Intro to Lipids and Membranes

What are 5 functions of membranes?
-isolate cells from toxic molecules
-assist in accumulation of nutrients
-causes cell metabolism – major site for energy production
-cell motion, reproduction, signal transduction, cell-to-cell interactions,etc.
-variety of different functions
Lipids spontaneously form ordered structures in water due to hydrophobic interactions
-very few lipids exist as monomers
-monolayers arrange lipid tails in air
-micelles bury nonpolar tails in the middle of a spherical structure
*reverse in nonpolar solvents
Micelles work well for fatty acids, but lipids, whose hydrophobic tails are significantly larger, cannot readily pack into a micelle.
So when you put lipids (phospholipids) into water, they will form bilayers with hydrophobic tails stuck on each other and hydrophilic heads exposed to the water.

-bilayers will close on themselves in water forming vesicles.
-may be unilamellar vesicles (liposomes) or multilamellar vesicles

-liposomes may have a use in medicine. you can put a particular enzyme within the liposome and then have it target a particular cell…like cancer cells!

Fluid Mosaic Model
-Singer and Nicolson
-The idea is that the membrane is a fluid mosaic or two dimensional solvent that forms by lipid bilayer and the proteins are embedded into this bilayer. All this is pretty flexible, meaning that lipids and proteins can both move within the plane of the membrane.
-Lipids and proteins can migrate (“diffuse”) in the bilayer
*can move a few nanometers per minute
-Frye and Edidin proved this (for proteins), using fluorescent-labelled antibodies
-Lipid diffusion has been demonstrated by NMR and EPR (electron paramagnetic resonance) and also by fluorescence measurements
What are the two types of membrane proteins according to the fluid mosaic model?
peripheral membrane proteins – attach to the outer surface of membranes

intrinsic membrane proteins – engulfed into the membrane and very often penetrate several times

You do not typically have lipids and proteins even distributed throughout the membrane – many times their distribution is asymmetric
-experiment done on lipids which found that certain lipids will tend to congregate in patches due to interactions with ions
-experiments also show that many proteins interact with each other and form in patches on the surface of the membrane; can be the same proteins or different proteins

-though it seems obvious that there tend to be different types of proteins on the inside and outside of a membrane….there also tend to be different lipids on the inside and outside of membranes

As a rule of thumb:
-phosphatidylcholine and spingomyelin are very abundant on the outside of the membrane
-phosphatidylethanolamine and phosphatidylserine are sitting on the inner lealet of the membrane
Flippases
-soul function is to maintain the asymmetric structure of membranes
-lipids can be moved from one monolayer to the other by way of flippase proteins
-some operate passively and do not require an energy souce; others work actively and require energy from hydrolysis of ATP
-active flippases can generate membrane asymmetry
Membranes by themselves require fluidity to be able to function.
-necessary because many proteins which are channels or transporters and so forth, must be able to move; freezing membrane will dramatically reduce proteins ability to function
-cooling membrane will eventually lead to solid crystal structure; when you heat up, it will become fluid again
What are two ways to control fluidity of a membrane?
-change content of unsaturated fatty acids in phospholipids
*must be cooled down much more to bring to crystal/solid state

-put cholesterol into bilayer
*solid molecule, like brick; restricts the mobility of tails
*at high temps cholesterol will prevent fluidity
*at low temps it will actually allow membrane to stay fluid by interrupting crystal structure

Membrane proteins (peripheral, integral, lipid anchoring)
-peripheral
*attached to outside or inside of protein mainly through electrostatic interactions
*can be dissociated with mild detergent treatment or high salt concentrations

-integral
*cannot extract readily without breaking membrane down with detergent or organic solvent
*often transmembrane, but not necessarily
*ex: glycophorin, bacteriorhodopsin
*in order to insert in membrane you have to get a hydrophilic protein into a hydrophobic solve – not very favorable

-lipid anchoring proteins
*have lipids component covalently attached
*one of the functions of this lipid component is to keep protein attached to membrane

Glycophorin
-simplest example of integral protein
-present in membrane of red blood cells and actually determines ABO blood type
-penetrates membrane just once
-structure that penetrates membrane is an alpha-helix
*remember, N and O are shielded within the helix, while R groups of hydrophobic AAs point towards the outside
Bacteria Rhodopsin
-penetrates membrane 7 times
-common structure (epinephrine penetrates membrane 7 times)
-Consists of 7 transmembrane helical segments with short loops that interconnect the helices
-helices are transmembrane, loops exposed to cytosol and extracellular fluid
Porins
-alternative way of inserting proteins into membranes
-found in gram negative bacteria and outer membrane of mitochondria
-penetrates membrane as beta sheets
-in beta strands, all oxygen and nitrogen engaged in forming hydrogen bonds are shielded from environment, and the side chains are exposed to the outside.
-as a result you form a barrel from layer of beta sheets
*outside has hydrophobic side chains which are readily soluble in lipid hydrophobic environment
*in the middle you will have a hole of certain size that allows certain compounds to go through.
-beta sheets in order to penetrate membranes need just 9-10 residues in contrast to 20-25 for alpha helices so it is actually more energy efficient.
These two different intermembrane protein arrangments allow proteins to penetrate membrane differently.
-beta barrels will make holes
-alpha helices make a narrow channel
Lipid-Anchored Membrane Proteins Are Switching Devices!

What are the 4 different types that have been found? (based on linkages)

-Amide-linked myristoyl anchors
*linkage in which myristate, a fatty acid, is attached to N terminal glycine residue of protein
*idea is that when myristate is attached to N terminal glycine, its inserted into membrane and this physically attaches protein to membrane.
-Thioester-linked fatty acyl anchors
*fatty acids (myristate, palmitate, stearate, oleate), though primarily palmitate, is attached to sulfhydryl group of cysteine residue.
-Thioether-linked prenyl anchors
*prenylation refers to linking of isoprene groups
*Proteins containing the C-terminal sequence CAAX (cysteine, aliphatic, x = any residue) can undergo prenylation reactions that place thioether-linked (a) farnesyl or (b) geranylgeranyl groups at the cysteine side chain. Prenylation is accompanied by removal of the AAX peptide and methylation of the carboxyl group of the cysteine residue, which has become the C-terminal residue.
*Isoprene groups include farnesyl (15-carbon, three double bond) and geranylgeranyl (20-carbon, four double bond) groups
-Glycosyl phosphatidylinositol anchors
*always attached to C-terminal residue
*many surface antigens, and adhesion molecules that are attached to membranes my GPI anchors
*GPI anchor is a whole phospholipid which is phosphatidylinositol that has number of polysaccharide chains that touch and finally protein
*2 interesting things about this type of anchor
=readily cleavable and allows outer membrane proteins to be released into circulation
=some anchors may be by themselves signaling molecules
What are the two major forces for molecules to move across a membrane?
-concentration gradient
*moves from high to low
*does not work if there is no transporter or something to help it move through membrane physically (hydropilic combounds)

-charge gradient
*comes when you transport charged molecules
*molecule will move towards highest gradient of opposing charge, or away from highest gradient of like charge

Facilitated Diffusion
-passive diffusion is very slow for hydrophilic compounds despite gradients
-for naturally occurring compounds, the membrane typically has specific transporters to help them cross the membrane
-may be a channel or enzyme, but you definitely need a protein to help it get across
-ex: glucose transporter
*proteins span membranes many times
*helices bind together forming channel in the middle through which glucose will be actually going through.
Active Transport
-used when you have to transport molecule against its gradient
-have to expend energy, usually through ATP
-ex sodium and calcium pumps
-ex osteoclasts
*pumps acid outside of the osteoclast to dissolve bone matrix
-ex MDR ATPase
*recognizes foreign molecules and pumps them out of the cells
*designed as a defense mechanism, but hurts efforts in drug therapy (like chemo)
*not specific so can work against multiple compounds/drugs
Primary and Secondary active transport
-primary active transport uses ATP as source of energy to move molecules

-secondary active transport uses ion gradients, like that created by the sodium pump, to cotransport molecules like amino acids and glucose across the membrane.
*Symport – ion and the amino acid or sugar are transported in the same direction across the membrane
*Antiport – ion and transported species move in opposite directions