Fundamentals I: 11:00-12:00Scribe: Ashley Brewington

Friday, September 4, 2009Proof:

Dr. WhikehartMembrane Lipids, Proteins, and CarbohydratesPage1 of 8

Membrane Lipids, Proteins & Carbohydrates [S1]:

  1. I hope that you didn’t think that the lecture that Dr. Pritchard gave was something that was sandwiched in between a series of lectures on lipids, actually what he was doing was summarizing everything we’ve been talking about on metabolism. The last lecture I gave was on the metabolism of lipids. So he was just trying to create a big picture for you to help you understand these very complex interactions.
  2. . Now, today actually we are not going to leave lipids we are going to go back and talk about lipids in relationship to membranes, proteins and carbohydrates. So what we’ll be looking at today will be the nature of cell membrane themselves, lipids in membranes, proteins in membranes, lipid-protein interactions and the presence of carbohydrates and what their roles happen to be.
  1. Cell membranes [S2]

In the early part of the 20th century, actually at the end of the 19th century, very little was known about cell membranes or membranes themselves of any type and there were a number of individuals that did make some attempts to begin work in this area. They were Overton, Langmuir, Gorter, Grendel, Danielli, and Davson.

  1. Figure 1.1 [S3]
  1. Overton found that membranes were lipid in nature and not easily penetrated.
  2. Langmuir developed a device in 1917 to study lipid layers spread out on thin films to observe their air-water interfaces.
  3. What I’ve put on the board is a diagram of some of Langmuir’s early work, what he did was make a very thin film of a fatty acid from a fatty acid monomer and layered it out over a water surface well that was only a means to get to an end the end was to take this thin membrane and transfer it to a solid surface to study the physical chemical properties of the interface not necessarily the lipid itself.
  4. Gorter and Grendel were the first to propose that membranes were made up of lipids as bilayers and did this by using Langmuir’s device.
  5. The proposal was that there was a hydrophilic part and a hydrophobic part of the lipid double layer and the hydrophobic part faced each other where as the hydrophilic parts were present at the water lipid interface.
  6. The bilipid model is often incorrectly attributed to Danielli and Davson
  7. The 1925 model that you see on the screen actually failed to account for any proteins associated with the membranes and that was the contribution of Danielli and Davson to show how the proteins were assembled into the lipids and what they did there.
  1. Bilipid Layer with Phospholipids [S4]
  2. Now we know that the bilipid layer exists and that the hypothesis proposed in 1925 was correct.
  3. The lipid layer is composed of two lipids facing foot-to-foot in which the red arrow points to the hydrocarbon tails of the fatty acids and the hydrophilic portions are the polar head groups exposed to the aqueous environment indicated by the blue arrows.
  4. These particular FA’s are really esters are composed of FA’s that are both saturated and unsaturated.

The unsaturated portion is indicated by these dark arrows or the gold arrows.

  1. Soap Bubbles [S5]
  2. Interestingly enough, bilipid layers in biological cells are formed in the way I just showed you. That is they are oriented against an aqueous environment.
  3. You can have bilipid layers that will arrange themselves in a nonpolar environment as well.
  4. Soap bubbles are a perfectly good example, but here the bilipid layers are backwards so to speak.
  5. The hydrophilic portions face each other and the hydrophobic portions face out into some environment and the environment in this case happens to be the air outside the bubble and the air inside the bubble.
  6. Air is hydrophobic in nature and you’ll find that a lot of gases as well such as nitrogen and oxygen are really hydrophobic.
  7. If I were to ask you something about soap bubbles, then maybe you can give me some idea of how the soap bubble is arranged.
  8. The arrangement of the bilipid layer is dominated by the prevailing need for the most stable, low energy association of the lipids. That is the key to this.
  1. Amphipathic Lipids [S6]
  2. Amphipathic lipids are lipids that have a hydrophobic and hydrophilic component to it.
  3. These particular kinds of lipids “spontaneously” form structures in water by hydrophobic bond formation.
  4. “Sponaneously” meaning they will go and form the most energetically favorable arrangement that the possibly can, or in other words, they will go to the lowest energy formation.
  5. Because like people amphipathic lipids are lazy and don’t want to exert a lot of energy as any particular time.
  6. The arrangement is not always a bilipid layer
  7. The micelle, structure on the left, is the structure that you get with the formation of FA’s, only FA’s, these are amphipathic containing a hydrophobic and hydrophilic section to them, but do not form a bilipid layer because there is no need to do that. They all come together, the tails come together on the inside and the head groups face the environment (water) on the outside.
  8. A bilipid layer is one that is usually formed by phospholipids.
  9. The phospholipids differ from the FA’s in that each phospholipid has two FA’s esterified to a glycerol molecule and they tend to make a more square formation rather than a circular one.
  10. You have two layers in which each individual layer will face some aqueous environment or polar environment on one side and on the other side.

Now these bilipid layers are somewhat unstable, that means if you have a bilipid layer and you get to the end of the layer it will want to curl up.

  1. Now these bilipid layers are somewhat unstable, that means if you have a bilipid layer and you get to the end of the layer it will want to curl up. It will curl up all by itself and form a structure that is called a liposome.
  2. A liposome is like a cell without any ingredients in it. You have phospholipids forming a bilipid layer and a hollow core on the inside with no nucleus, organelles or cytoplasm.
  3. The interesting thing about liposomes is that one-time drug companies were interested in using them as devices to deliver a drug to an individual cell.
  4. It was a very attractive device because it was easy to put drugs inside and make a liposome. All you have to do is put all the ingredients together in an aqueous environment and sonicate it and you get liposomes.
  5. The liposme will go to a cell and fuse with its plasma membrane and deliver the contents of the liposome to the inside of the cell.
  6. Problem is how do you target specific drugs to specific cells. You can probably conclude to yourself that this could be a disaster and it has been. The only way out of this is to find protein receptors and lipoproteins that will bind to the kind of cells that you want to deliver the drug to. Well, that has been a nightmare. Interest in liposomes first developed by 1970 and has not gone anywhere.
  1. Micelles, bilipid layers, and liposomes[S7]
  2. Read this slide
  3. What About Sphingolipids and Cholesterol in Membranes? [S8]
  4. We have other lipids that we know about besides phospholipids. These are also incorporated into membranes and they are sphingolipids and cholesterol.
  5. A sphingolipid is part of a bilipid layer and fits in just like a phospholipid would, but they also contribute a different polar head group especially with the gangliosides and that’s what makes them different and useful in cells as part of the cell membrane.
  6. The diagram that you see on the right hand side is an example of that is lactosylceramide, which is a ceramid with two lactose sugars bound to it and this a device to have sugars incorporated into membranes and a perfectly good example of that is to have these sugars added as immunological recognition devices for a cell.
  7. We see where the sugars are and the nonpolar portion below that which is usually the part of the bilipid layer, this is the sphingo part which extends into the bilipid layer itself and it could be in this particular case an amino derivative of a FA plus a sphingosine.
  8. Cholesterol [S9]
  9. Cholesterol is a flat molecule, you can see two representations of it here.
  10. We also know that the ring portion is relatively rigid and will wedge itself between two phospholipids.
  11. When the Lipid Composition is Modified [S10]
  12. This would be an example of what happens in half of a lipid bilayer and here is a better diagram than before. Here is a diagram of the cholesterol molecule found in your textbook wedged between phospholipids.
  13. What it is doing there is to make the bilipid layer more rigid. That is the take home lesson. It becomes more and more rigid as you add more cholesterol to the bilipid layer of the membrane.
  14. Stiffening of the membrane is important for certain types of membranes.
  15. It is important to the plasma membranes in particular, and is not important in membranes that you find in the matrix of the mitochondria.
  16. As a matter of fact you really don’t want to stiffen the mitochondrial membrane up for certain reasons which will be explained later.
  17. The cholesterol molecules tends to be present more on the outside of the bilipid layer than the inside.
  1. The Length and Unsaturation of Fatty Acids That Make Up Membrane Phospholipids [S11]
  2. The length and unsaturation of FA’s that make up the membrane phospholipids may have or do have a very strong effect on membrane structure and its properties.
  3. It is apparent that the unsaturation of FA’s causes two effects: membrane thinning and membrane disorder.
  4. Membrane disorder can be translated as an increase in membrane fluidity.
  5. Here are two examples, a saturated lipid bilayer membrane with saturated hydrocarbons and you can see that it is a relatively structured series of molecules. The other has a series of unsaturated hydrocarbon tails and it is much more randomized and it is also a little thinner than the saturated series of hydrocarbons.
  6. In real life we have a mixture of these two extremes because cells have special needs for degrees of fluidity and nonfluidity depending on what they need to carry out.
  7. For example, membrane fluidity is an important quality in membrane functions like transport.
  8. Why? Because the proteins involved in the transport process have to be able to move back and forth to achieve their function.
  9. Properties of Transition in Bilipid Membranes [S12]
  10. Other properties that we need to know about in bilipid membranes are known as transition. There are two forms of transition : state transition and motion transition.
  11. This is a property of change in the structure of a membrane.
  12. As I told you before phospholipids and sphingolipids with greater unsaturation have a more disorganized structure and add more fluidity to a membrane.
  13. Phospholipids and sphinglipids at a higher temperature do the same thing.
  14. The conversion of a membrane lipid to a more liquid state occurs at what is known as its transition temperature or transition state and forms a liquid crystal.
  15. We know all about liquid crystals because some of our watches have liquid crystals and that is essentially the same thing.
  16. Some portions or FA’s in bilipid layers are above the transition temperature and some are below it. The amount of unsaturated FA’s vs. the amount of saturated FA’s determines the overall flexibility of a membrane because of the existence of some FA’s in the liquid crystal state and some in the paracrystalline or gel state.
  17. Motion Transition [S13]
  18. In addition, there is also motion transition.
  19. Lipid components of a membrane like molecular components of all our cells are under constant motion or if you want to call it vibration. The amount of vibration depends upon composition and temperature.
  20. The lipids that are present in a bilipid layer tend to diffuse away from where ever they were at any point in time.
  21. Now generally we used to say that a phospholipid on one side of a bilipid layer is there forever, but within the past ten years it has been discovered that there is an enzyme called flippase.
  22. I will not go into its molecular weight or properties, but it will take a phospholipid and move it from one side of a bilipid layer to the other.
  23. One important function of this is to take a membrane that is just being formed and move a phospholipid from one side to the other to help a membrane become organized.
  24. It is also for maintenance, supposed a certain phospholipids are lost by a certain process or by damage and when those phospholipids are needed on that particular side of the bilayer, then flippase will transfer them to that side of the bilayer.
  1. Other Properties That Lipids Impose on Membranes [S14]
  2. We also have a property called asymmetry and this is a property in which the lipid composition of a transverse section, across the bilipid layer, can be varied between the outside and inside of a membrane.
  3. Now the example here is in the red blood cell or erythrocyte, this asymmetry imposes certain membrane curvature or immune specificity to a certain cell.
  4. For example, if you have a sphingolipid that will incorporate sugar molecules for the purpose of immunological identification you would want most of that on the outside of the membrane. If it was on the inside how can it impart immunological specificity to the cell.
  5. Another example, is phosphatidylcholine being largely on the outside and phosphatidylethanolamine being on the inside.
  6. These particular phospholipids cause the membrane to assume a certain curvature. The cell will either be more round than flat.
  7. There are variations of this because cells do have different types of shapes.
  8. There is the biconcave shape that you get in red blood cells which is important for the transport of oxygen and is due to the fact that certain phospholipids are present on certain sides of the plasma membrane of the cell.
  1. Asymmetry From Membrane to Membrane in a Cell [S15]
  2. A little bit more on asymmetry, you see some examples of the different lipid compositions of various membranes.
  3. I have two examples of differences that are pointed out with arrows where green indicates cholesterol and the purple indicates the phosphatidylethanolamine.
  4. Comparing the plasma membrane and mitochondrial membrane. Mitochondrial membrane deals with things like oxidative phosphorylation and has to have a high concentration of proteins that can allow that process to take place in order to carry out the oxidation reduction reactions and must have certain amount of freedom meaning it must have a flexible membrane.
  5. The plasma membrane is no so constrained even though it does involve uptake, it has to be a bit more rigid.
  6. So you find for example, that the cholesterol concentration is higher in the plasma membrane than the mitochondrial membrane.
  7. The phosphatidylethanolamine contains components that allow for more flexible membrane (phospholipids with more double bonds) and so you will find that the mitochondria has a higher concentration of phosphatidylethanolamine than the plasma membrane.
  1. The Role of Proteins in Membranes? [S16]
  2. Danielli and Davson worked together on membranes in the 1930-40s and showed the presence and role of proteins in membranes and the important role of proteins in membranes was to be able to carry out transport across the plasma membrane itself.
  1. 1937 and 1940’s [S17]
  2. Cell function and survival couldn’t exist if you did not have cell transport.
  3. So what they first proposed in 1937, was taking a globular protein and rolling it on the surface, either side of the plasma membrane and allowing some invagination of that protein where that protein has a large sequence of apolar amino acids.
  4. Well, that was fine but it didn’t go far enough and later on they said that the apolar amino acids need to do is go all the way thorough the membrane to provide pores so substance could go all the way through the membrane.
  5. That idea stuck for a while and didn’t completely explain everything, but was an improvement.
  1. 1960-1972 [S18]
  2. So, a little later S.J. Singer from 1960-1972, proposed something called the fluid mosaic model.
  3. The fluid mosaic model added to the overall bit of knowledge that was known about membranes something called an intrinsic protein and extrinsic protein and in addition introduced the location of carbohydrates on proteins themselves.
  4. This model also included how transport would take place and later development of that included pores and a rocking protein that would trap a substance to be transported from one side of the membrane and push it through to the other side, an alternative model to the pore model.
  5. But all this was due to S.J. Singer.
  1. Intrinsic Proteins [S19]
  2. We have intrinsic proteins, also called integral membrane proteins and these are proteins that are firmly anchored in the bilipid layer.
  3. As with anything else, there is some exceptional material that will be presented later on.
  4. Generally most people will define an intrinsic protein as a protein that is anchored firmly in the bilipid layer.
  5. They pass through the layer and are involved in transport and hormone reception.
  6. Hormone reception becoming a concept that became really important later on.
  7. The example that we are looking at here is a protein called glycophorin.