 Hello, hi everybody. So we have a little bit to finish up from the last lecture. So in terms of the pace of the lecture, sometimes we get a little bit behind, and I think it's nice to have questions in lecture. But I might try by each midterm we got to get caught up because there's a certain set of topics on the midterm. So today it's okay. Today it's more descriptive. So don't feel like you have to come up with questions. Say what is that? It's what's on the screen. But please continue to be as interactive. It makes a pleasure actually to teach a class. So just a few reminders. So there's a variety of events coming up. So invariably for some of these due dates, people forget certain things. So the sapling, when the sapling becomes available, and when it's due, is posted on the Sapling Problem Set page as well as on the saplinglearning.com webpage. So we've set the midterm, is that right? 8 to 10. Yeah. So the midterm review session led by Sarah Taylor and crew is on a Sunday, 8 to 10. We had earlier said I think 7 to 9, but the room was not available. So this is in the Science Center. I have office hours this Thursday and next Thursday, right after class. And the TAs are available to you as contract employees. They're free to make an ad hoc appointment to them. They're required to have office hours, but they don't have it at a particular time. But for one-on-one tutoring, they're available. Of course, you've got conference sections. You can go to more than one conference section if you choose. We have one more sapling problem set. It's due date coincides a little bit close to when the midterm is, but it perhaps will help you in the studying for the midterm. So we've also received notice about the midterm exam location. So this is untenable in terms of taking exams. You're all packed in here like sardines. So we're going to spread out pretty much almost every other seat. So to do that, I need your cooperation. So people with last names from A to M, letter M, come here. And for the others, N through Z, you go to this new place called BERT. So BERT 130, that's Hunter Labs, but they now call it BERT. So I guess hasn't been, nobody has donated money yet to name the new lecture hall. So some of you will be hanging out in BERT for the midterm. Okay, so let's get going. Lots of covers. So we looked at, we started to think about the regulation of this awesome 10 of the 17-fold rate enhancement. How are these enzymes regulated? And a big theme topic throughout the semester will be the regulation of pathways. And so one obvious thing is pathways are sets of enzymes that pass each substrate, each product is the next enzyme substrate. And so you have a cascade. Each of these E1 is a different enzyme than E2 is different than E3 and so forth. And here we have a pathway. These are the metabolites that are worked upon by the enzymes. So we're starting with 3N and we're converting that to isoleucine. So in the amino acid lecture, I will learn in perhaps gory detail about some of the enzymes, a little bit more details. But I wanted to introduce the regulatory logic. So you don't want to convert all your 3N to isoleucine. And then you wouldn't have 3N to make proteins. You need 20 amino acids to make proteins. So as the amount of isoleucine increases, it blocks the first step in the pathway. So do you think that makes sense? It would be less efficient to block perhaps later in the pathway because these metabolites here would tend to accumulate. So we need 3N or isoleucine, not BC or D, right, in the cells. We need those to be available. So this is a concept that you'll see throughout the semester of feedback inhibition. Here's a beautiful enzyme that's catalyzing the transfer of a carbonyl group to an aspartate amino acid to make this molecule. This is called aspartate transcar mammalase, ATC. And so this enzyme, this is the first step in the pathway to make the primitive nucleotide CTP and UTP. And so this enzyme is allosterically inhibited by the end product. So this is the same thing I just showed you, feedback inhibition. But the inhibition itself is inhibited by ATP, a purine nucleotide. Can you imagine why? What would be the rationale why such a balance would be evolved? So what are these molecules used to make DNA or RNA, UTP? So do we need just primitive nucleotides to make DNA? Can we make DNA without purine nucleotides? So the regulation here is balancing the pools of primidines and purines. And this is very similar to the ways that we were thinking about hemoglobin. Now, hemoglobin is not an enzyme. This is an enzyme. Chemical structures are actually changed. But these regulators are allosteric regulators. They're changing the shape of, in this case, the quaternary structure of this enzyme affecting the rate. And so this is a picture of the feedback inhibition. So many steps lead to, after the ATCA enzyme, to the production of CTP that feeds back. But this is actually what's going on here. We have a transition between two states. In this case, it's a little bit different than the way we thought about hemoglobin. What you could say is the open state is the productive state. That's the state that can catalyze the reaction. In this state, a more closed down state is the inhibited state. And so when CTP molecules bound to the regulatory subunits in these flesh tones over here, that causes the enzyme to curl up and not to be able to catalyze the reaction. When CTP is not bound, the enzyme opens up. It's in a more optimal configuration to catalyze the reaction. And so it's a bit confusing because ATP is actually competing at the regulatory site. So you would think if anything binds to a site that causes an enzyme to close down, that it would cause the same effect. But in this case, if ATP binds, the enzyme opens up at this site. And if CTP binds, it closes down. So this is a hetero-oligomer of polypeptides. It's a little bit hard to see. You have two clusters of three catalytic subunits. These are the three catalytic here, three catalytic here. And then the regulatory subunits, you have actually three clusters of two. And you're only seeing two of those here. The other one's pointing back. So it's having this change of shape catalyzed by binding at a site other than the active site affecting what's happening at the active site. So similar to what we saw in hemoglobin. And so what this does is it has the effect of changing the apparent binding affinity of the enzyme for its substrates. And so the binding affinity is increased, right? So remember, the apparent binding affinity is increased. The K.5 is decreased. Remember, it's akin to a dissociation constant and the inverse of an association constant. So affinity is being increased at the enzyme active site for its substrates. When ATP binds at the regulatory site, when CTP binds at the regulatory site, the K.5 increases and therefore the apparent binding affinity decreases. Now these enzymes don't follow strictly McHale, Smith, and Kinetics because they're not hyperbolic curves. The double reciprocal plot would be nonlinear. So you can't really call these a KM. You can call them a K.5. So a certain number of substrate molecules is necessary to fill half the binding sites irrespective of the shape of the curve. And so these particular regulars are affecting the KM and perhaps slightly also affecting the Vmax. But predominantly, the effect on KM is what's important here. You can have a mixture of those two. So that was a little example of some of the regulatory mechanisms that we'll see throughout the semester. But now we're going to switch entirely gears to look at a new language. We've so far looked at the language of proteins, the words, amino acids. Today we're going to be learning the words in the language of lipids. And so there's a variety, perhaps a more diverse array of different words in this language. And so we're going to be looking at the structures of some of those lipids. And you might get a feeling of panic as you see some pretty complicated structures. You're like, good gosh, are you going to just have me write this? I generally have a really bad memory. And so I don't do things to you that I wouldn't want done to me as much as I can. And so I'll try to highlight as we go along what I think is important. So we'll look at the ways in which these lipid words associate into the sentences of more complex structures such as micelles, bilayers. We'll look at different composition of these structures. We'll look at just sort of the wonderful world of lipids. We have some of the chemical properties of these lipids, phase transitions, the concept of fluidity of the membrane. And then we'll look at the fundamental lesson you'll learn today is that membranes are impermeable barriers. In effect, they're resistors. You cannot have molecules easily pass across them. And because they're impermeable barriers, you can set up a lack of symmetry in the type of molecules on either side of the barrier. So we can have protein molecules as we see transporters that can affect the population of molecules on either side of this barrier. And we'll think about this in some of the dynamic terms a bit at the end. Okay, so now a wonderful world of lipids. So we have the most simple lipid is a fatty acid. So you remember amino acids have an amino group and an acid group. Here we just have a fatty group with long aliphatic alkyl chain. And then we have an acid group, the carboxylate. So these are called fatty acids. They can be fully saturated. In other words, not have a double bond. Or they can be partially or multiply unsaturated. So here we have a lipid that has a double bond. Naturally occurring lipids are predominantly in this thermodynamically unfavored configuration of this double bond. You might say that breaks the laws of physics. Well enzymes manipulate which reactions occur. And so enzymes are the molecules that put these double bonds in there. And as it turns out there's benefits to the structures these lipids are going to make to have cis double bonds instead of trans double bonds. So let's think about the fatty acid in the way it might associate with other fatty acids. So here we see a bit of angst, right? So we have these long alkyl chains. You can imagine that there's hydrophobic interactions, a lot of hydrophobic interactions between these chains. But at the top in the terms of fatty acids these would all be negatively charged functionalities all packed into a tight space. So if these were not sort of repelling each other then the membrane would just be a solid, right? You have all these hydrophobic interactions, but because you have this polar head group there's a bit of repulsion and attraction. So that increases the fluidity of the membrane and the fluidity of the membrane is very important to the function of the membrane as we'll see today. So look at what happens when we introduce a unsaturated fatty acid in the cis configuration of this double bond. Do you see how it creates a gap? It tends to disassociate the number of hydrophobic interactions that occurred when we didn't have cis double bond. So you have some hydrophobic interactions here, but then after the kink there's less hydrophobic interactions. So these kinks introduced in lipids affect the fluidity as well. You increase the fluidity of the membrane. What if you had a trans double bond? With a trans double bond you would just have a jog, right? You wouldn't have a kink in the chain. So you would still have a large number of hydrophobic interactions in a trans fatty acid. So it's beneficial to control fluidity by having cis double bond. Okay, this is overwhelming. Don't you think so? First instinct is good. Gosh, do I have to know it all? What's on the exam? Well, let me tell you the things that are important. So first thing, you need to recognize what this means over here. So this is the number of carbon atoms in the fatty acid. So each fatty acid has a number of carbon atoms, colon, the number of double bonds. So in this case there's no double bonds in 12 carbon atoms, including the carboxylate carbon atom. We number a fatty acid starting from the carboxylate as carbon 1 up to carbon 12. You have a systematic name, in this case, in decanoic acid. And then we have a more common name, loric acid. And then we have associated melting points. Okay, a melting point. We'll come back to melting points later on in the lecture. So you can see different fatty acids have different lengths and different number of unsaturations or double bonds. So for example, let's focus. One of the most common fatty acids is this stearic acid. So cow meat has a lot of this carbon 18, 18 carbon long, fully saturated fatty acid. So this is Mr. Cowl comes in. But Mr. Cowl can also have double bonds in this 18 carbon long fatty acid. And we call it oleic acid, and linoleic acid, and linolenic acid. And so with each of these we're adding additional double bonds. And the nomenclature here is specifying which carbon the double bond is attached to. Starting from carbon 1 at the carboxylate, going along to carbon 9, in this case, you have a double bond. Here you have 9, 12, and 9, 12, and 15 perhaps. If I can read it correctly. So there's different numbers of unsaturations and different names, systematic names. So the ones that are most important, most common are the stearic acid, octodecanoic acid, and some of these unsaturated forms. Because those, as it turns out, are not exclusively, you know, the only fatty acids found in membranes, but they predominate in many membranes. Okay, so we'll come back to that. And now we're just going to look at, we've looked at different classes of fatty acids. And now we're going to look at, not just one molecule, classes of molecules. So what if you condense a fatty acid with a fatty alcohol? You're like, what is a fatty alcohol? That's like when you drink too much beer, you get a fatty alcohol right now. So this is a ester-linked fatty acid. Fatty acid, you have a carboxylate, and you have a long aliphatic chain with an alcohol functionality. So it's condensed to make an ester. So this is a fun ole-ole-ate, ole-ate. So this has properties of waxes. So beeswax is the condensation of palmitic acid with one triacid, and all. And so that fatty alcohol condensed with this acid makes beeswax a nice comfy home to collect your honey. So waxes are important in your epidermis. If you didn't have waxes, if you took a shower, you'd get all bloated up, right? But whales would sink to the bottom of the ocean without waxes in their heads. They have the oil, as it turns out, triacilglycerol is mixed with waxes. And this is important as they go to deeper depths. It gets colder, and they wouldn't want to have a change in buoyancy. They want to be neutrally buoyant at whatever temperature. So this is important. The whole head of a sperm whale is actually full of a mixture of waxes and triacilglycerols that you'll see in a bit. So this is just one family. There can be any diversity of different substituents between this acid part and the alcohol part. And these are just some particular examples that might be familiar to us. So I mentioned the sperm whale triacilglycerol. So here's glycerol. Remember glyceraldehyde? Do you remember how that's how we specified the stereochemistry in amino acids? So here we have a similar molecule, glycerol. And you say, oh, that's also an alcohol. I guess you could even call it a fatty alcohol. It's not very fatty. It's sort of short. But each of these alcohol positions can condense with a different fatty acid to make ester linkages. And so this carbon is called prochiral. Do you guys remember what that means, prochiral from Orgo? So it's sort of conditionally chiral. It can be achiral if these two substituents are identical. But if they're different, for the case of many triacilglycerols, they're actually different substituents. This would be a chiral center. And so this is one example of many a plethora of different substituents that can be on the trace of glycerol. Here we have two fully saturated fatty acids of different lengths, and then one unsaturated fatty acid. And so it makes this tripod shape. Reminds me of this. Sort of scary. It's got these deserate eyes that shoot out at you. But it helps you to remember the structure. So in general, it is common to have this one unsaturated fatty acid in the middle. And these are important in storage of fats, as we'll see later in the course. And so lipids form compartmentation for various parts of the cell. So in each of these components, organelles of the cell, you have a certain combination of lipids in the bilayers that regulates the properties of that bilayers. Some of these compartments, for example, need to pinch off little vesicles or to fuse little vesicles. You might imagine, okay, you might need a different complement of lipids to optimize that process through evolution. Also, we want to regulate the transport of molecules across these membranes and these organelles. And we'll see throughout the semester different transporter molecules that are important in this process. So lipids are very important in spatially defining the cell. What goes where? If we didn't have lipids, it'd just be with one large gamish, right? And we can't do some of the things the cell needs to do. Okay, so coming back to fatty acids. So you can have, remember we talked about the sugar cage globular, or the water cage and globular proteins, and you sort of squish down that water cage to form the globular protein. Similar forces drive the structures formed by fatty acids. So here you have, you can imagine, a very unfavorable configuration. You have just a single fatty acid. These water molecules are scared to death, right? They're making all kinds of interactions with other water molecules, and they're not able to make hydrophobic interactions with this long alkyl change. So this really decreases the entropy of the water molecules. Water wants to have more entropy and be changing configuration more fluidly. But then what we can do is we can begin to stack these hydrophobic aliphatic sections of these fatty acids, such that we're decreasing the number of unfavorable interactions with water per fatty acid molecule. Do you see that? So although we still have unfavorable interactions around the periphery here, in between we have a nice stable hydrophobic core, right? But this is still not perfect. We still have an ordered water cage around the outside of this. So what can happen as you gradually increase the concentration of fatty acids? You can begin to make a sphere of myself. Very similar to the structure of a globular protein. So on the surface you have polar, in this case a charged group. And in the middle you have all this hydrophobic core. So it's a planetoid of hydrophobic goodness. Instead of iron you have these long alkyl chains. And the water molecules now really are quite happy. If anything they could interact with these polar head groups, the carboxylate groups. And so at a certain concentration referred to as the critical mice cell concentration, these forms of fatty acids will spontaneously occur. Fatty acids can make mice cells other detergent molecules with a similar structure perhaps a different head group can also make mice cells. So these are some of the structures that can form. So with the fatty acid, the single alkyl chain, if you think about it the head group's a little bit bigger, right? So you have a carboxylate that's sort of wide, and then you have an alkyl chain that's a little bit more narrow. So you can think of this in terms of a cone, a wedge shape. And so you can imagine how that would fit into a sphere, a planetoid of hydrophobic goodness. But if you have two alkyl chains attached to a head group, that's harder to imagine how could that make a cone shape. That's more likely to make a cylindrical shape. So we're not able to adopt a mice cell configuration if we have two alkyl chains. We make what's called a bilayer. And you can take this even further. So you can take the bilayer and wrap it around such that none of the hydrophobic surface here is exposed to the environment. And this is very similar to what cells are doing. So cells have polar head group surface. They've got this hydrophobic layer. And then on the inside you have hydrophilic surface, another polar surface. And so these are called lipid vesicles. Typically when you make these in the lab, you just have one type of lipid. And you're going to see today that it's important for cells to have a large mixture of different lipids to prevent catastrophic phase transitions in these bilayers. So these are just the wonderful world of structures that can be spontaneously occur when you get the right amounts and the right types of lipid molecules. Any questions so far? I'm sort of buzzing through because I think it's less homogenic, is that the word? Yes, a vesicle is... Well, a bilayer could occur, perhaps if you... Yeah, so that's an interesting question. What triggers the transition? Probably the concentration of the lipids is going to play, but it's sort of hard for me to predict just what would be the amount or what would trigger that process. So a lot of times people can layer bilayers like on a glass slide. And so in that case, you're controlling it artificially because you want it to be planar. So if you want a lot of studies to ascertain the physical properties of bilayers are in glass slides. And the assumption is that perhaps that might extrapolate to the lipid vesicle configuration. Great, good question. All right, so if you're going to make a bilayer, if you remember just one thing from this lecture, remember that the bilayer is an impermeable barrier to passage of molecules. The gross majority of molecules cannot pass over this bilayer. You need a series of mechanisms to regulate what molecules are on what side of the bilayers. And so later on in the lecture we're going to be looking at all these different mechanisms of transport. But so this is a necessary sort of consequence of the fact that we have this barrier. We need to be able to transport molecules across the barrier. And so I've made a last minute change to the slide. Some of you might have a different picture here. Please adjust yours. If you've printed today then you won't. You should be good. So this is L glycerol 3-phosphate. Does that numbering bother some people? It bothers me a little bit. Proline has a higher atomic number than oxygen. It's a convention. So this is just what people call this. You could actually name it in either direction. If you named it starting with carbon 1 here, so if you said it would be a D glycerol 1-phosphate. So if you name it from here, and remember the convention for DNL naming is to put carbon 1 at the top. You would sort of rotate this around and the alcohol, the hydroxyl group, would be on the not left, be on the D side of things. And when we draw, you'll see a lot of these products of condensation of glycerol, L-grysol, 3-phosphate with a variety of lipids. When they're drawn here, be aware they're not specifying stereochemistry. So they're not saying where this hydrogen atom is. But in reality, this chain goes this way and this chain goes that way. It just doesn't fit in the picture as well to do it like that. And so this is glycerol-phospholipids. Again, a widely various set of lipid substituents can occur at each of these positions. So we've condensed two fatty acids with glycerol and then we have a phosphate here. And that phosphate can be substituted with a variety of different molecules. And so this is a cylindrical type of lipid. And so this type of lipid is what we see in the lipid bilayer. And so you can have a saturated or an unsaturated fatty acid in the side chain here. And so you can have different lengths. It really is not a single molecule, but a class of molecules. And so if you consider the pKa of the phosphate and some of these head groups, you can calculate a net charge. And so this table is actually from the last edition of the textbook. I find the new edition just makes a simple thing really complicated. So I decided to go back to this older version. So if you have a hydrogen atom here, well actually that would be ionized at physiological pH. And so that would have a negative 2 in that charge. But you can have a variety of substituents at this X position. So the parent member is called phosphatidic acid. Again, phosphatidic acid is not one molecule. It's a class of molecules that has this head group but with a variety of different alkyl chains. So you can have phosphatidyl ethanolamine. So as it turns out, this molecule is called ethanolamine. And then that's a zwitterionic molecule. So it has zero net charge. Phosphatidylcholine is another common class of lipids in bilayers. It has this structure, a quaternary nitrogen is there. So that's positively charged as well. You can stick a serine on backwards. So through the alcohol group, you can condense that alcohol with the phosphate to get phosphatidylserine. So the amino acid, of course, we know in gory detail that amino acids are zwitterionic. They have both a positive and negative charge. You have a phosphatidylglycerol. So you can have a glycerol molecule here, and you can attach another glycerol molecule. You can attach a sugar residue through phosphoester bond. So here is a phosphatidyl nostril 4-5 bisphosphate. And you'll see that that's an important lipid in cell signaling processes. And we'll know in next lecture in gory detail how to number sugars, but we number the sugars from the acetal, I believe this hemiacetal carbon right here. And so 1, 2, 3, 4, 5 bisphosphate, phosphatidyl nostril. We can also make a very elaborate structure where we have a total of three glycerol molecules, one here in the parent group, and then one here and one here. And this has a total of four aliphatic chains, two here and then two additional ones here. So cardiolipin is an important lipid in the heart tissue and cardiomyocytes. So these are all classes of molecules that we're likely to see throughout this semester. So those are the glycerophospholipids. I should highlight the names. Phosphoglyceride is a synonym for a glycerophospholipid. I don't know that I highlighted that, but you can also have glyceroglycolipids. So instead of having a phosphate, have a sugar residue attached to the glycerol. And so this is one of those slides where it creates a little bit of panic. You're like, okay, so you're going to tell us, draw DGDD. No, I'm not going to ask you to do that. That would be cruel and unusual punishment. But you should know what is a glyceroglycolipid. Well, that's where you've attached a sugar residue to the glycerol head chain. So you should know the invariant part of this molecule. So in here, this can vary. So we have glycerol attached here to carbon one of this sugar residue. And you can have different sugars, and those sugars can be decorated with different substitutions. So this is another class of lipids that don't have a phosphate group inside of them. Okay, well, if we can condense things to make an ester, we can also condense things to make an ether. So a wide variety of lipids are called ether lipids because they have a lipid molecule attached to an ether linkage. So here we have a plasmalagen, a carbon one, right? It has an ether linkage to this particular side chain. It has an ester linkage at carbon two, has a phosphate, and then a choline. We just learned that this structure is called choline. You can also have a platelet activating factor, ether link lipid as well. And here you have just a very small fatty acid, about as small as you can get. And you have choline as well. So this is just to introduce to you the concept that we can put these things together in any of the ways you could probably imagine. So we can attach these molecules through ether linkages. Okay, here's another wild and crazy etherlinked lipid. Look at this. Doesn't this look just crazy? You have a glycerol molecule here and one over here. And then you have this long aliphatic chain that's got all these carbon atoms hanging off of it. And so this is similar to a bilayer. It's sort of a bilayer in a molecule. So it's a normal glycerol phospholipid. You have two surfaces of a bilayer and there's not a covalent attachment between the two leaflets of the bilayer. But here, this is fully covalent. So it makes sense that this is going to be stronger, more rigid, more impervious to harsh conditions. So the types of organisms that have these types of lipids are exposed to torturous conditions in Yosemite National Park. So you have archaea bacteria. And so this is actually, you know, so you can see the water is boiling. And with normal glycerol phospholipids, if you were to go into the boiling water, well, you know, you would dissolve. That would not be good. But these organisms are perfectly happy to hang out in nearly boiling water because of the structure of the lipids in their membrane. And so actually in this picture, a human being is right here. So these organisms are a huge colony of these organisms and we're little in comparison. So these are amazing organisms and they get their strength from the lipids in their membranes. Okay, so I mentioned this idea of phosphatidonastatol being important in signaling. So I wanted to introduce a set of enzymes that are important in these signaling processes. So here we have phosphatidonastol 4, 5, this phosphate. And you have phospholipase enzymes that can cleave ester linkages. So people are a little bit confused sometimes when they see this name. Why do you call it a phospholipase? It's not breaking a bond to a phosphate. The molecule has a phosphate. So these are lipases. They break bonds between lipids and head groups. And so this is a phospholipid, so they call it a phospholipase. So phospholipase A1 cleaves ester linkage here, A2 ester linkage here, and phospholipase C cleaves this phosphate ester here. And so this is the important one in cell signaling processes. So the cleavage of this molecule leads to the production of what's called diacylglycerol, right, and 1, 4, 5, enosatol, phosphonosatol. So IP3, enosatol-tryphosphate. And so we have a set of enzymes that add each of these phosphates. They're called kinases. Their substrates are not proteins. Their substrates are lipid molecules, and they add transfer of phosphate to the sugar residue at the 4 and the 5 position. Okay, and then we have this phospholipase, and that cleaves this bond, making enosatol-1, 4, 5, trisphosphate and diacylglycerol. Both of these molecules converge in our necessary for the activation of a critical signaling protein called protein kinase C. So enosatol-1, 4, 5, trisphosphate binds to receptors on the endoplasmic reticulum and causes them to release stores of calcium, which then causes import of calcium from outside of the cell. And then both of these molecules come together and allosterically activate protein kinase C, which is a protein kinase. So its substrates are to phosphorylate other proteins. So this is an important... Lipid molecules are important in bilayers, but they're also important in processes that regulate the communication within cells and between cells and their environment. Any question on phospholipases? All right. You have two questions from the audience. Sure. So one is about... I'm reading this correctly, is why the polar head groups, one of my cell are reticul, is formed. Why the polar head groups don't decrease entropy when they interact with water? Why don't they decrease entropy of the molecule, the micelle, or the water? Yeah, I'm sorry, the water. They... Well, compared to what? I can't ask follow-up questions. Yeah. It would depend what you're comparing it to. I assume compared to interactions with the hydrophobic valophatic chain. Yeah, I don't know that I'm getting the question. Yeah, I'm sorry. Simon, the second one is why do ether lipids are better for organisms in harsh conditions? Ah, because of the covalent bond. So normal lipids form a bilayer, where there's a non-covalent surface of interaction between the two leaflets of the bilayer. I didn't really highlight that so much. So do you see this? So in between these... This is called a leaflet, and this is called leaflet. You usually have an inner and outer leaflet. And so at this interface between the two leaflets, you have non-covalent bonds here. And so if you want to dissolve a membrane, you heat it up, right? That's going to cause more motions, and this is going to be less stable, right? Then if you have covalently linked both sides of both leaflets of the bilayer. That's a great question. I'm sorry I didn't understand the other. Okay. The other? All right, little people in the Yosemite. Okay, so wonderful lipids. We've looked at the function in membrane, some of the structures, the sentences we can put together. Okay, let's continue this. We haven't seen them all. There's more to come. Yes, there's more. So we've looked at so far the glycerol lipids, triacylglycerols, glycerofospholipids. So the foundation is the glycerol molecule, and that's being decorated in a variety of different ways. But now we're going to look at the single lipids. Don't these seem mysterious? You're like, what secrets are held in the single lipids? So they sort of have this L shape in fatty acid, and then they have a head group. So what are the mysteries? Are they from Egypt? The single lipids. So single lipids, if you look at their character, right, they're similar to the glycerol phospholipids if you squint sort of, you know, the only real difference is a little bit different way to connect these. So instead of having an ester linkage here, we have a carbon-carbon bond. And here instead of having an ester, well, we have an amide bond, right, that's formed. So, you know, this is not glycerol. This is a molecule called sphingocene. But, you know, in terms of the properties, you have two alphatic chains, right? And they can be substituted in different ways. So we can have double bonds. So I lie a little bit. Sometimes I lie a little bit. So before, I said, well, exclusively, we really have cis-double bonds in these lipids. But in the single myelin, they tend to have trans-double bonds. And because they have a trans-double bond, they look a little bit different than the glycerol phospholipids. So these have a kink in them, whereas these are linear. So the sphingolipids are found a lot in neural tissue in your brain. And for whatever reason, it's necessary to have this type of structure for those lipids for neurons to function, right? But it sort of looks like the legs. I think people named it according to a late-night dream. So this is the legs in the head group looking out into Cairo. So these are the sphingolipids. Another wide class of lipids that we look at, you might not even think that these are lipids. And these are called sterols. And so sterols, we'll be looking actually at the enzymatic pathways in which sterols are assembled because it's just so beautiful and elegant. But today, we're just learning about what they look like when they're finally produced. And so the founding member, the capstone of the sterols, is the cholesterol. And so do you see how this has properties of a similar to a fatty acid? You have a, in this case, polar, not charged, but a polar head group. And then you have a long aliphatic chain. It's a bit more boldest, right? So it's sort of large and fat. But it's just a long aliphatic chain. So the way these things interact with the membrane is they plop down in the membrane. They tend to nestle into where you have unsaturated fatty acids in that kink. And they sort of lubricate the membrane. So without cholesterol, the membranes would be less stable. So this helps to make the membranes more fluid because it can sort of move around in that membrane. And so this is the founding member of many different molecules that can be derived from cholesterol. So you might have heard of steroid molecules, not typically found in membranes, but important in regulating cellular processes. So you can take cholesterol. And you can see how they might be derived from cholesterol because they have the fundamental skeleton of the thing. It's similar. You have these fused rings and different substituents and different numbers of double bonds. So these molecules are produced and then bound to receptors. And these receptors then go into the cell and activate transcription. So we have steroid... These are called steroid hormones. There's another name for them. But they're very important regulators in functions of cells. But they're all derived. They're lipids and they're derived from cholesterol. And you can sort of see. It makes sense that they probably wouldn't hang out in lipid bilayers because they've got... Many of them have a polar group on either side. They wouldn't say, no, I don't think so. You're not hydrophobic enough or polar enough. Another type of lipids often derived from steroids or in the class of steroids are vitamins. So vitamins are things that we get from our environment or some case we make them. But in many cases we get them from veggies and other things that we consume. And anytime a name has an all in it, it's a steroid-like molecule, so retinol, these other all vitamins. And the one thing that I'd like to remember, I made this large because I like to ask questions about it. So the way that these steroid-like molecules are assembled is from the building block of isoprene. So you can sort of see the isoprenes in here, right? And we'll be looking later on in the course how chemically we can combine isoprene units to make sterile molecules. So isoprenes are commonly found in or used to make steriles. So lipids, vitamins. Okay, so let's get more back to the function. So you're exhausting and exhaustive. Look at all the different types of lipids. Let's do some cool stuff with this. Let's consider this critical property of the bilayer of the lack of permeability. How can we use this to our advantage in a cell? So think about lipids. Remember I said they have a bit of angst. They have a love-hate relationship with this bilayer. They have these oftentimes they can have charge-charge repulsion with the head groups if many lipids have negatively charged head groups. But then they have this nice snug, long, hydrophobic positive interactions with the side chains. And so it makes a certain amount of sense that a lipid molecule is going to be able to move around in this bilayer. When you draw a picture like this, it seems like it's static. But actually, these lipid molecules are moving phenomenally quickly. And so the width of this bilayer is about three nanometers. So lipid molecules are moving one micrometer, orders the magnitude faster or in longer distances than the distance across the bilayer. So they're whipping around in E. coli cell. One lipid molecule can circumnavigate the cell every two seconds. So this is not some static thing that sits around. It's very dynamic. We also want to control the types of lipids that are on either side, the bilayer, the inner leaflet or the outer leaflet. And so there's a class of enzymes. So you can imagine, well, this is not favorable. It's not favorable for this polar head chain to say, I'm just go dive through the bilayer. It'd be like, oh, it's so greasy inside. It gets to the other side, it would be more happy, perhaps. So this happens very slowly. It takes days, half-time of days to cross the bilayer. So we're going to need a help. We need an alternate pathway to make this even possible. So these are the funniest names of enzymes out there. So we have flip bases and flop bases and scramble bases. It makes me think of lunch or breakfast, actually. And so flip bases and flop bases are arbitrarily named according to which way the lipid is going. So the flip bases, lipids are coming in into the inner surface of the bilayer. And the flop bases are going out. They could have been arbitrary. They could have named it either way. This requires energy. So you need to have conformational changes of this protein in order to accommodate the transit of the lipid molecule to the other side of the bilayer. And so this conformational change of the flip base and flop base is catalyzed by the highly exergonic hydrolysis of ATP. A recurring theme we'll see throughout the semester. So we hydrolyze an ATP and that use of energy drives the accumulation of a defined whatever set of lipids that are necessary on each surface. So certain cellular functions require the lipids to be on a certain surface. So we looked at a perfect example. Signaling, lipid molecules involved in signaling. How many phospholipases are cleaving, you know, these sugar containing phospholipids on the outside surface of the cell? That wouldn't accommodate the function. So those types of lipids need to be on the inner surface. And then we have scrambleases. Perhaps sometimes you want to remove the lack of similarity between the two leaflets. So this is amazing speed of movement and amazingly slow diffusion across the bilayer. Okay. So this is a little movie. And if there's a minor miracle, it'll actually play. Okay. So that's one micron. Right? Two of those is how the circumference of an E. coli cell and we've labeled the lipid. And you see it moving around? It's an amazingly dynamic process. This is a sea of lipids and things are moving around a lot here. And then in the next movie, you can see it's sort of like ping-pong. All right. Oh, no. So here we're actually looking at the fluidity of the membrane. We're going to take some laser tweezers. We're going to grab us some membrane and then we're going to whip it around. And you could just see the graduate students doing this work was just smiling. Watch this. So we're going to grab this projection of the membrane. Here come the laser tweezers. Yay. Let's grab the membrane. Here we go. You pull it out. You see how fluid that is? Now watch. Whee! Back and forth. And isn't this fun? My advisor is going to let me graduate now that I can play with the membrane. Maybe it was late night in the lab. But that's spectacular. I mean, the thing you would think, for most things you grab like you grab your skin, there's a certain amount of pliability. You can't be whipping it around like that. It's almost like a liquid. It's a semi-liquid, right? So it's important to control this fluidity, this biochemical property. Look at this lack of symmetry of the composition of the inner and the outer bilayer. So we have certain types of lipids that occur on the, tend to occur on the inner surface. And this lack of symmetry is catalyzed by these flip-based, flop-based enzymes. So this is an erythrocyte, red blood cell. You can see, you know, it's not a hugely shocking fact that the lipids involved in signaling tend to occur on the inside of the cell. But then other types of lipids tend to occur on the outside of the cell. And so this is important, you might imagine. Perhaps two cells need to find each other. And so the type of lipids that exist in that interface could affect the efficacy of the interaction between those two types of cells. And so this is what nature has created for us, a dynamic set of lipids. It's not the same on both leaflets. Okay. So I'm gonna take a breath and come back to the, thinking about the physical property of the melting point. Okay. And so we just saw the graduate student whipping around the bilayer. There's a lot of fluidity there. But how can we control that fluidity? So each fatty acid has associated with it a melting point, the point at which it goes from a solid to a liquid. Right? The melting point increases as we increase the length. Well, that makes sense, right? Because we increase the length, we're gonna have more and more hydrophobic interactions. So they're gonna tend to pack together and be less fluid. Right? So that makes sense. But then what happens when we have these double bonds? Looking at this picture, you could guess it. Right? So if you add a unsaturated fatty acid, that's gonna dramatically decrease and very dramatically decrease the melting point. So just adding a single double bond to steric acid brings a melting point from 70 degrees centigrade to 13 degrees centigrade. You put two and three, you bring it down even further. And so you might think, again, oh, perhaps there's a static composition in the bilayer. And perhaps there's a certain assortment of lipids. It makes sense that there wouldn't be just one. If there were just one, you get to 70 degrees and all your cells would just go, and the graduate student would pull on it and nothing would happen. So we wanna have a mixture so we don't have this very defined point in which we have catastrophic changes in the properties of the bilayer. So there's a mixture of different lipids. So how does the cell regulate this process, you might wonder? And so this is the transition that I was referring to. So you have what's referred to as the paracrystalline state. This is what you're melting. So this is similar to the solid state. It's not completely solid. There's a little bit of movement, but it goes through transformation as you increase the heat, for example, to a fluid state where the bilayer is moving all around. This would be bad if this happened in your cells. This kind of sudden change in the properties of the bilayer. And so we put a mixture of lipids in these bilayers to prevent this. This is bad. Put bad on the slide. So this is called the paracrystalline state and the fluid state, and we have mixtures of lipids to prevent this from happening in cells. We don't wanna have just sudden changes into a different state. But the cell actively regulates the complement of lipids in the bilayer. You might think once a cell is made, well that's how it is, and that's the type of lipids in the bilayer. But the cell actually senses the environment. So if it gets cold, you might wanna have more hydrophobic interactions, right? So this is an example of an E. coli bacterial cell. And we're looking at the ratio of unsaturated to saturated amino acid. So you take the cell and you gradually warm it up. See, you're decreasing the unsaturated amino acids and increasing the saturated amino acids because as it warms up, you have more thermal vibrations of the lipids in the bilayer. And so you need longer, or you need less kinks in those lipids so they can pack tighter and maintain a constant state of fluidity. We don't want changes in fluidity. We want this to be a constant. So we need to respond to the environment. As the temperature goes up, we change the type of fatty acids. We could do this also by the length of the fatty acids, but remember the melting point table? The biggest and most dramatic changes in melting point were by adding double bonds. And so that's exactly, this E. coli cell is literally changing the degree of saturation of the lipids in its bilayer as it responds to the environment. Does this make sense? Yes. Yeah, yeah. So all of these lipids come from pathways. They are the final products of pathways. And so we can allosterically regulate the enzymes in those pathways to cause different pathways to become activated. So the details will come to us in a later lecture. We actually come back to this topic, look at those pathways. But allosteric regulation of the enzymes in the pathways that make these lipids can change how they're produced. Right? So it's pretty cool. Yes. There'll be more hydrophobic interactions. So that counteracts the thermal vibrations caused by warming up the cell. So it's a good thing. The goal here for the cell is constant fluidity. All the functions of the cell are calibrated to a certain degree of fluidity of the membrane. And if you change that, you'll change the function of a vast number of proteins. That are interacting with membranes. Another question. Ah, great question. I don't know the answer. Inzones can be very quick. These pathways can be highly amplified. But I don't have a specific answer here. Is it seconds, minutes, hours? I don't know. That's a great question. They might be in the textbook, though. Yes. Right. So that's inappropriate aggregation in your blood vessels. Right? So in this case, yeah, it's sort of, I don't know, I have problems accepting that. So why is it a plasticizer? Why does it cause it to be more fluid? Doesn't it make up more hydrophobic interactions? It is a fact. So if you do biophysical measurements of a membrane as the membrane incorporates more cholesterol, it does become more fluid. But I find it surprising as well. Contraintuitive. Yes. I guess perhaps you could do a calculation of all the interactions in these lipids and I don't know why it necessarily has to be one way or the other. It probably has to do with the types of lipids in there. And each one has a certain melting point. It's not a continuous range of melting points. That's just speculation, but perhaps there's probably a discontinuous, nonlinear range of melting points, perhaps. Good question. So this is the most sort of, you got to think about it a little bit. And for clicker, you got to think about this. I want to make sure you understand this slide. All right. So we're going to get to clicker in a moment. But right now, we're going to finish up on lipids, the lipid composition. So if you have different types of membranes for different organelles, they have a different complement of lipids and cholesterol. So for example, plasma membrane tends to have a lot of cholesterol, whereas these other types of membranes have less cholesterol. And so you're tempted to think, why did evolution set it up like this? I don't know that we know for sure. I mean, we're able to observe cells and maybe the plasma membrane, the functions of transporting proteins are, it's important to have a certain level of fluidity for transport processes, but I don't know specifically. But it is a true case that not only can we regulate the response of the environment, the types of lipids, but we can also, for different types of membranes, that in case different organelles, those have different complements of lipids. And I'm in likewise. So one point here is that I just want to make sure you read this. So not my responsibility or Alex's responsibility for you to look for the green light and for you to check afterwards to see if your grade is properly recorded. It's your responsibility, okay? So if your clicker is not working, get out the paper, write your banner ID and the letter and give it to Alex, okay? Everybody answered. It's just no hard clicker question for lipids. I mean, what's the guy going to do? So what are you guys... Oh, wait, wait, don't say it. We're still collecting papers. They got one more. Anybody else? No green light. Harvest side points. So two more just over here. Yep, give it to Alex please. Thank you. Sorry for the stress. Technology. Okay. So we do have the next part of the lecture is much more engaging, more intellectually challenging. So I want to get to that. The answer here... Oh, it's off. Okay. Okay. You guys rock. So smart. All right. So we're going to... I tend to have these at the end because it gets... it is a nice break actually. So let's dive right in. The next part is going to require a little bit more concentration. So drink your coffee and let's go. Okay. So different types of cells also have different compositions in their bilayers. And so here we're going to begin to think about not only are there lipids in bilayers, but there's proteins. And so here's, you know, different myelin, human myelin chief that has this composition. Whereas, you know, yeast and different organisms have a different composition in their bilayers. And so this is also, but not every organism or every type of cell is the same. So not very surprising. I'm not going to ask you to recapitulate this table, but you should be familiar. That's another way in which lipids and proteins vary. So let's switch gears to proteins and how they work as transporters. So we have different types of transporters. So we have just a transporter. We can transport ions across the bilayer. So a charged ion, either polar or even charged, it's not just going to dive through and just try to make it through the bilayer alone. It needs a lower energy pathway. We can pass information across the bilayer. So through a conformational change, we can have the binding of a signal causing a conformational change in a receptor, and that conformational change could lead to new binding surfaces being exposed here, allowing you to activate certain proteins or actually to turn this enzyme on. So some receptors that bind to a ligand are enzymatic. They convert a substrate to a product. Many of them are kinases. And so when bound to the ligand, the change in conformation causes that kinase activity to turn on. So we're passing ions, information, or we can regulate the passage of molecules through the bilayer. And so we can use a ligand to gate the channel. So this is a allosteric switch to whether it's good to pass the ion through or not. So you might imagine, we're not going to want to just have an open pour and just have things pouring in. The cell is changing, but the pour stays open. You're going to want to regulate that. And that regulation is done allosterically. So remember an allosteric regulator changes the shape of a protein. And so in this case, the change of shape changes the ability of ions to pass through. And so we can describe proteins associated with membranes as peripheral or integral. An integral membrane protein is hard to remove from a bilayer, whereas a peripheral membrane, or peripheral protein associated with a membrane is easier. So you can just change the ionic strength, or the pH, and that might cause peripheral proteins to come off. Integral proteins, you literally have to pry the membrane apart, dissolve it in detergents, warm it up, add chaotic, chaos-causing agents to the membrane. And so this is important to define the different types of interactions a protein can make with a bilayer. So integral membrane proteins are firmly associated. They can only be removed by literally prying the bilayer apart, dissolving that bilayer in detergents or organic solvents. The peripheral membrane proteins are defined as more transient and weaker interactions with the bilayers. And so those can be hydrogen bonds or electrostatic interactions. But you're not literally going across the bilayer. And so integral proteins that are anchored to a membrane by a covalently attached lipid are integral because it takes harsh treatments of the bilayer to remove them. So I think in the last picture, you see this. See this protein? A bunch of amino acid polypeptide attached to a lipid molecule covalently. And that lip, although not a single amino acid is inserted in the bilayer, this is still integral in the sense of what it takes to get that protein off the bilayer. You have to dissolve the membrane or break a covalent bond. That's a harsh treatment. Okay, so these are definitions. So integral membrane proteins can be associated in the groups. Again, don't panic about, okay, I'm not going to ask you, okay, draw a type four and you better get it exactly right. Just realize that there's different ways to integrate in the bilayer. It's very common to have an alfihelix. You might think about why it would be good to have an alfihelix across the bilayer. So the polypeptide backbone, hydrophobic or hydrophilic. Polypeptide backbone. Amide bond. Oxygen. Nitrogen. That's hydrophilic. So why would you want to have an alfihelix across the bilayer? I'll let you guys engage. Yes. Come on, exactly, because you can control. You can, for example, put hydrophobic side chains. And so those are pointing out, remember, perpendicularly to the axis of the helix. So that's perfect. That'll keep these lipid molecules making unfavorable interactions with the polypeptide backbone. You can also have something beta sheets. So you can have what's called a beta barrel. And that can span the bilayer. Because in that structure as well, you have side chains that are perpendicular. So you can put hydrophobic side chains. And as here, you're probably going to want charged side chains as you cross the interface. Because that might make favorable interactions with the polar head groups. And so there's different types of integral membrane proteins. Very good intuition. So here's some examples. So here is a aquaporin, as it turns out. It's a protein that transports water molecules across the bilayer. And here you can see there's lots of alfihelicins color-coded to this plot. So down here, we have a plot called a hydropathy plot. A hydropathy plot measures or indicates the degree of hydrophobicity of each amino acid. So amino acid 10 through 250 in this case, going from the end to the C term. Each amino acid is either hydrophobic or hydrophilic. And you can plot this on this chart. And you see the periodicity to the hydropathy plot. So it's cyclic. The reason it's cyclic is because every time it gets to the end, it tends to have hydrophilic amino acids. And then as it crosses, hydrophobic, hydrophilic. And so you have this periodicity. And just by looking at this kind of plot, you can predict how many times the polypeptide backbone crosses the bilayer, right? Because you have these cycles. And alfihelics has a certain length according to a number of amino acids. So there's periodicity here. It's not perfect because some alfihelicities extend a little bit beyond the bilayer and some have a little bit of random coil in between. So hydropathy plots are useful to make predictions of number of times across bilayer. So these proteins are bobbing around like buoys in Narragansett Bay. They're sitting in this sea of lipids. Remember, the lipids are moving one micron per second. And they're just sort of bouncing around here. And they tend to move less quickly than the lipid molecules. But they can diffuse. So for example, when you're immune cells, your T-cell finds an antigen presenting cell. The receptors in that cell cluster together. And they have this inward flow of proteins. And that movement is important in immune recognition. So the fluid mosaic model is the idea of buoys of proteins bobbing around in the sea of lipids. Right. And so you can look at the fluidity, movement of proteins just similarly to how you looked at the fluidity of lipids. So instead of labeling lipids, we can label the proteins. We can put some kind of colored fluorescent indicator onto the proteins. And then we can take two different types of cells, cause them to fuse together. So if those buoys are anchored, you know, on the bottom of Narragansett Bay, and they're not moving, this will be the end of the story. But when people do these style of experiments, they find the intermeshing of the buoys. And so the protein molecules are also diffusing around in the bilayer. You can do a similar experiment where you ablate with a laser. So you have a fluorescently labeled cell. You shoot a beam of laser and you ask the question, does the ablated section reconstitute with the colored protein? So you quench the fluorescence in some of the area of the cell. And if it wasn't fluid, if the proteins were anchored, that would just stay like it was. So it's another way to do that experiment. Okay, so there's different types of transporters. Uniport transports one molecule. You can have co-transport where two molecules are moved across the bilayer. If it's a simport, the molecules are going in the same direction. If it's an anti-port, the molecules go in an opposite direction. Semantic definition. So we have simports and antiports for co-transport and we have a uniport. So here are different types of transporters. You can have facilitated diffusion. We're going to think in terms of the electrochemical gradient. So you can have concentration differences of the molecule being transported in the outside and the inside of the cell. And that can help to drive molecules across the bilayer. And so in facilitated transport, a molecule is recognized as a high specificity by the transport protein and then moved down this gradient into the cell. In primary active transport, we're driving the endergonic transport of a molecule against the electrochemical gradient by the highly exergonic hydrolysis of ATP. So we're using the chemical energy released upon hydrolysis of a phosphate from an ATP molecule to change the conformation of the protein and literally push proteins like you would in a well. You push the proteins across the bilayer. So it's primary active transport if ATP hydrolysis occurs on the same protein that's transporting the molecule. It's secondary active transport if it's not an enzyme. If there is no ATP hydrolysis. So in secondary active transport, one molecule moves with the gradient and the other molecule moves against the gradient. So one molecule is moving with the gradient from a zone of high concentration to low concentration. The other molecule is moving against the gradient from a zone of low concentration to high concentration. We can have ion channels that ions pass through. We need high specificity, but these are moving down the gradient. Or we can encapsulate an ion in a protein and that can provide a capsule to get this molecule across the bilayer. So all kinds of different transports. Okay, let's think about this process thermodynamically. What is delta G prime knot? Delta G prime knot, standard change in free energy of a transporter. What is the molecule over here compared to over here? Is it different chemically? Delta G prime knot is zero. So delta G is not zero. Delta G depends on the concentration of the molecule on the other side of the bilayer. So normally you see you have some kind of value. Only a small number of molecules are actually able to pass directly over the bilayer and that would have a huge thermodynamic barrier. So drug molecules are often engineered to try and optimize the chances that they can diffuse across the bilayer. You have to have the perfect mix of hydrophobic character to the molecule. CO2 is an example of a molecule that can diffuse across the bilayer, but water cannot diffuse as easily. And so here what the protein transporter is doing is providing a lower energy pathway through which the molecule can span the bilayer. It's lowering delta G double dagger and therefore accelerating rate according to the Arrhenius equation. Rate is consistent with the height of the barrier. Decrease the barrier, increase the rate. Delta G prime knot is zero because it's the same molecule. It's not a chemical transformation, but there is a barrier to this process. So here's the aquapor in. Here are the water molecules. It goes through this gauntlet of death. Look at this. You have histidine amino acids pointed at this channel. And the water, it's like a funnel. And right at the end, they have to go through the gauntlet of death. Look at these histidines. Water molecule at a time can pass through here. It's very important that we don't let any old molecule pass through here. Only water molecules. So this provides the selectivity. The transporter must bind the molecule and allow it to come through because we need to differentiate what's coming through. We also might want to regulate the amount of water in a cell so we can gate the aquapor in. We can cause a conformational change. It causes the channel to close down. So glucose is transported across the membrane and transported down the electrochemical gradient. So glucose molecule binds to its transporter. That binding event causes the conformational change in the protein. And once the conformation is changed, it now has decreased affinity for the glucose molecule and it's released. Once glucose is released, that also triggers a change in conformation. So this thing is seesawing back and forth. Every seesaw movement transports one glucose molecule across the bilayer. And so here is the look at the hydrophobic nature of the amino acid side chains. The glucose transporter, and so you can see in general, we have hydrophobic amino acids in the core. As we span the bilayer, and we have much more charged and polar amino acids on the outside. These helices that span the bilayer are amphipathic. So amphipathic means that they have a hydrophobic surface and a hydrophilic surface. The hydrophilic surface actually context glucose. You can't have a hydrophobic surface touching the glucose molecule. It has a lot of hydroxyl groups that wouldn't work. And so these things are associating with each other, providing this alternate energy path, lower energy pathway. So each of these helices are clustered together. And in the center, these are all hydrophilic amino acids. They bind the glucose and are reducing this transformation. Whereas the outside, making contact with the lipid molecules, are hydrophobic amino acids. So this is referred to as amphipathic helix. Because the protein must first bind, it's saturable and it's specific. Does that sound familiar? Right? This is exactly like Michela's Minton kinetics. But here, instead of transforming a molecule, we're causing its transport. So a certain concentration of glucose molecules will fill half the transport molecules. And once we filled all the transport molecules, we'll be transporting glucose at the optimal rate. So we can define, in this case, we don't call it a KM, we call it KT for transport. So Michela's Minton didn't get their due respect here. But a certain number of molecules is required to fill half the binding sites. And when half the glucose-transporter binding sites are filled, that transported phenomena is occurring at its half-maximal rate. So it's saturable and it's specific, therefore it has kinetics similar to Michela's Minton kinetics. And you might imagine, if it's allosterically regulated, if it's a gated channel, it's probably going to have a sigmoidal curve. And it's not going to quite fit this hyperbolic shape and therefore not really fit Michela's Minton kinetics. So I think we'll do this as the last one. So this is your red blood cells. Remember we talked about carbon dioxide and picking up carbon dioxide and protons in your tissue, putting them on covalently hemoglobin. Turns out there's not enough hemoglobin molecules to carry all the carbon dioxide. And carbon dioxide gas is much less soluble than bicarbonate iron. So we're going to use the red blood cells as a factory to convert the carbon dioxide into another form in your tissues that is more soluble in your plasma. That way you can get enough CO2 out of your tissues. So in the tissues, CO2 diffuses across the bilayer and enzyme lowers the activation barrier converting carbon dioxide to bicarbonate. Bicarbonate has been transported out as chloride iron is transferred in. So it turns out the concentration of bicarbonate, not surprisingly, is very high in your plasma. So this co-transport is causing the endorganic transport of bicarbonate to be more favorable. But then when you get to the lungs, you just reverse things. This enzyme lowers an activation barrier. Once you've achieved the transition state energy, there's an equal probability the enzyme would go in either direction. It's reversible. So you then let the bicarbonate back into the red blood cell in your lungs. The enzyme working in reverse recreates the carbon dioxide molecule and that's transported out. This is a real-world example of the relevance of transport. We'll come back to this in the next lecture.