 And when we talk about metabolism-- 25 00:01:01,650 --> 00:01:03,630 the most interesting part of this course-- 26 00:01:03,630 --> 00:01:06,300 after spring break, we're going to delve 27 00:01:06,300 --> 00:01:09,930 into this in a lot more detail. 28 00:01:09,930 --> 00:01:12,950 But sugars are also very important to understand 29 00:01:12,950 --> 00:01:14,640 nucleic acid structure. 30 00:01:14,640 --> 00:01:17,870 Turns out, membranes are essential for signal 31 00:01:17,870 --> 00:01:19,040 transduction. 32 00:01:19,040 --> 00:01:21,770 Those are the two major topics that Professor Yaffe 33 00:01:21,770 --> 00:01:25,110 is going to talk about for the rest of his time. 34 00:01:25,110 --> 00:01:27,710 And so this year, we're going to try something different 35 00:01:27,710 --> 00:01:31,260 and have me introduce at least some of these topics 36 00:01:31,260 --> 00:01:35,600 now, which will deal with some redundancies that otherwise 37 00:01:35,600 --> 00:01:37,880 might have existed and maybe set it up 38 00:01:37,880 --> 00:01:42,830 better for him to discuss some of the lectures coming up. 39 00:01:42,830 --> 00:01:45,710 Now today's lecture is unfortunately topically 40 00:01:45,710 --> 00:01:48,530 a little bit disjointed. 41 00:01:48,530 --> 00:01:53,240 But it still has important information about biochemistry. 42 00:01:53,240 --> 00:01:56,330 And it will help us all speak the same language, 43 00:01:56,330 --> 00:02:01,790 both for the upcoming lectures from Professor Yaffe as well 44 00:02:01,790 --> 00:02:06,140 as things that I will start off with when I come back. 45 00:02:06,140 --> 00:02:09,720 And it's a nice way to ease back in after the exam. 46 00:02:09,720 --> 00:02:10,220 OK. 47 00:02:10,220 --> 00:02:14,340 So what is a carbohydrate or a sugar? 48 00:02:14,340 --> 00:02:18,460 So let's break this down, and so it's carbohydrate. 49 00:02:25,400 --> 00:02:29,420 So a carbohydrate is effectively carbon 50 00:02:29,420 --> 00:02:32,310 in some ratio with water. 51 00:02:32,310 --> 00:02:36,590 So all carbohydrates have the same chemical formula, Cn 52 00:02:36,590 --> 00:02:38,030 and H2On. 53 00:02:38,030 --> 00:02:42,500 And so there are some deviations from this in biology. 54 00:02:42,500 --> 00:02:45,890 Sometimes, you can introduce a heteroatom, phosphate, sulfur, 55 00:02:45,890 --> 00:02:47,600 nitrogen, et cetera. 56 00:02:47,600 --> 00:02:49,820 Technically, these are not carbohydrates, 57 00:02:49,820 --> 00:02:53,840 although they're often lumped together with carbohydrates. 58 00:02:53,840 --> 00:02:55,160 And why would nature do this? 59 00:02:55,160 --> 00:02:57,710 Because it changes some of the chemical properties 60 00:02:57,710 --> 00:03:01,160 that can be useful for either structural or signaling 61 00:03:01,160 --> 00:03:02,090 reasons. 62 00:03:02,090 --> 00:03:05,120 You may encounter these later-- 63 00:03:05,120 --> 00:03:06,608 certainly, in other classes. 64 00:03:06,608 --> 00:03:08,900 But we're not going to talk much more about that today. 65 00:03:08,900 --> 00:03:11,783 We really just kind of focus on the base carbohydrate, 66 00:03:11,783 --> 00:03:15,200 Cn H2On structure. 67 00:03:15,200 --> 00:03:21,510 Now carbohydrates come in different forms. 68 00:03:21,510 --> 00:03:27,420 And so they can come as single units with that structure. 69 00:03:27,420 --> 00:03:30,068 These are so-called monosaccharides. 70 00:03:35,370 --> 00:03:38,730 Or these single units can come together 71 00:03:38,730 --> 00:03:40,080 to form various polymers. 72 00:03:43,290 --> 00:03:50,470 And those polymers could be two units, so-called disaccarides-- 73 00:03:53,720 --> 00:03:56,690 so two sugars stuck together. 74 00:03:56,690 --> 00:03:59,900 Or many units, and so sometimes, those 75 00:03:59,900 --> 00:04:12,300 are referred to as oligosaccharides 76 00:04:12,300 --> 00:04:13,980 or polysaccharides. 77 00:04:19,990 --> 00:04:22,089 And there's really no clear distinction 78 00:04:22,089 --> 00:04:26,020 between a few chains together, oligosaccharides, 79 00:04:26,020 --> 00:04:27,880 many polysaccharides. 80 00:04:27,880 --> 00:04:30,350 They're somewhat used interchangeably. 81 00:04:30,350 --> 00:04:32,320 Now you guys have almost certainly 82 00:04:32,320 --> 00:04:34,320 heard of many of these things. 83 00:04:34,320 --> 00:04:35,830 So what's a monosaccharide? 84 00:04:35,830 --> 00:04:38,840 So a good example of that is glucose. 85 00:04:38,840 --> 00:04:40,930 So glucose is, of course, the main sugar 86 00:04:40,930 --> 00:04:43,240 that exists in your blood. 87 00:04:43,240 --> 00:04:45,430 What's a disaccharide? 88 00:04:45,430 --> 00:04:48,470 So a common one is sucrose. 89 00:04:48,470 --> 00:04:53,110 So sucrose is a disaccharide-- so two sugars stuck together, 90 00:04:53,110 --> 00:04:54,580 glucose plus fructose. 91 00:04:54,580 --> 00:04:57,310 You've probably heard of both of those things before. 92 00:04:57,310 --> 00:04:59,072 Sucrose is, of course, table sugar. 93 00:04:59,072 --> 00:05:01,030 It's what you would have mixed into your coffee 94 00:05:01,030 --> 00:05:02,590 if you had that this morning. 95 00:05:02,590 --> 00:05:07,150 And an example of a polysaccharide is starch-- 96 00:05:07,150 --> 00:05:09,460 so what's in a potato. 97 00:05:09,460 --> 00:05:10,660 All right. 98 00:05:10,660 --> 00:05:13,210 Now clearly, these are all sugars. 99 00:05:13,210 --> 00:05:15,370 They're all relevant to a human diet. 100 00:05:15,370 --> 00:05:16,690 You would never confuse-- 101 00:05:16,690 --> 00:05:19,600 you've probably not knowingly tasted glucose. 102 00:05:19,600 --> 00:05:22,090 It's not particularly sweet. 103 00:05:22,090 --> 00:05:23,530 Sucrose is much sweeter. 104 00:05:23,530 --> 00:05:26,890 And a potato is not necessarily sweet at all. 105 00:05:26,890 --> 00:05:32,590 Yet, this really points out that how these sugars are built, 106 00:05:32,590 --> 00:05:34,840 the different structures matter a lot. 107 00:05:34,840 --> 00:05:39,280 They matter for things like how you taste them in your diet. 108 00:05:39,280 --> 00:05:43,030 And how carbohydrates are built also 109 00:05:43,030 --> 00:05:46,270 matters a lot for all kinds of aspects of biology, which 110 00:05:46,270 --> 00:05:49,720 is why it's somewhat important to at least understand 111 00:05:49,720 --> 00:05:52,450 some of what we're talking about-- a common language 112 00:05:52,450 --> 00:05:57,430 about how to describe these molecules. 113 00:05:57,430 --> 00:05:58,770 OK. 114 00:05:58,770 --> 00:06:03,480 So the simplest biological sugars 115 00:06:03,480 --> 00:06:07,720 have three carbons that are least commonly used. 116 00:06:07,720 --> 00:06:11,220 These are referred to as trioses. 117 00:06:11,220 --> 00:06:13,890 And if we take the general formula, 118 00:06:13,890 --> 00:06:20,220 C3 H2O3, and we say, what are two ways that we 119 00:06:20,220 --> 00:06:22,530 can satisfy that formula? 120 00:06:22,530 --> 00:06:24,790 There's two main ways we can do it. 121 00:06:24,790 --> 00:06:27,120 One is like this. 122 00:06:34,980 --> 00:06:36,140 So if you add up-- 123 00:06:36,140 --> 00:06:38,660 count all the carbons, hydrogens, and oxygens, 124 00:06:38,660 --> 00:06:44,120 you will see that that is C3 H2O3. 125 00:06:44,120 --> 00:06:47,120 This molecule is called glyceraldehyde. 126 00:06:50,920 --> 00:06:51,640 OK. 127 00:06:51,640 --> 00:06:53,920 And the other way we can do this is like this. 128 00:07:02,080 --> 00:07:02,580 OK. 129 00:07:02,580 --> 00:07:05,280 Again, three carbons, three waters. 130 00:07:05,280 --> 00:07:07,920 If you add up all the atoms, this molecule 131 00:07:07,920 --> 00:07:09,810 is called dihydroxyacetone. 132 00:07:16,110 --> 00:07:22,050 And these are sugar-- same chemical formula, 133 00:07:22,050 --> 00:07:23,820 different chemical structure. 134 00:07:23,820 --> 00:07:25,020 That has a term. 135 00:07:25,020 --> 00:07:26,220 That's called an isomer. 136 00:07:29,470 --> 00:07:32,680 And it turns out that we can chemically 137 00:07:32,680 --> 00:07:36,290 interconvert these molecules in the following way. 138 00:07:36,290 --> 00:07:46,390 And so if we carry out this chemistry, 139 00:07:46,390 --> 00:07:48,867 we will get this intermediate. 140 00:08:10,270 --> 00:08:10,930 OK. 141 00:08:10,930 --> 00:08:15,550 And that will allow you to interconvert glyceraldehyde-- 142 00:08:15,550 --> 00:08:20,620 this aldehyde-- with this ketone, dihydroxyacetone. 143 00:08:20,620 --> 00:08:23,650 And the enzyme class that carries this out 144 00:08:23,650 --> 00:08:27,280 is a class of enzymes called isomerases. 145 00:08:27,280 --> 00:08:29,080 And this is exactly the chemistry 146 00:08:29,080 --> 00:08:32,470 that that enzyme would use to interconvert 147 00:08:32,470 --> 00:08:37,390 these two forms of this triose, this three-carbon sugar 148 00:08:37,390 --> 00:08:39,049 molecule. 149 00:08:39,049 --> 00:08:40,370 All right. 150 00:08:40,370 --> 00:08:43,940 Now if we look at dihydroxyacetone, 151 00:08:43,940 --> 00:08:45,950 there's no stereocenter here. 152 00:08:45,950 --> 00:08:47,780 What do I mean by a stereocenter? 153 00:08:47,780 --> 00:08:50,540 That's a carbon that has-- reminder from 512-- 154 00:08:50,540 --> 00:08:54,920 carbon that has four different non-equivalent substituents 155 00:08:54,920 --> 00:08:56,010 around it. 156 00:08:56,010 --> 00:08:58,520 However, if you look at glyceraldehyde, 157 00:08:58,520 --> 00:09:02,330 that carbon in the middle is a stereocenter-- 158 00:09:02,330 --> 00:09:04,590 four non-equivalent groups around it. 159 00:09:04,590 --> 00:09:07,640 And so there's two ways that I can draw glyceraldehyde. 160 00:09:07,640 --> 00:09:08,960 I can draw it like this. 161 00:09:28,770 --> 00:09:32,070 Or I could draw it like this. 162 00:09:44,780 --> 00:09:46,890 All right-- so two different ways. 163 00:09:46,890 --> 00:09:51,830 So the one on the left here is D-glyceraldehyde. 164 00:09:58,580 --> 00:10:01,850 And the one on the right is L-glyceraldehyde. 165 00:10:08,160 --> 00:10:08,660 OK. 166 00:10:08,660 --> 00:10:10,820 And I know that this is review. 167 00:10:10,820 --> 00:10:12,800 Some people are good at seeing these things. 168 00:10:12,800 --> 00:10:13,990 Some people are not. 169 00:10:13,990 --> 00:10:15,530 I brought a couple of models here. 170 00:10:15,530 --> 00:10:19,970 Here's this-- the blue and the brown 171 00:10:19,970 --> 00:10:23,510 are two stereocenters with four different constituents on them. 172 00:10:23,510 --> 00:10:26,450 No way you can twist these around to make them identical 173 00:10:26,450 --> 00:10:27,740 molecules. 174 00:10:27,740 --> 00:10:28,890 Why does this matter? 175 00:10:28,890 --> 00:10:31,850 Well, enzyme-active sites are going 176 00:10:31,850 --> 00:10:35,330 to fit this molecule different than this molecule. 177 00:10:35,330 --> 00:10:37,820 And this is why these stereoisomers 178 00:10:37,820 --> 00:10:41,882 matter so much for biology. 179 00:10:41,882 --> 00:10:44,090 It's also something that's really hard to accomplish. 180 00:10:44,090 --> 00:10:46,048 If you think about how do you actually generate 181 00:10:46,048 --> 00:10:48,890 stereoisomers if you were in an organic chemistry lab-- 182 00:10:48,890 --> 00:10:49,940 really hard. 183 00:10:49,940 --> 00:10:51,620 But biology does this all the time. 184 00:10:51,620 --> 00:10:53,870 And the real reason is because it's 185 00:10:53,870 --> 00:10:57,260 enzymes that ultimately catalyze these interconversions. 186 00:10:57,260 --> 00:10:59,450 And different stereoisomers will fit differently 187 00:10:59,450 --> 00:11:02,450 into enzyme-active sites. 188 00:11:02,450 --> 00:11:05,120 Now the way that I've been drawing these sugars 189 00:11:05,120 --> 00:11:08,240 is a convention called a Fischer projection. 190 00:11:08,240 --> 00:11:12,260 And when drawn in this way, the convention is that-- 191 00:11:12,260 --> 00:11:16,450 so you put the carbonyl towards the top. 192 00:11:16,450 --> 00:11:21,820 If the OH group, the alcohol, is pointing to the right, 193 00:11:21,820 --> 00:11:26,560 that's D. If it's pointing to the left, that's L-- 194 00:11:26,560 --> 00:11:31,600 and so D, pointing to the right, L, pointing to the left. 195 00:11:31,600 --> 00:11:38,050 Like amino acids, biology has chosen one stereochemistry 196 00:11:38,050 --> 00:11:41,950 for most biological sugars because, of course, 197 00:11:41,950 --> 00:11:43,420 enzymes act on them. 198 00:11:43,420 --> 00:11:49,580 And so in biology, it's D, sugars. 199 00:11:49,580 --> 00:11:53,900 This is in contrast to L, amino acids. 200 00:11:53,900 --> 00:11:56,620 And so if you can remember that sugars are D, 201 00:11:56,620 --> 00:11:58,480 you know that amino acids are the opposite. 202 00:11:58,480 --> 00:12:00,250 Or if you remember that amino acids are L, 203 00:12:00,250 --> 00:12:02,830 you can remember that sugars are the opposite. 204 00:12:02,830 --> 00:12:04,030 OK. 205 00:12:04,030 --> 00:12:05,200 All right. 206 00:12:05,200 --> 00:12:11,410 So dihydroxyacetone is really the only sugar 207 00:12:11,410 --> 00:12:15,670 with three or more carbons that doesn't have a chiral center. 208 00:12:15,670 --> 00:12:17,320 Everything else will. 209 00:12:17,320 --> 00:12:22,390 And so if I go to a four-carbon sugar-- 210 00:12:22,390 --> 00:12:23,560 draw a couple of them here. 211 00:12:35,930 --> 00:12:36,430 OK. 212 00:12:36,430 --> 00:12:42,290 So here's C4 H2O4 drawn with a ketone. 213 00:12:42,290 --> 00:12:45,170 If you look at this, this is a chiral center. 214 00:12:45,170 --> 00:12:47,290 OH group is pointing to the right. 215 00:12:47,290 --> 00:12:48,730 This is a D sugar. 216 00:12:48,730 --> 00:12:51,280 If I had drawn it with the OH group on this side, 217 00:12:51,280 --> 00:12:52,790 it would be an L sugar. 218 00:12:52,790 --> 00:12:53,290 All right. 219 00:12:53,290 --> 00:12:58,000 If I draw this sugar as a different isomer, this time 220 00:12:58,000 --> 00:13:13,130 with an aldehyde, now, we encounter a bit of an issue 221 00:13:13,130 --> 00:13:17,280 because now I have one, two stereocenters. 222 00:13:17,280 --> 00:13:18,200 OK. 223 00:13:18,200 --> 00:13:21,140 So if there's two stereocenters, that 224 00:13:21,140 --> 00:13:25,850 means there's 2 to the n ways that I can 225 00:13:25,850 --> 00:13:28,610 draw this as a stereoisomer. 226 00:13:28,610 --> 00:13:32,270 And so you can see that this could get really complicated 227 00:13:32,270 --> 00:13:33,990 very quickly. 228 00:13:33,990 --> 00:13:40,430 Now this sugar obviously is a D because I drew both OH groups 229 00:13:40,430 --> 00:13:42,080 pointing to the right. 230 00:13:42,080 --> 00:13:46,190 But you can imagine, I could draw one this way 231 00:13:46,190 --> 00:13:47,430 or one that way. 232 00:13:47,430 --> 00:13:49,920 And so how do if it's a D or an L sugar? 233 00:13:49,920 --> 00:13:54,350 And so the convention is that whether or not 234 00:13:54,350 --> 00:13:58,130 a sugar is designated as D or L refers 235 00:13:58,130 --> 00:14:01,280 to the stereocenter that is furthest away 236 00:14:01,280 --> 00:14:03,180 from the carbonyl. 237 00:14:03,180 --> 00:14:06,350 And so this is the relevant stereocenter 238 00:14:06,350 --> 00:14:08,120 that says it's a D sugar. 239 00:14:08,120 --> 00:14:08,870 So what do I mean? 240 00:14:08,870 --> 00:14:09,470 I can draw. 241 00:14:09,470 --> 00:14:13,040 And then any other sugar would have a different name. 242 00:14:13,040 --> 00:14:14,290 And so what do I mean by that? 243 00:14:14,290 --> 00:14:16,260 Let me draw this a few different ways. 244 00:14:16,260 --> 00:14:18,920 And so the sugar that I've drawn here 245 00:14:18,920 --> 00:14:21,125 is referred to as D-erythrose. 246 00:14:24,170 --> 00:14:24,670 All right. 247 00:14:24,670 --> 00:14:40,170 If I draw it, now the OH group on the carbon 248 00:14:40,170 --> 00:14:42,910 furthest from the carbonyl pointing to the left, 249 00:14:42,910 --> 00:14:44,850 so this would be L-erythrose. 250 00:14:50,330 --> 00:14:55,130 And if I draw it differently by altering 251 00:14:55,130 --> 00:15:05,390 the stereochemistry of this carbon, 252 00:15:05,390 --> 00:15:06,750 now, it has a different name. 253 00:15:06,750 --> 00:15:09,080 And so this carbon is D-- 254 00:15:09,080 --> 00:15:12,750 or this sugar is D-threose. 255 00:15:12,750 --> 00:15:13,780 All right. 256 00:15:13,780 --> 00:15:14,400 Makes sense. 257 00:15:14,400 --> 00:15:19,230 If I flip the OH group to this side, it would be L-threose. 258 00:15:19,230 --> 00:15:19,730 All right. 259 00:15:19,730 --> 00:15:22,760 So lots of possibilities. 260 00:15:22,760 --> 00:15:27,890 Turns out nature only uses a subset of the stereoisomers 261 00:15:27,890 --> 00:15:30,320 and makes them relevant for biology. 262 00:15:30,320 --> 00:15:32,510 For example, D-erythrose is something 263 00:15:32,510 --> 00:15:35,000 that you will encounter when we talk about metabolism 264 00:15:35,000 --> 00:15:36,290 later in the course. 265 00:15:36,290 --> 00:15:39,950 D-threose, as far as I know, is not used in biology. 266 00:15:39,950 --> 00:15:42,230 I would never say, it's never used in biology. 267 00:15:42,230 --> 00:15:43,520 Never bet against biology. 268 00:15:43,520 --> 00:15:45,330 It can do absolutely everything. 269 00:15:45,330 --> 00:15:47,370 There's always an exception somewhere. 270 00:15:47,370 --> 00:15:49,370 But, in general, D-threose is not 271 00:15:49,370 --> 00:15:55,330 something that really exists, at least commonly in nature. 272 00:15:55,330 --> 00:15:56,230 All right. 273 00:15:56,230 --> 00:15:58,420 So if we go through, and you look 274 00:15:58,420 --> 00:16:04,210 at all of these different sugars that I've drawn, 275 00:16:04,210 --> 00:16:07,660 you can see that they either have an aldehyde, 276 00:16:07,660 --> 00:16:11,240 or they have a ketone somewhere in the molecule. 277 00:16:11,240 --> 00:16:11,740 All right. 278 00:16:11,740 --> 00:16:17,990 So if you have an aldehyde, these sugars 279 00:16:17,990 --> 00:16:22,370 are generically referred to as aldoses. 280 00:16:22,370 --> 00:16:23,420 All right. 281 00:16:23,420 --> 00:16:26,990 And if you have a ketone, an internal carbonyl, 282 00:16:26,990 --> 00:16:31,950 these are generically referred to as ketoses. 283 00:16:31,950 --> 00:16:35,780 Now you might say, as you start getting 284 00:16:35,780 --> 00:16:40,792 to longer and longer sugars, you can put the ketone anywhere 285 00:16:40,792 --> 00:16:42,500 along the sugar, and there would be a ton 286 00:16:42,500 --> 00:16:44,660 of different possibilities. 287 00:16:44,660 --> 00:16:47,300 But it turns out ketoses always have 288 00:16:47,300 --> 00:16:53,270 the carbonyl as the second carbon in from the end. 289 00:16:53,270 --> 00:16:56,870 The reason for that is because biology, 290 00:16:56,870 --> 00:16:59,450 as you will see when we talk about metabolism, 291 00:16:59,450 --> 00:17:03,300 interconverts these via isomerase reactions. 292 00:17:03,300 --> 00:17:05,599 And so you can't use an isomerase reaction 293 00:17:05,599 --> 00:17:08,000 to interconvert a ketose and an aldose 294 00:17:08,000 --> 00:17:12,510 unless the ketone is one carbon away from the end of the sugar. 295 00:17:12,510 --> 00:17:14,569 And so this fact really limits some 296 00:17:14,569 --> 00:17:19,910 of the diversity of ketoses that can actually exist in nature. 297 00:17:19,910 --> 00:17:22,720 Now most important biological sugars, 298 00:17:22,720 --> 00:17:24,970 at least the most common ones, end up 299 00:17:24,970 --> 00:17:28,300 having six carbons or five carbons. 300 00:17:28,300 --> 00:17:32,500 And these are referred to as hexoses or aldose-- 301 00:17:32,500 --> 00:17:34,240 or pentoses. 302 00:17:34,240 --> 00:17:37,400 Sorry-- so much nomenclature. 303 00:17:37,400 --> 00:17:37,900 OK. 304 00:17:37,900 --> 00:17:40,030 So six-carbon sugars are hexoses. 305 00:17:40,030 --> 00:17:42,820 Five-carbon sugars are pentoses. 306 00:17:42,820 --> 00:17:46,060 And if we just talk about the hexoses, 307 00:17:46,060 --> 00:17:48,910 you're very familiar with a couple of them. 308 00:17:48,910 --> 00:17:53,080 And so one that we mentioned earlier, blood sugar-- 309 00:17:53,080 --> 00:17:54,670 glucose looks like this. 310 00:18:12,910 --> 00:18:15,900 So this molecule is D-glucose. 311 00:18:19,950 --> 00:18:23,460 It's a D sugar because the stereocenter here 312 00:18:23,460 --> 00:18:27,525 furthest from the carbonyl-- 313 00:18:27,525 --> 00:18:29,140 OH group points to the right. 314 00:18:29,140 --> 00:18:31,290 So it's a D sugar. 315 00:18:31,290 --> 00:18:34,560 It's an aldose because it has an aldehyde. 316 00:18:34,560 --> 00:18:35,370 All right. 317 00:18:35,370 --> 00:18:40,710 And it's a hexose because it has 1, 2, 3, 4, 5, 6 carbons. 318 00:18:40,710 --> 00:18:43,680 And what makes it glucose is the stereochemistry 319 00:18:43,680 --> 00:18:49,380 of these other sites that end up being-- this 320 00:18:49,380 --> 00:18:52,020 is the molecule glucose. 321 00:18:52,020 --> 00:18:56,580 Now if I carry out that isomerase reaction 322 00:18:56,580 --> 00:19:05,377 that I showed you earlier, it'll give me this intermediate. 323 00:19:11,221 --> 00:19:13,000 Let me draw the whole thing. 324 00:19:23,510 --> 00:19:26,340 Then I interconvert that aldose. 325 00:19:26,340 --> 00:19:27,845 Now, it becomes a ketose. 326 00:19:41,340 --> 00:19:49,770 This is D-fructose-- another common sugar that 327 00:19:49,770 --> 00:19:52,080 certainly is in the news a lot. 328 00:19:52,080 --> 00:19:55,260 D sugar because the stereocenter for this 329 00:19:55,260 --> 00:19:58,930 from the carbonyl points to the right, makes it a D sugar. 330 00:19:58,930 --> 00:20:00,030 It's a ketose. 331 00:20:00,030 --> 00:20:01,560 It's a hexose. 332 00:20:01,560 --> 00:20:04,050 This organization of other stereocenters 333 00:20:04,050 --> 00:20:06,490 is what makes it D-fructose. 334 00:20:06,490 --> 00:20:07,650 All right. 335 00:20:07,650 --> 00:20:10,620 Now if we go through and we count these, 336 00:20:10,620 --> 00:20:15,120 there's four stereocenters in glucose, three stereocenters 337 00:20:15,120 --> 00:20:16,380 in fructose. 338 00:20:16,380 --> 00:20:17,820 That means 2 to the n. 339 00:20:17,820 --> 00:20:23,190 There are 16 ways I can make ketose aldose, eight 340 00:20:23,190 --> 00:20:25,980 ways I can make a ketose-- 341 00:20:25,980 --> 00:20:28,620 I'm sorry-- an aldose hexose, eight ways 342 00:20:28,620 --> 00:20:32,340 I can make a ketose hexose. 343 00:20:32,340 --> 00:20:35,160 Among those, that's even 12 different ways 344 00:20:35,160 --> 00:20:40,440 that I can have D hexoses that are a ketose or an aldose. 345 00:20:40,440 --> 00:20:42,400 Could be very, very complicated. 346 00:20:42,400 --> 00:20:49,200 But it turns out, fructose is the only D hexose ketose that 347 00:20:49,200 --> 00:20:51,870 really is relevant for nature. 348 00:20:51,870 --> 00:20:55,530 There's only two other molecules related to glucose-- 349 00:20:55,530 --> 00:21:00,720 only two other hexose aldoses that are sugars used in nature. 350 00:21:00,720 --> 00:21:03,510 You've probably heard of these as well. 351 00:21:03,510 --> 00:21:06,330 I'll draw them to illustrate a different point. 352 00:21:25,100 --> 00:21:26,030 OK. 353 00:21:26,030 --> 00:21:35,710 So this molecule is galactose, an important sugar 354 00:21:35,710 --> 00:21:37,930 found in milk. 355 00:21:37,930 --> 00:21:39,430 All right. 356 00:21:39,430 --> 00:22:03,810 If I draw a glucose next to it again, glucose-- 357 00:22:06,720 --> 00:22:24,230 so these two sugars differ here, the stereochemistry there. 358 00:22:24,230 --> 00:22:28,260 So they are, I guess, isomers of each other. 359 00:22:28,260 --> 00:22:30,630 There's a special name for it I will get to in a second. 360 00:22:30,630 --> 00:22:32,750 So that's how galactose is related to glucose. 361 00:22:32,750 --> 00:22:36,140 The other major aldose hexose is this one. 362 00:22:54,070 --> 00:22:55,495 This is D-mannose. 363 00:22:59,600 --> 00:23:05,840 This differs from glucose here-- 364 00:23:05,840 --> 00:23:08,000 that carbon at that carbon. 365 00:23:08,000 --> 00:23:10,430 By the way, by convention, the way 366 00:23:10,430 --> 00:23:13,640 that you number carbon in sugars is you 367 00:23:13,640 --> 00:23:16,820 start with the end that's closest to the carbonyl, either 368 00:23:16,820 --> 00:23:21,270 the aldehyde or the carbon one up from the ketone, 369 00:23:21,270 --> 00:23:26,780 so this would be carbon 1, 2, 3, 4, 5, 6. 370 00:23:26,780 --> 00:23:30,440 So galactose differs from glucose at carbon 4. 371 00:23:30,440 --> 00:23:36,320 Glucose differs from mannose at carbon 2. 372 00:23:36,320 --> 00:23:40,020 These isomers have names in relation to each other. 373 00:23:40,020 --> 00:23:45,930 And so two sugars that differ by one part of stereochemistry-- 374 00:23:45,930 --> 00:23:49,430 so galactose to glucose or glucose to mannose-- 375 00:23:49,430 --> 00:23:52,310 are called epimers. 376 00:23:52,310 --> 00:23:58,670 And these can be or converted by enzymes called epimerases. 377 00:23:58,670 --> 00:24:01,910 We'll talk about how these work later in the course. 378 00:24:01,910 --> 00:24:05,510 And so glucose is an epimer of galactose. 379 00:24:05,510 --> 00:24:07,850 Glucose is an epimer of mannose. 380 00:24:07,850 --> 00:24:11,210 Mannose is not an epimer of galactose 381 00:24:11,210 --> 00:24:12,965 because mannose and galactose differ 382 00:24:12,965 --> 00:24:16,580 at both carbon 2 and carbon 4-- 383 00:24:16,580 --> 00:24:19,520 their stereochemistry. 384 00:24:19,520 --> 00:24:21,060 Why is that relevant? 385 00:24:21,060 --> 00:24:22,790 Because if you're going to interconvert 386 00:24:22,790 --> 00:24:24,890 galactose and mannose, you would have 387 00:24:24,890 --> 00:24:28,220 to do it in two steps, two different epimerase reactions 388 00:24:28,220 --> 00:24:32,300 to interconvert those two sugars. 389 00:24:32,300 --> 00:24:34,570 All right. 390 00:24:34,570 --> 00:24:35,170 Great. 391 00:24:35,170 --> 00:24:42,330 So we've discussed now all of the major hexoses 392 00:24:42,330 --> 00:24:43,350 that nature uses. 393 00:24:43,350 --> 00:24:48,210 We've discussed all the major trioses that nature uses. 394 00:24:48,210 --> 00:24:51,690 The other major length of sugars that 395 00:24:51,690 --> 00:24:53,580 ends up being important in biochemistry 396 00:24:53,580 --> 00:24:56,190 is the five-carbon sugars, the pentoses. 397 00:24:56,190 --> 00:24:59,910 And so I want to mention a couple pentoses 398 00:24:59,910 --> 00:25:06,900 and do so in a way that will allow me to basically solidify 399 00:25:06,900 --> 00:25:09,990 some of the nomenclature that I've gone through. 400 00:25:09,990 --> 00:25:13,650 And, of course, one of the five carbon sugars-- ribose-- 401 00:25:13,650 --> 00:25:16,770 is really critical to nucleic acid structure, which 402 00:25:16,770 --> 00:25:19,740 is one of the reasons why we're talking about this 403 00:25:19,740 --> 00:25:21,060 at this point in the course. 404 00:25:29,510 --> 00:25:30,010 OK. 405 00:25:30,010 --> 00:25:32,200 So this is a pentose-- 406 00:25:32,200 --> 00:25:37,060 1, 2, 3, 4, 5-carbon sugar, D-ribose. 407 00:25:40,490 --> 00:25:40,990 All right. 408 00:25:40,990 --> 00:25:43,630 It's a D sugar because the stereocenter furthest 409 00:25:43,630 --> 00:25:46,870 from the carbonyl points to the right. 410 00:25:46,870 --> 00:25:50,350 It's an aldose because it has an aldehyde group. 411 00:25:50,350 --> 00:25:55,382 I can act on this with an isomerase. 412 00:25:55,382 --> 00:25:58,830 I'm not going to draw out the isomerase reaction again. 413 00:25:58,830 --> 00:26:00,840 It's exactly what I drew before. 414 00:26:00,840 --> 00:26:06,210 If I did this, now, I turn this into a ketose. 415 00:26:17,100 --> 00:26:19,590 Pentose because it's five carbons. 416 00:26:19,590 --> 00:26:21,960 Ketose because as a ketone. 417 00:26:21,960 --> 00:26:24,180 D sugar because the stereocenter furthest 418 00:26:24,180 --> 00:26:26,820 from the carbonyl points to the right. 419 00:26:26,820 --> 00:26:29,040 This has a name of D-ribulose. 420 00:26:31,690 --> 00:26:32,570 All right. 421 00:26:32,570 --> 00:26:36,010 And it turns out that there is an important epimer 422 00:26:36,010 --> 00:26:39,170 of D-ribulose that's found in nature. 423 00:26:39,170 --> 00:26:43,330 The epimer changes the stereochemistry at carbon 3-- 424 00:26:43,330 --> 00:26:45,520 carbon 1, 2, 3, 4, 5. 425 00:26:45,520 --> 00:26:46,960 That's carbon 3. 426 00:26:46,960 --> 00:26:53,420 And so if this was acted on by an epimerase that did that, 427 00:26:53,420 --> 00:27:00,440 you get this sugar, also a pentose, 428 00:27:00,440 --> 00:27:10,520 also a ketose, a D sugar, but an epimer of ribulose. 429 00:27:10,520 --> 00:27:11,975 It's called D-xyulose. 430 00:27:14,960 --> 00:27:21,260 It's an epimer because xyulose ribulose 431 00:27:21,260 --> 00:27:26,120 are epimers because they differ by stereochemistry only at one 432 00:27:26,120 --> 00:27:28,290 position. 433 00:27:28,290 --> 00:27:29,850 I'll just say, right off the bat, 434 00:27:29,850 --> 00:27:33,910 you should not memorize names of sugars and their structures. 435 00:27:33,910 --> 00:27:37,260 These are things you can look up in books. 436 00:27:37,260 --> 00:27:38,940 The point of going through all this 437 00:27:38,940 --> 00:27:42,960 is just to expose you to some of the nomenclature, 438 00:27:42,960 --> 00:27:45,570 remind you about stereochemistry. 439 00:27:45,570 --> 00:27:47,980 I realize, these are basic things. 440 00:27:47,980 --> 00:27:50,970 Many of you have already encountered this. 441 00:27:50,970 --> 00:27:52,470 Some of you find this very easy. 442 00:27:52,470 --> 00:27:57,390 Some people find these sort of spatial things more difficult. 443 00:27:57,390 --> 00:28:00,570 This is very well-reviewed, though, in textbooks 444 00:28:00,570 --> 00:28:05,350 or other places online if you need to look it up. 445 00:28:05,350 --> 00:28:08,040 But the key thing is just to remember this nomenclature 446 00:28:08,040 --> 00:28:12,510 because it'll make it easier for us to talk about sugars later 447 00:28:12,510 --> 00:28:13,660 in the course. 448 00:28:13,660 --> 00:28:16,000 All right let's take a short break, 449 00:28:16,000 --> 00:28:19,410 so I can get some board space back. 450 00:28:19,410 --> 00:28:24,490 And then we'll build off some of these concepts in a minute. 451 00:28:24,490 --> 00:28:29,890 I've been drawing all of these sugars as straight chains. 452 00:28:29,890 --> 00:28:32,350 But you probably know, from high school 453 00:28:32,350 --> 00:28:36,730 or from looking at DNA or RNA that the ribose there 454 00:28:36,730 --> 00:28:41,080 is not a straight chain but, instead, forms a ring. 455 00:28:41,080 --> 00:28:44,470 And, in fact in solution, particular aqueous 456 00:28:44,470 --> 00:28:49,870 solution, sugars, particularly five carbons and longer almost 457 00:28:49,870 --> 00:28:51,400 always exist as rings. 458 00:28:51,400 --> 00:28:53,770 And there is a very clear reason for this. 459 00:28:53,770 --> 00:28:57,910 And you probably remember, from 512 Organic Chemistry 460 00:28:57,910 --> 00:29:01,900 that alcohols will react with aldehydes and ketones 461 00:29:01,900 --> 00:29:03,860 in solution. 462 00:29:03,860 --> 00:29:06,760 And so here, I have a model of glucose and fructose 463 00:29:06,760 --> 00:29:08,810 if you want to come and play with them. 464 00:29:08,810 --> 00:29:12,130 And so if you look at this, and you just look at the model, 465 00:29:12,130 --> 00:29:17,440 you see that this oxygen, right here, this alcohol, in space, 466 00:29:17,440 --> 00:29:22,300 is very close or can be moved to be very close to this aldehyde 467 00:29:22,300 --> 00:29:24,040 here on the end of the molecule. 468 00:29:24,040 --> 00:29:26,200 Or the same thing here with fructose. 469 00:29:26,200 --> 00:29:32,680 Here's a alcohol very close in space with this ketone. 470 00:29:32,680 --> 00:29:33,190 All right. 471 00:29:33,190 --> 00:29:39,450 So what happens in this situation? 472 00:29:39,450 --> 00:29:40,940 Well, if you have-- 473 00:29:40,940 --> 00:29:43,080 this is a review of organic chemistry. 474 00:29:43,080 --> 00:29:43,580 OK. 475 00:29:43,580 --> 00:29:46,760 So here's any generic aldehyde. 476 00:29:46,760 --> 00:29:48,980 Here's some alcohol. 477 00:30:02,930 --> 00:30:03,430 OK. 478 00:30:03,430 --> 00:30:07,090 So those things react. 479 00:30:07,090 --> 00:30:16,950 You end up with this so-called hemiacetol. 480 00:30:20,310 --> 00:30:30,825 Or the same thing if I do it with a ketone and an alcohol-- 481 00:30:36,600 --> 00:30:45,846 now you get this hemiketal. 482 00:30:49,780 --> 00:30:50,890 OK. 483 00:30:50,890 --> 00:30:54,430 Now given that you have an alcohol reacting 484 00:30:54,430 --> 00:30:57,280 with a carbonyl, an aldehyde, or ketone 485 00:30:57,280 --> 00:30:59,710 on the same molecule, well, what happens 486 00:30:59,710 --> 00:31:03,610 is you effectively get a ring with oxygen 487 00:31:03,610 --> 00:31:07,550 being one of the components of the ring. 488 00:31:07,550 --> 00:31:11,020 And so if we draw this for glucose-- 489 00:31:35,220 --> 00:31:37,740 so this is D-glucose. 490 00:31:37,740 --> 00:31:38,690 All right. 491 00:31:38,690 --> 00:31:47,380 And so if the alcohol here on carbon 1, 2, 3, 4, 5 492 00:31:47,380 --> 00:31:53,170 interacts with the aldehyde on carbon 1-- 493 00:31:53,170 --> 00:31:56,530 you can play with the model and see that that is 494 00:31:56,530 --> 00:31:58,780 the one that's close in space-- 495 00:31:58,780 --> 00:32:01,855 you now end up getting this. 496 00:32:22,930 --> 00:32:31,887 This, where you get a ring between carbons 1 and 5. 497 00:32:31,887 --> 00:32:33,220 We can number the rest of them-- 498 00:32:33,220 --> 00:32:37,390 2, 3, 4, 5, 6. 499 00:32:37,390 --> 00:32:38,380 All right. 500 00:32:38,380 --> 00:32:43,180 Or if I turn this molecule so that you can now draw it 501 00:32:43,180 --> 00:33:16,200 in a slightly more chemically proper way, 502 00:33:16,200 --> 00:33:18,180 now, you basically have-- 503 00:33:18,180 --> 00:33:26,460 this is carbon 1, 2, 3, 4, 5, 6. 504 00:33:26,460 --> 00:33:33,570 So carbon 5, oxygen from carbon 5 now bound to carbon 1 505 00:33:33,570 --> 00:33:37,500 gives you this six-membered ring structure. 506 00:33:37,500 --> 00:33:41,430 The six-membered ring structure is reminiscent of an organic 507 00:33:41,430 --> 00:33:45,330 molecule called a-- 508 00:33:45,330 --> 00:33:46,740 looks like that-- called pyran. 509 00:33:50,280 --> 00:33:54,540 And so this six-membered ring in a sugar 510 00:33:54,540 --> 00:33:59,540 is also referred to as a pyranose. 511 00:33:59,540 --> 00:34:01,120 All right. 512 00:34:01,120 --> 00:34:03,300 So that's the first thing. 513 00:34:03,300 --> 00:34:08,030 The second thing is that this whole business 514 00:34:08,030 --> 00:34:10,219 can be very tedious to draw. 515 00:34:10,219 --> 00:34:12,830 In fact, it's very tedious drawing sugars in general, 516 00:34:12,830 --> 00:34:16,219 as I'm sure you would agree with me if you're taking notes 517 00:34:16,219 --> 00:34:17,540 during this lecture. 518 00:34:17,540 --> 00:34:24,679 And so oftentimes, these pyranoses, like glucose, 519 00:34:24,679 --> 00:34:26,840 are drawn with shorthand. 520 00:34:26,840 --> 00:34:37,440 And the shorthand is as follows, where I basically represent 521 00:34:37,440 --> 00:34:40,980 the OH groups as simply lines. 522 00:34:40,980 --> 00:34:44,610 And so this is another shorthand to draw 523 00:34:44,610 --> 00:34:47,760 that pyranose form of glucose-- 524 00:34:47,760 --> 00:34:54,810 again, carbon 1, 2, 3, 4, 5, 6. 525 00:34:54,810 --> 00:34:57,240 Now the last thing is you can see I didn't draw the OH 526 00:34:57,240 --> 00:34:59,100 group there on carbon 1. 527 00:34:59,100 --> 00:35:02,790 And that's because by making this pyranose ring, 528 00:35:02,790 --> 00:35:04,920 if you look at carbon 1, I have now 529 00:35:04,920 --> 00:35:08,700 generated a new stereocenter. 530 00:35:08,700 --> 00:35:13,650 And so carbon 1 now has four non-equivalent groups on it, 531 00:35:13,650 --> 00:35:17,490 which means I could draw the OH group, that carbon 1, 532 00:35:17,490 --> 00:35:18,700 in two different ways. 533 00:35:18,700 --> 00:35:21,150 And so this is carbon 1. 534 00:35:21,150 --> 00:35:24,900 I could draw it such that the OH group points down. 535 00:35:24,900 --> 00:35:32,610 Or I could draw it such that the OH group points up. 536 00:35:32,610 --> 00:35:35,250 And those are two different molecules. 537 00:35:35,250 --> 00:35:38,400 And so there's a naming convention for this too. 538 00:35:38,400 --> 00:35:41,490 And so if the OH group points down 539 00:35:41,490 --> 00:35:46,350 using this way of drawing the molecule, it's called alpha. 540 00:35:46,350 --> 00:35:51,390 If the OH group points up, it's called beta. 541 00:35:51,390 --> 00:35:52,710 All right. 542 00:35:52,710 --> 00:35:56,970 And so this alpha versus beta ends up 543 00:35:56,970 --> 00:36:03,620 being structurally different because it puts, basically, 544 00:36:03,620 --> 00:36:10,040 that OH group pointing in a very different position in space. 545 00:36:10,040 --> 00:36:13,640 So if I make a ring here with glucose, 546 00:36:13,640 --> 00:36:17,060 and the OH group is pointing here versus there-- 547 00:36:17,060 --> 00:36:19,280 very different position in space. 548 00:36:19,280 --> 00:36:20,900 And this has implications for how 549 00:36:20,900 --> 00:36:23,420 you build bonds for disaccharides 550 00:36:23,420 --> 00:36:26,750 and polysaccharides that make structural differences. 551 00:36:26,750 --> 00:36:30,680 And we'll cover this a lot when we get to metabolism. 552 00:36:30,680 --> 00:36:34,220 But it should be very clear now that glucose, 553 00:36:34,220 --> 00:36:37,130 if you just take a solution of glucose and put it in water, 554 00:36:37,130 --> 00:36:39,020 it is not one thing. 555 00:36:39,020 --> 00:36:41,870 There's actually multiple different forms it can have. 556 00:36:41,870 --> 00:36:46,520 It could be as I drew it up here with the OH group pointing 557 00:36:46,520 --> 00:36:47,360 down. 558 00:36:47,360 --> 00:36:52,160 This would be alpha D-gluco-- 559 00:36:52,160 --> 00:36:57,990 because it's glucose-- pyranose, because it's 560 00:36:57,990 --> 00:37:01,860 in the pyranose ring form. 561 00:37:01,860 --> 00:37:05,370 I could have drawn it with the OH group pointing up-- 562 00:37:05,370 --> 00:37:06,900 different molecule. 563 00:37:06,900 --> 00:37:09,670 That would be Beta D-glucopyranose. 564 00:37:15,530 --> 00:37:22,040 Or it could just be the open-chain D-glucose 565 00:37:22,040 --> 00:37:24,410 that I was drawing earlier. 566 00:37:24,410 --> 00:37:25,310 All right. 567 00:37:25,310 --> 00:37:29,690 All of those are perfectly legitimate ways for glucose 568 00:37:29,690 --> 00:37:31,640 to exist in solution. 569 00:37:31,640 --> 00:37:32,510 All right. 570 00:37:32,510 --> 00:37:35,480 Now it turns out, that in reality, about a third 571 00:37:35,480 --> 00:37:36,770 of it in solution is this. 572 00:37:36,770 --> 00:37:38,120 Two thirds is that. 573 00:37:38,120 --> 00:37:40,340 And a trace amount is this. 574 00:37:40,340 --> 00:37:41,600 OK. 575 00:37:41,600 --> 00:37:47,000 And that has to do with just what 576 00:37:47,000 --> 00:37:49,910 is more favorable forms or not. 577 00:37:49,910 --> 00:37:54,710 But which form it's in actually matters for structural reasons, 578 00:37:54,710 --> 00:37:56,520 as we'll see later in the course. 579 00:37:56,520 --> 00:38:02,480 Now the final complexity is that this ring is not flat. 580 00:38:02,480 --> 00:38:08,570 So if I actually take glucose here and make a form of it-- 581 00:38:08,570 --> 00:38:11,100 so here's my glucose molecule. 582 00:38:11,100 --> 00:38:14,030 There's no way for me to make this completely flat, 583 00:38:14,030 --> 00:38:16,190 say, like benzene. 584 00:38:16,190 --> 00:38:19,220 And so there's really two different pyranose 585 00:38:19,220 --> 00:38:22,400 confirmations that can be formed. 586 00:38:22,400 --> 00:38:24,740 I'll try to draw them, but they're harder to draw. 587 00:38:24,740 --> 00:38:29,720 Here's a overhead that you can look at if it's easier. 588 00:38:29,720 --> 00:38:38,370 But basically, you can have it form this so-called boat form 589 00:38:38,370 --> 00:38:46,100 or this so-called chair form. 590 00:38:46,100 --> 00:38:52,250 So there's the boat versus the chair conformation. 591 00:38:52,250 --> 00:38:57,260 Glucose, turns out, prefers the chair conformation. 592 00:38:57,260 --> 00:38:59,180 And there's an interesting thing that 593 00:38:59,180 --> 00:39:05,150 comes from this because if you take beta D-glucopyranose, 594 00:39:05,150 --> 00:39:09,470 it turns out of all the hexoses that 595 00:39:09,470 --> 00:39:15,350 exist in possible hexoses that exist, the form of a hexose 596 00:39:15,350 --> 00:39:19,430 aldose that best spreads out all the hydroxyl groups 597 00:39:19,430 --> 00:39:27,000 is beta D-glucopyranose, the more common one in solution. 598 00:39:27,000 --> 00:39:28,230 Why does this matter? 599 00:39:28,230 --> 00:39:31,460 Well, because if this has this reactive aldehyde bound up 600 00:39:31,460 --> 00:39:34,070 in this stable ring structure, it's 601 00:39:34,070 --> 00:39:37,890 less likely to react with other aldehydes in the cell. 602 00:39:37,890 --> 00:39:42,200 And so this is likely why nature chose D-glucose as the most 603 00:39:42,200 --> 00:39:45,830 common storage sugar-- why it's sugar in your blood-- 604 00:39:45,830 --> 00:39:51,260 because it's the most stable hexose that's out there. 605 00:39:51,260 --> 00:39:54,890 And it's not just a random reason 606 00:39:54,890 --> 00:39:57,200 that nature picked this one, but actually 607 00:39:57,200 --> 00:40:00,350 because of real chemical stability 608 00:40:00,350 --> 00:40:02,810 issues for why it's there. 609 00:40:02,810 --> 00:40:04,040 All right. 610 00:40:04,040 --> 00:40:09,890 Now I want to mention that ketoses also can form rings. 611 00:40:09,890 --> 00:40:11,810 And I'm going to use this as an example 612 00:40:11,810 --> 00:40:14,495 to show you that a ketose-- so here's fructose. 613 00:40:31,810 --> 00:40:33,690 So this is D-fructose. 614 00:40:33,690 --> 00:40:37,260 And so it turns out, this can form two possible rings. 615 00:40:37,260 --> 00:40:40,200 It can form a five-membered ring or six-membered ring. 616 00:40:40,200 --> 00:40:42,900 So how do I form a five-membered ring? 617 00:40:49,610 --> 00:40:54,770 So if I take the carbon here from carbon 1, 2, 3, 4, 618 00:40:54,770 --> 00:41:00,500 5, the hydroxyl from carbon 5, form a ring there. 619 00:41:00,500 --> 00:41:03,020 Now, I get this molecule. 620 00:41:25,350 --> 00:41:30,410 So there's 1, 2, 3, 4, 5, 6. 621 00:41:30,410 --> 00:41:30,950 OK. 622 00:41:30,950 --> 00:41:34,220 Or if I now turn this so that I draw it 623 00:41:34,220 --> 00:41:52,270 in the way you're probably more used to seeing it, 624 00:41:52,270 --> 00:41:55,250 I'm going to use the shorthand here. 625 00:41:55,250 --> 00:42:03,750 So this here would be carbon 1, 2, 3, 4, 5, 6-- 626 00:42:03,750 --> 00:42:09,030 hydroxyl from carbon 5, forming a bond to carbon 2. 627 00:42:09,030 --> 00:42:13,140 That creates a new stereocenter at carbon 2. 628 00:42:13,140 --> 00:42:15,430 OH group is pointing up. 629 00:42:15,430 --> 00:42:19,230 So this is a beta sugar-- 630 00:42:19,230 --> 00:42:20,890 OH group pointing up. 631 00:42:20,890 --> 00:42:28,410 So this here, as I drew it, would be beta D-fructo 632 00:42:28,410 --> 00:42:31,260 and this is a furanose. 633 00:42:31,260 --> 00:42:32,100 Why is that? 634 00:42:32,100 --> 00:42:35,190 Because the organic molecule that's 635 00:42:35,190 --> 00:42:39,240 a five-membered ring with an oxygen in it is a furan. 636 00:42:39,240 --> 00:42:44,340 And so the five-membered ring is referred to as a furanose-- 637 00:42:44,340 --> 00:42:46,545 beta D-fructofuranose. 638 00:42:46,545 --> 00:42:48,570 If I'd drawn it with the OH group pointing down, 639 00:42:48,570 --> 00:42:51,610 it would be alpha D-fructofuranose. 640 00:42:51,610 --> 00:42:52,110 OK. 641 00:42:52,110 --> 00:42:54,840 There's another possible ring I can do. 642 00:42:54,840 --> 00:43:01,590 And instead, I take the hydroxyl from carbon 6 and do that-- 643 00:43:01,590 --> 00:43:02,700 form a ring. 644 00:43:02,700 --> 00:43:04,680 Now I'm going to form a six-membered ring. 645 00:43:31,530 --> 00:43:37,450 We have carbon 1, 2, 3, 4, 5, 6. 646 00:43:37,450 --> 00:43:39,420 So if I now turn this-- 647 00:44:00,380 --> 00:44:04,540 so this, here, now I drew the OH group pointing down. 648 00:44:04,540 --> 00:44:08,270 So it's an alpha. 649 00:44:08,270 --> 00:44:10,430 If I drew it up, it would be beta. 650 00:44:10,430 --> 00:44:13,000 So this is alpha D-fructofuranose-- 651 00:44:19,230 --> 00:44:24,240 six-membered ring version of fructose, five-membered ring 652 00:44:24,240 --> 00:44:25,780 version of fructose. 653 00:44:25,780 --> 00:44:30,900 So lots of non-equivalent ways I can draw fructose. 654 00:44:30,900 --> 00:44:33,990 And it turns out, these actually matter. 655 00:44:33,990 --> 00:44:38,670 And they matter for real things that probably matter to you. 656 00:44:38,670 --> 00:44:42,150 And so I brought with me here two different sweeteners. 657 00:44:42,150 --> 00:44:43,740 This is corn syrup. 658 00:44:43,740 --> 00:44:44,970 This is honey. 659 00:44:44,970 --> 00:44:47,220 Has anyone ever-- I'm sure most of you have had honey. 660 00:44:47,220 --> 00:44:50,740 Has anyone ever tasted corn syrup? 661 00:44:50,740 --> 00:44:51,240 Come on. 662 00:44:51,240 --> 00:44:53,300 Someone's tasted corn syrup. 663 00:44:53,300 --> 00:44:55,290 Is a sweet? 664 00:44:55,290 --> 00:44:57,250 Which one's sweeter? 665 00:44:57,250 --> 00:45:00,102 Honey, by far-- much, much, much sweeter. 666 00:45:00,102 --> 00:45:02,560 I used to have it so you guys could come up and taste them. 667 00:45:02,560 --> 00:45:03,640 But I couldn't come up with a way 668 00:45:03,640 --> 00:45:05,020 to do that in a sanitary way. 669 00:45:05,020 --> 00:45:05,990 So I gave up on it. 670 00:45:05,990 --> 00:45:09,280 But nonetheless-- much sweeter than that. 671 00:45:09,280 --> 00:45:11,157 It turns out, these are not pure fructose. 672 00:45:11,157 --> 00:45:12,490 They're a combination of sugars. 673 00:45:12,490 --> 00:45:14,650 But their composition is actually, 674 00:45:14,650 --> 00:45:17,620 from a chemical standpoint, similar amounts 675 00:45:17,620 --> 00:45:20,260 of fructose in each one. 676 00:45:20,260 --> 00:45:26,450 And it turns out that honey is beta D-fructopyranose. 677 00:45:26,450 --> 00:45:30,910 Whereas corn syrup is beta D-fructofuranose. 678 00:45:30,910 --> 00:45:31,810 All right. 679 00:45:31,810 --> 00:45:35,575 So same sugar, different structure-- one's a furanose, 680 00:45:35,575 --> 00:45:37,090 one's a pyranose-- 681 00:45:37,090 --> 00:45:41,020 tastes very differently to you, one being much more sweet, 682 00:45:41,020 --> 00:45:43,720 one being much less sweet. 683 00:45:43,720 --> 00:45:44,950 OK. 684 00:45:44,950 --> 00:45:49,330 Now, of course, I also want to talk about ribose because it's 685 00:45:49,330 --> 00:45:51,760 the thing that you guys are going 686 00:45:51,760 --> 00:45:56,120 to talk about the most next because it's in nucleic acids. 687 00:45:56,120 --> 00:45:59,380 And so just as a reminder, here's ribose. 688 00:45:59,380 --> 00:46:01,870 It's a aldose and a pentose. 689 00:46:09,610 --> 00:46:12,550 So this is D-ribose. 690 00:46:12,550 --> 00:46:17,500 Ribose, as from high school, forms five-membered rings. 691 00:46:17,500 --> 00:46:22,360 That's because it links alcohol on carbon 4 692 00:46:22,360 --> 00:46:24,760 to the aldehyde on carbon 1. 693 00:46:24,760 --> 00:46:29,290 And that gives you this ring structure. 694 00:46:39,970 --> 00:46:41,090 Now number of carbons-- 695 00:46:41,090 --> 00:46:44,180 1, 2, 3, 4-- 696 00:46:44,180 --> 00:46:47,900 alcohol on carbon 4, forming a ring 697 00:46:47,900 --> 00:46:49,790 to the aldehyde on carbon 1. 698 00:46:49,790 --> 00:46:52,820 This is carbon 5. 699 00:46:52,820 --> 00:46:56,370 And the way I drew it, OH group pointing up-- 700 00:46:56,370 --> 00:47:04,220 so this is beta D-ribofuranose would be the proper way 701 00:47:04,220 --> 00:47:05,220 to have it. 702 00:47:05,220 --> 00:47:08,615 And so when you guys talk about this in DNA, 703 00:47:08,615 --> 00:47:11,060 well, the base is going to be linked to carbon 1. 704 00:47:11,060 --> 00:47:14,270 It's going to replace that hydroxyl group-- 705 00:47:14,270 --> 00:47:17,900 the beta form of the hydroxyl group with the nitrogen. 706 00:47:17,900 --> 00:47:20,270 And you're going to talk about bonds being from the 5 707 00:47:20,270 --> 00:47:21,850 prime or the 3 prime end. 708 00:47:21,850 --> 00:47:23,630 Those are those hydroxyl groups. 709 00:47:23,630 --> 00:47:25,610 That's where 5 prime and 3 prime comes 710 00:47:25,610 --> 00:47:30,510 from because those are the 5 and the 3 position on ribose. 711 00:47:30,510 --> 00:47:32,520 Great. 712 00:47:32,520 --> 00:47:33,370 Perfect. 713 00:47:33,370 --> 00:47:33,870 All right. 714 00:47:33,870 --> 00:47:42,090 So we will return to carbohydrates in great detail. 715 00:47:42,090 --> 00:47:47,130 We'll discuss how we combine different carbohydrates, 716 00:47:47,130 --> 00:47:49,830 different monosaccharides to make disaccharides 717 00:47:49,830 --> 00:47:53,880 and polysaccharides, how these different structural properties 718 00:47:53,880 --> 00:47:58,560 end up mattering for things like energy storage 719 00:47:58,560 --> 00:48:02,790 as well as to produce various structural molecules that 720 00:48:02,790 --> 00:48:07,410 can be important for different cells in organisms. 721 00:48:07,410 --> 00:48:11,610 But for the remaining part of the class today, 722 00:48:11,610 --> 00:48:14,640 I really want to shift topics to now discuss 723 00:48:14,640 --> 00:48:18,810 a completely different class of biomolecules. 724 00:48:18,810 --> 00:48:20,640 And that's lipids. 725 00:48:20,640 --> 00:48:23,370 So as I said earlier, the reason we're going to do this 726 00:48:23,370 --> 00:48:28,530 is because coming up for Professor Yaffe's lectures, 727 00:48:28,530 --> 00:48:32,010 lipids end up being really important molecules 728 00:48:32,010 --> 00:48:34,320 for various aspects of cell signaling. 729 00:48:34,320 --> 00:48:36,760 They're also very important for energy transduction, 730 00:48:36,760 --> 00:48:41,250 which is why I'm talking about them as well. 731 00:48:41,250 --> 00:48:43,350 We will spend a lot of time later talking 732 00:48:43,350 --> 00:48:45,270 about lipids in great detail. 733 00:48:45,270 --> 00:48:47,310 But really, what I want to focus on today 734 00:48:47,310 --> 00:48:50,820 is how lipids are used to create membranes. 735 00:48:50,820 --> 00:48:55,500 That is, barriers that really separate the outside world 736 00:48:55,500 --> 00:48:59,070 from the inside of cells, things that make compartments 737 00:48:59,070 --> 00:49:02,910 within cells, and these things also 738 00:49:02,910 --> 00:49:07,390 end up being surfaces where you can have key things happen. 739 00:49:07,390 --> 00:49:09,120 So you reduce spatial complexity if you 740 00:49:09,120 --> 00:49:12,030 go to a 2D surface versus a 3D surface, 741 00:49:12,030 --> 00:49:15,510 which is part of why they're so important in signal 742 00:49:15,510 --> 00:49:16,890 transduction. 743 00:49:16,890 --> 00:49:20,160 But to talk about these, I have to introduce, first 744 00:49:20,160 --> 00:49:24,450 of all, what is a lipid? 745 00:49:24,450 --> 00:49:27,180 So lipids are a class of molecules. 746 00:49:27,180 --> 00:49:29,280 You think of them as fats. 747 00:49:29,280 --> 00:49:32,340 That's how they fit in the nutritional standpoint of what 748 00:49:32,340 --> 00:49:34,060 we'll all talk about. 749 00:49:34,060 --> 00:49:36,630 But first, I want to give a general definition 750 00:49:36,630 --> 00:49:38,760 of what a lipid is. 751 00:49:38,760 --> 00:49:41,280 Now there's lots of different classes of lipids. 752 00:49:41,280 --> 00:49:44,020 And we'll cover these later in the course. 753 00:49:44,020 --> 00:49:46,380 But in general, at the highest level, 754 00:49:46,380 --> 00:49:50,250 almost all lipids consist of two pieces-- 755 00:49:50,250 --> 00:49:53,760 consist of something called a fatty acid that's 756 00:49:53,760 --> 00:49:58,710 esterified to an alcohol. 757 00:49:58,710 --> 00:50:01,710 So what is a fatty acid? 758 00:50:01,710 --> 00:50:07,770 Well, a fatty acid is really any molecule that 759 00:50:07,770 --> 00:50:11,790 has a carboxylic acid group. 760 00:50:11,790 --> 00:50:14,160 So there's a carboxylic acid. 761 00:50:14,160 --> 00:50:17,100 And then hooked up to that carboxylic acid 762 00:50:17,100 --> 00:50:25,380 is a whole bunch of saturated hydrocarbons-- alkyl chains. 763 00:50:25,380 --> 00:50:31,410 Oftentimes, fatty acid would be drawn like this because of all 764 00:50:31,410 --> 00:50:34,590 the different saturated chains. 765 00:50:34,590 --> 00:50:39,060 So acid group on one end, really greasy alkyl 766 00:50:39,060 --> 00:50:41,390 chain on the other. 767 00:50:41,390 --> 00:50:47,520 And it turns out that this can be esterified to an alcohol. 768 00:50:47,520 --> 00:50:51,320 So here's just a generic alcohol. 769 00:50:51,320 --> 00:50:59,710 And as you certainly remember from organic chemistry, 770 00:50:59,710 --> 00:51:03,640 I can use an alcohol and an acid to form an ester. 771 00:51:03,640 --> 00:51:05,170 And so then that would be-- 772 00:51:11,310 --> 00:51:17,370 so now, I have, basically, this ester bond. 773 00:51:17,370 --> 00:51:21,180 And this is esterification between some lipid species 774 00:51:21,180 --> 00:51:26,130 and a fatty acid, in general, is the general thing 775 00:51:26,130 --> 00:51:31,360 that leads to a class of molecules known as lipids. 776 00:51:31,360 --> 00:51:37,290 Now why this is particularly useful for biology 777 00:51:37,290 --> 00:51:44,130 is that this long, greasy alkyl chain is not water soluble. 778 00:51:44,130 --> 00:51:46,980 So if you take oil from your cabinet-- 779 00:51:46,980 --> 00:51:49,800 canola oil, or olive oil, or whatever, and you 780 00:51:49,800 --> 00:51:52,880 pour it in water, what happens? 781 00:51:52,880 --> 00:51:54,330 It doesn't mix together. 782 00:51:54,330 --> 00:51:58,870 You get these globs of oil floating in the water. 783 00:51:58,870 --> 00:52:02,730 And so if you want to form a barrier between two 784 00:52:02,730 --> 00:52:06,330 aqueous compartments, a way to do this is to have, 785 00:52:06,330 --> 00:52:08,370 basically, a hydrophobic layer-- 786 00:52:08,370 --> 00:52:15,150 a membrane-- basically separate the two aqueous compartments, 787 00:52:15,150 --> 00:52:17,980 the inside and the outside of the cell. 788 00:52:17,980 --> 00:52:23,940 Now if I take a lipid, and the lipid, I form this ester, 789 00:52:23,940 --> 00:52:29,020 and it doesn't have a charged group anywhere on the molecule, 790 00:52:29,020 --> 00:52:35,250 this is sometimes referred to as a neutral lipid. 791 00:52:35,250 --> 00:52:37,050 So what do I mean by neutral lipid? 792 00:52:37,050 --> 00:52:41,580 It's basically taking this, which is intrinsically 793 00:52:41,580 --> 00:52:44,220 a fatty acid, is intrinsically a polar molecule. 794 00:52:44,220 --> 00:52:48,120 It has this carboxylic acid on the end of it. 795 00:52:48,120 --> 00:52:50,910 But if I form an ester linkage to an alcohol, 796 00:52:50,910 --> 00:52:53,370 and the alcohol has no charge on it, 797 00:52:53,370 --> 00:52:55,710 now, it's just this really greasy molecule. 798 00:52:55,710 --> 00:52:58,860 And it turns out, that's exactly what is in your olive oil 799 00:52:58,860 --> 00:53:01,590 or in your canola oil in your cabinet. 800 00:53:01,590 --> 00:53:05,310 It's basically a bunch of fatty acids-- 801 00:53:05,310 --> 00:53:09,780 long alkyl chains esterified to non-charged alcohols, 802 00:53:09,780 --> 00:53:14,550 which makes this neutral lipid, this greasy molecule, which 803 00:53:14,550 --> 00:53:17,910 is great for energy storage in plants, something 804 00:53:17,910 --> 00:53:19,600 we'll talk about. 805 00:53:19,600 --> 00:53:21,570 But if you want to mix it together with water, 806 00:53:21,570 --> 00:53:22,920 it doesn't do very well. 807 00:53:22,920 --> 00:53:24,420 So if you go to make salad dressing, 808 00:53:24,420 --> 00:53:26,340 and you pour your oil in with your vinegar, 809 00:53:26,340 --> 00:53:28,770 the aqueous thing, you get a bunch of droplets. 810 00:53:28,770 --> 00:53:30,875 You don't get a solution. 811 00:53:30,875 --> 00:53:32,250 And if you think about it, that's 812 00:53:32,250 --> 00:53:35,220 not very good at generating an interface between two 813 00:53:35,220 --> 00:53:36,065 compartments. 814 00:53:36,065 --> 00:53:37,440 You just get a bunch of droplets. 815 00:53:37,440 --> 00:53:39,390 You actually don't get an interface 816 00:53:39,390 --> 00:53:41,760 between two compartments. 817 00:53:41,760 --> 00:53:43,410 So if you want to make an interface, 818 00:53:43,410 --> 00:53:44,970 you have to do something different. 819 00:53:44,970 --> 00:53:50,100 You need to have a charge group, like on this fatty acid, that's 820 00:53:50,100 --> 00:53:55,360 going to be happy sticking towards the aqueous side 821 00:53:55,360 --> 00:53:59,440 and then a hydrophobic portion that's going to be happy 822 00:53:59,440 --> 00:54:02,410 sticking to form a barrier. 823 00:54:02,410 --> 00:54:05,350 Effectively, that's what soap is. 824 00:54:05,350 --> 00:54:08,380 So if you want to wash your hands and use canola oil, 825 00:54:08,380 --> 00:54:11,013 it doesn't work very well, right? 826 00:54:11,013 --> 00:54:13,180 But what happens if you have a bunch of greasy stuff 827 00:54:13,180 --> 00:54:16,330 in the water, and you take a drop of dawn dish detergent 828 00:54:16,330 --> 00:54:17,740 and drop it in the water? 829 00:54:17,740 --> 00:54:21,160 You get this immediate barrier that forms 830 00:54:21,160 --> 00:54:24,460 across the top as you get this film where you basically 831 00:54:24,460 --> 00:54:26,980 align up all these charged hydrophilic parts 832 00:54:26,980 --> 00:54:31,000 to the aqueous pieces and all of the greasy parts 833 00:54:31,000 --> 00:54:33,550 to the hydrophobic side. 834 00:54:33,550 --> 00:54:38,860 So you end up having this nice charged molecule 835 00:54:38,860 --> 00:54:40,840 with this long hydrophobic part. 836 00:54:40,840 --> 00:54:49,320 So you have a hydrophilic end and a hydrophobic end. 837 00:54:52,830 --> 00:54:55,890 And that ends up being very useful as a way 838 00:54:55,890 --> 00:55:01,800 to form an aqueous hydrophobic interface. 839 00:55:01,800 --> 00:55:04,590 And this is exactly how soap works, 840 00:55:04,590 --> 00:55:06,750 as I'm sure you've learned in other classes. 841 00:55:06,750 --> 00:55:09,570 As an aside, you know how you make soap? 842 00:55:09,570 --> 00:55:14,490 You basically boil the neutral lipids in lye. 843 00:55:14,490 --> 00:55:16,350 So boiling it in base, if you remember 844 00:55:16,350 --> 00:55:19,620 from organic chemistry, will break the ester linkage. 845 00:55:19,620 --> 00:55:21,630 And now, you have these molecules. 846 00:55:21,630 --> 00:55:23,610 And that's how you make soap. 847 00:55:23,610 --> 00:55:24,570 All right. 848 00:55:24,570 --> 00:55:27,180 Now the way biology does this is they actually 849 00:55:27,180 --> 00:55:32,040 assemble lipids that effectively have that same property, where, 850 00:55:32,040 --> 00:55:36,990 basically, they have a head group from the alcohol that's 851 00:55:36,990 --> 00:55:41,040 charged with a hydrophobic group contributed 852 00:55:41,040 --> 00:55:42,920 from these fatty acids. 853 00:55:42,920 --> 00:55:48,460 And this is what allows things to assemble into membranes. 854 00:55:48,460 --> 00:55:52,980 And so membranes are largely made up of-- and certainly, 855 00:55:52,980 --> 00:55:56,400 for the purposes of the upcoming lectures-- 856 00:55:56,400 --> 00:55:58,240 so-called phospholipids. 857 00:56:01,140 --> 00:56:03,063 So what is a phospholipid? 858 00:56:03,063 --> 00:56:07,800 Well, on a phospholipid, the alcohol part of the lipid 859 00:56:07,800 --> 00:56:13,110 is derived from a molecule called glycerol. 860 00:56:13,110 --> 00:56:15,540 So glycerol looks like this. 861 00:56:28,060 --> 00:56:28,560 OK. 862 00:56:28,560 --> 00:56:30,060 So this is glycerol. 863 00:56:30,060 --> 00:56:31,480 All right. 864 00:56:31,480 --> 00:56:35,470 3-carbon molecule-- three alcohols-- 865 00:56:35,470 --> 00:56:36,520 actually, very similar. 866 00:56:36,520 --> 00:56:39,100 If you look back in your notes to dihydroxyacetone, 867 00:56:39,100 --> 00:56:42,160 the difference between glycerol and dihydroxyacetone 868 00:56:42,160 --> 00:56:45,790 is carbon 2 was an alcohol here before it was a ketone. 869 00:56:45,790 --> 00:56:49,720 Turns out, that's where glycerol comes from-- dihydroxyacetone 870 00:56:49,720 --> 00:56:52,120 getting turned into glycerol. 871 00:56:52,120 --> 00:56:54,010 And basically, what a phospholipid 872 00:56:54,010 --> 00:56:58,080 is that you esterify a fatty acid 873 00:56:58,080 --> 00:57:03,390 to two of the alcohols and another phosphate 874 00:57:03,390 --> 00:57:08,560 and charged alcohol to the other hydroxyl group. 875 00:57:08,560 --> 00:57:09,670 So what do I mean by that? 876 00:57:09,670 --> 00:57:15,630 So like this-- so here's ester linkage number one. 877 00:57:15,630 --> 00:57:18,330 I'll draw it this way just for ease. 878 00:57:18,330 --> 00:57:20,790 This is the carbon 2. 879 00:57:20,790 --> 00:57:24,990 Here's ester linkage number two. 880 00:57:24,990 --> 00:57:29,580 So that's a fatty acid esterified to 2 and 3. 881 00:57:29,580 --> 00:57:35,550 And then you now make an ester to a phosphate. 882 00:57:39,870 --> 00:57:43,100 So there's a phosphate at carbon 1. 883 00:57:43,100 --> 00:57:47,420 And then you make another ester linkage to another alcohol 884 00:57:47,420 --> 00:57:50,810 where R equals alcohol. 885 00:57:50,810 --> 00:57:54,800 And that alcohol also turns out to be charged. 886 00:57:54,800 --> 00:58:02,000 And so the most common phospholipid membrane lipid 887 00:58:02,000 --> 00:58:12,840 is a molecule called phosphatidylcholine, 888 00:58:12,840 --> 00:58:17,975 which is that structure with the R group being this alcohol. 889 00:58:29,540 --> 00:58:31,410 And so this is choline. 890 00:58:34,580 --> 00:58:41,250 OK and so phosphatidylcholine is basically that structure. 891 00:59:10,750 --> 00:59:11,250 OK. 892 00:59:11,250 --> 00:59:14,800 So this is phosphatidylcholine drawn out in all of its glory. 893 00:59:14,800 --> 00:59:22,760 So you have this hydrophilic end. 894 00:59:22,760 --> 00:59:24,500 Here, you have a hydrophobic end. 895 00:59:30,260 --> 00:59:30,960 That's good. 896 00:59:30,960 --> 00:59:32,780 This can point towards the aqueous side. 897 00:59:32,780 --> 00:59:35,660 This can point towards the greasy side 898 00:59:35,660 --> 00:59:38,780 and make a nice interface. 899 00:59:38,780 --> 00:59:43,190 And so another common word for phosphatidylcholine-- 900 00:59:43,190 --> 00:59:44,750 an old word for it-- 901 00:59:44,750 --> 00:59:48,580 is a molecule called lecithin. 902 00:59:48,580 --> 00:59:52,235 Anyone read the labels on the side of your food ever? 903 00:59:52,235 --> 00:59:53,830 In fact, there's an old commercial 904 00:59:53,830 --> 00:59:55,090 that makes fun of this. 905 00:59:55,090 --> 00:59:56,030 Soy lecithin? 906 00:59:56,030 --> 00:59:56,530 What's that? 907 00:59:56,530 --> 00:59:59,050 I don't want that in my ice cream. 908 00:59:59,050 --> 01:00:00,190 Well, what is lecithin? 909 01:00:00,190 --> 01:00:01,450 It's phosphatidylcholine. 910 01:00:01,450 --> 01:00:03,830 It's basically, why is this done? 911 01:00:03,830 --> 01:00:05,950 Well, it's commonly added to ice cream 912 01:00:05,950 --> 01:00:07,090 because what is ice cream? 913 01:00:07,090 --> 01:00:09,490 And it's an emulsion between fat, 914 01:00:09,490 --> 01:00:14,650 the cream, and sugar, which is pretty aqueous solubles, 915 01:00:14,650 --> 01:00:17,860 you might guess, from looking at what we drew earlier today. 916 01:00:17,860 --> 01:00:20,050 And so by putting phosphatidylcholine 917 01:00:20,050 --> 01:00:24,640 in your ice cream, you basically stabilize this emulsion 918 01:00:24,640 --> 01:00:26,680 between the aqueous and the fat face. 919 01:00:26,680 --> 01:00:28,930 So this is why it's added to lots of food. 920 01:00:28,930 --> 01:00:32,350 And in fact, it says, soy lecithin, an emulsifier. 921 01:00:32,350 --> 01:00:34,360 That's why it's added. 922 01:00:34,360 --> 01:00:35,650 You can do this yourself. 923 01:00:35,650 --> 01:00:38,363 This is a common trick that many cooks know. 924 01:00:38,363 --> 01:00:40,780 So if you make salad dressing, you put a little mayonnaise 925 01:00:40,780 --> 01:00:42,760 in your salad dressing. 926 01:00:42,760 --> 01:00:43,750 Why do you do that? 927 01:00:43,750 --> 01:00:45,130 Well, mayonnaise has eggs in it. 928 01:00:45,130 --> 01:00:47,410 Eggs have a lot of phosphatidylcholine. 929 01:00:47,410 --> 01:00:50,920 That mayonnaise that you add into your salad dressing 930 01:00:50,920 --> 01:00:53,410 basically stabilizes that emulsion 931 01:00:53,410 --> 01:00:57,100 between the oil in the vinegar and helps it be more stable 932 01:00:57,100 --> 01:01:00,830 and distribute better across your lettuce. 933 01:01:00,830 --> 01:01:02,410 I think my favorite example of this 934 01:01:02,410 --> 01:01:04,450 is, who's made chocolate chip cookies? 935 01:01:04,450 --> 01:01:05,740 Everyone's done this, right? 936 01:01:05,740 --> 01:01:06,560 OK. 937 01:01:06,560 --> 01:01:08,930 So how do you make chocolate chip cookies? 938 01:01:08,930 --> 01:01:13,120 So you take butter and sugar, all right, fat, and something 939 01:01:13,120 --> 01:01:14,200 that's very non-- 940 01:01:14,200 --> 01:01:17,035 very polar, aqueous soluble sugar. 941 01:01:17,035 --> 01:01:18,410 And you try to mix them together. 942 01:01:18,410 --> 01:01:19,840 You have to cream it, right? 943 01:01:19,840 --> 01:01:21,680 And that takes forever. 944 01:01:21,680 --> 01:01:23,350 It's a pain in the butt. 945 01:01:23,350 --> 01:01:25,767 It doesn't come together very well. 946 01:01:25,767 --> 01:01:27,850 And so you get lazy, and you put the egg in there. 947 01:01:27,850 --> 01:01:31,000 And then it all comes together really, really easily. 948 01:01:31,000 --> 01:01:32,020 So why is that? 949 01:01:32,020 --> 01:01:35,130 The egg is the emulsifier that actually brings it together. 950 01:01:35,130 --> 01:01:36,880 Whereas trying to get the butter and sugar 951 01:01:36,880 --> 01:01:39,070 to make your cookies nice and fluffy 952 01:01:39,070 --> 01:01:40,760 is a lot harder because you're really 953 01:01:40,760 --> 01:01:42,760 trying to get these two things to come together. 954 01:01:42,760 --> 01:01:45,820 And it's basically this exact chemistry 955 01:01:45,820 --> 01:01:50,540 that's taking place when you're doing that act of cooking. 956 01:01:50,540 --> 01:01:51,520 All right. 957 01:01:51,520 --> 01:01:57,850 Now phosphatidylcholine is not the only phospholipid 958 01:01:57,850 --> 01:01:59,890 found in membranes. 959 01:01:59,890 --> 01:02:05,150 But I'll just quickly mention what a couple of the other ones 960 01:02:05,150 --> 01:02:05,650 are. 961 01:02:13,070 --> 01:02:18,050 So it turns out you have two other major phospholipids 962 01:02:18,050 --> 01:02:21,110 by abundance and then another phospholipid that's 963 01:02:21,110 --> 01:02:22,610 really important for signaling. 964 01:02:22,610 --> 01:02:24,750 So I'll list them here first. 965 01:02:24,750 --> 01:02:43,710 So there's phosphatidylserine, phosphatidylethanolamine 966 01:02:43,710 --> 01:02:45,870 These, it turns out, phosphatidylcholine, 967 01:02:45,870 --> 01:02:48,150 phosphatidylserine, phosphatidylethanolamine is 968 01:02:48,150 --> 01:02:51,390 what makes up the majority of the phospholipids in cell 969 01:02:51,390 --> 01:02:52,420 membranes. 970 01:02:52,420 --> 01:02:55,440 And then there is a much more minor phospholipid 971 01:02:55,440 --> 01:02:59,770 that's important for signaling, which is phosphatidylinositol. 972 01:03:02,280 --> 01:03:03,330 So what are these? 973 01:03:03,330 --> 01:03:05,340 So serine, ethanolamine, and inositol 974 01:03:05,340 --> 01:03:09,540 are just, like choline, different alcohols 975 01:03:09,540 --> 01:03:12,540 that can be esterified to the phosphate in exactly 976 01:03:12,540 --> 01:03:15,810 the same way I did for phosphatidylcholine. 977 01:03:15,810 --> 01:03:18,840 And so what's phosphatidylserine? 978 01:03:18,840 --> 01:03:21,720 So if you remember, this is-- 979 01:03:30,600 --> 01:03:34,230 so this here is the amino acid serine. 980 01:03:34,230 --> 01:03:35,760 I'm sure you remember that. 981 01:03:35,760 --> 01:04:00,230 If we esterify that alcohol to the phosphate, 982 01:04:00,230 --> 01:04:02,180 there's phosphatidylserine. 983 01:04:15,310 --> 01:04:16,930 What's ethanolamine look like? 984 01:04:16,930 --> 01:04:20,125 So ethanolamine is this alcohol. 985 01:04:29,920 --> 01:04:32,020 So that's ethanolamine. 986 01:04:40,570 --> 01:04:49,130 Esterify the alcohol to phosphate. 987 01:04:49,130 --> 01:04:52,100 Esterify that phosphate to glycerol-- 988 01:05:02,310 --> 01:05:05,040 phosphatidylethanolamine. 989 01:05:05,040 --> 01:05:08,820 And the last one is inositol. 990 01:05:08,820 --> 01:05:10,680 So what is inositol? 991 01:05:10,680 --> 01:05:22,070 Well, inositol is a six-membered hydrocarbon rain 992 01:05:22,070 --> 01:05:31,370 where all of the carbons have an alcohol on them. 993 01:05:31,370 --> 01:05:35,990 And so if I esterify one of these alcohols on inositol, 994 01:05:35,990 --> 01:05:36,640 versus a-- 995 01:05:41,580 --> 01:05:45,530 if I esterify one of those alcohols 996 01:05:45,530 --> 01:05:50,420 to a phosphate to make phosphatidylinositol, 997 01:05:50,420 --> 01:05:54,530 turns out that that ends up being a really useful lipid 998 01:05:54,530 --> 01:05:56,150 to make for signaling. 999 01:05:56,150 --> 01:05:59,540 And you'll hear a lot about this in a couple of contexts, 1000 01:05:59,540 --> 01:06:03,360 I believe, from Professor Yaffe later in the course. 1001 01:06:03,360 --> 01:06:03,860 All right. 1002 01:06:03,860 --> 01:06:12,695 So this having these general membrane structure 1003 01:06:12,695 --> 01:06:16,370 or this general phospholipid structure, 1004 01:06:16,370 --> 01:06:20,360 where you have this hydrophobic part of the molecule 1005 01:06:20,360 --> 01:06:22,970 and the hydrophilic part of the molecule 1006 01:06:22,970 --> 01:06:26,870 is really what allows these lipids to come together 1007 01:06:26,870 --> 01:06:34,070 to form these membrane bilayers that are often drawn like this. 1008 01:06:34,070 --> 01:06:37,130 And so when I'm drawing it like this, 1009 01:06:37,130 --> 01:06:40,670 basically, what those two wavy lines 1010 01:06:40,670 --> 01:06:47,420 represent-- those are the hydrophobic fatty acid parts, 1011 01:06:47,420 --> 01:06:53,865 and this is the hydrophilic so-called head group. 1012 01:06:57,270 --> 01:07:03,222 That's the charged alcohol stuck on to the end of the-- 1013 01:07:03,222 --> 01:07:07,200 to the phosphate on the glycerol. 1014 01:07:07,200 --> 01:07:09,240 And these, of course, assemble such 1015 01:07:09,240 --> 01:07:12,930 that all of the hydrophilic head groups 1016 01:07:12,930 --> 01:07:18,075 face an aqueous compartment on either side. 1017 01:07:23,050 --> 01:07:23,740 OK. 1018 01:07:23,740 --> 01:07:30,770 And then you have this nice hydrophobic portion 1019 01:07:30,770 --> 01:07:31,860 that is a membrane. 1020 01:07:31,860 --> 01:07:34,520 And that is what can create a barrier 1021 01:07:34,520 --> 01:07:39,720 between two different compartments in cells. 1022 01:07:39,720 --> 01:07:41,430 Now just to put this into context 1023 01:07:41,430 --> 01:07:45,660 for the protein structure stuff that you learned about-- 1024 01:07:45,660 --> 01:07:47,580 so when you learn about all the different ways 1025 01:07:47,580 --> 01:07:49,230 you have protein structure, of course, 1026 01:07:49,230 --> 01:07:51,090 you have amino acids that are hydrophilic, 1027 01:07:51,090 --> 01:07:53,180 amino acids that are hydrophobic. 1028 01:07:53,180 --> 01:07:55,650 If you have a protein that's floating in solution, 1029 01:07:55,650 --> 01:07:57,810 it has all the hydrophilic parts on the outside 1030 01:07:57,810 --> 01:08:00,520 and the hydrophobic parts in the middle. 1031 01:08:00,520 --> 01:08:02,760 But you can imagine that you can also 1032 01:08:02,760 --> 01:08:06,060 have proteins that assemble to sit such 1033 01:08:06,060 --> 01:08:09,812 that they span the membrane-- so hydrophobic parts that interact 1034 01:08:09,812 --> 01:08:11,520 with the hydrophobic part, the inner part 1035 01:08:11,520 --> 01:08:14,860 of the membrane-- hydrophilic parts on either side. 1036 01:08:14,860 --> 01:08:16,020 These can form channels. 1037 01:08:16,020 --> 01:08:20,279 These can form all kinds of ways to move stuff across membranes. 1038 01:08:20,279 --> 01:08:24,300 Or you might think that you might have a protein that 1039 01:08:24,300 --> 01:08:26,430 does something like that-- 1040 01:08:26,430 --> 01:08:29,620 hydrophobic part on one end, hydrophilic part on the other, 1041 01:08:29,620 --> 01:08:34,109 and really floats on the surface of a membrane. 1042 01:08:34,109 --> 01:08:37,290 And you will see that the way cell signaling works 1043 01:08:37,290 --> 01:08:41,880 is that you basically have lots of protein complexes 1044 01:08:41,880 --> 01:08:43,560 that will assemble that membranes-- 1045 01:08:43,560 --> 01:08:45,479 two dimensional space. 1046 01:08:45,479 --> 01:08:49,290 Those membranes, as well as some of the membrane lipids, 1047 01:08:49,290 --> 01:08:52,529 can then act as messengers that allow cells to carry out 1048 01:08:52,529 --> 01:08:53,760 various signals. 1049 01:08:53,760 --> 01:08:58,109 And I will leave that to Professor Yaffe to talk about. 1050 01:08:58,109 --> 01:08:59,430 All right. 1051 01:08:59,430 --> 01:09:02,970 So I will also come back and discuss membranes 1052 01:09:02,970 --> 01:09:06,300 in a very different context in the context of metabolism 1053 01:09:06,300 --> 01:09:08,100 during the second half of the course 1054 01:09:08,100 --> 01:09:10,439 because it turns out, a lot of energy transduction 1055 01:09:10,439 --> 01:09:12,960 and the way cells really store energy also 1056 01:09:12,960 --> 01:09:17,029 ends up being really important with respect to membranes. 1057 01:09:17,029 --> 01:09:17,529 All right. 1058 01:09:17,529 --> 01:09:18,321 You guys are lucky. 1059 01:09:18,321 --> 01:09:20,955 You get out a little bit early today. 1060 01:09:20,955 --> 01:09:22,330 This is the first time that we've 1061 01:09:22,330 --> 01:09:24,550 done the lectures this way, so I don't have my timing 1062 01:09:24,550 --> 01:09:26,590 quite right. 1063 01:09:26,590 --> 01:09:31,330 But I will see you guys again for metabolism 1064 01:09:31,330 --> 01:09:33,360 after spring break.