Chapter 7: Phospholipids

plasma membrane is aka

cell membrane

Where else can you find plasma membranes?

in any membrane bound organelle

• ER

• golgi

• mitochondria, etc

unsaturated fatty acids

• have a kink from a C=C double bond

saturated fatty acid

• has more hydrogens since it has no C=C double bonds

• has a linear shape

common fatty acids

• stearic acid

• oleic acid

• palmitic acid

• all have a np tail and polar head group

  • all are carboxylic acids with hydrocarbon tails

triacylglycerols

• a higher order lipid

• glycerol with a np hydrocarbon tail and an ester

how fat droplets are formed

a droplet forms off of the phospholipid bilayer of the smooth ER

phospholipids

lipids in the membrane

• all have a phosphate, fatty acids tails (unsaturated and/or saturated), a glycerol, and some group at the top

• the group at the top is what differentiates different phospholipids

• usually neg charged

• have a hydrophillic head (polar) and hydrophobic tail (np)

phospholipids in the plasma membrane

phosphatidylethanolamine: on the cytosol side

phosphhatidylserine: on the cytosol side

phosphatidylcholine: mostly on the ECM side (inside the cell)

sphingomyelin: mostly on the ECM side (inside the cell)

sphingosine: membrane lipid that is not a phospholipid, and only has one tail

• ALL have 2 tails

membrane lipid that is not a phospholipid

sphingosine

• it just lacks a phosphate

cholestrol

steroid with a polar head group (a singular O) and a np hydrocarbon tail

• makes the membrane stiff by preventing nearby phospholipid tails from moving

• it is rigid due to the ring structure of the steroid part

ways for lipids to form in aqueous environments

micelle: sphere shaped

• forms from cone shaped lipids

lipid bilayer

• forms from cylinder shaped lipids

both form to minimize hydrophobic interactions of the lipid tails with water (water wants to surround lipids since it is energetically favorable)

phospholipid forming a sphere/ liposome

• does this when the edges of the bilayer are exposed to water, which is not favorable

• forms a liposome, which has water inside and outside of it

• two liposomes will not fuse together unless forced to do so

What characteristics would make a membrane more stiff?

long hydrocarbon chains (higher VDWs)

saturated fatty acids (higher VDWs)

more cholesterol (makes membrane stiff by limiting flexion and rotation of tails)

forming the membrane into a repeatable pattern shape to make a tight fit

ways phospholipid tails can move

flexion: tails wagging

rotation: tails spinning around the axis of their head

lateral diffusion moving left/right

flip-flop: is very rare, occurs when the top layer flips to the bottom due to flipase

• flipping is not favorable since it would require the sides to go on the opposite side, which is not energetically favorable for them, and the polar head would have to push thru the np layer below it

lipid rafts

rigid domains of the phospholipid bilayer formed to bring certain groups of proteins together

• this region has longer hydrocarbon tails, more cholesterol, etc

integral membrane proteins

span thru the membrane

• often have an alpha helix that spans thru the membrane bilayer, made up of np AAs

• can also be a beta barrel

lipid anchored proteins

have a lipid dipped into the lipid bilayer to anchor the protein at the membrane’s surface

peripheral proteins

are bound to integral proteins on the interior of the plasma membrane and are on one side of the membrane

lipid anchors

• helps proteins stick in the membrane and be peripheral

• also lets the protein know what side of the membrane it is on

• ex: myristoyl, palmitoyl, and farnesyl anchors, all which are on the cytosol side

GPI (Glycosylphosphatidylinositol)

• a common form of lipid anchor

• allows protein to attach on the extracellular side

• added in the ER and targets the exoplasmic leaflet, where the np region of the bilayer is exposed to water

almost every AA in the alpha helix of an integral membrane protein is

np

How integral membrane proteins fold

1) they immediately go to their location in the membrane since their np regions do not like being in the aqueous environment of the cytosol

2) then they fold up

hydropathy plots

• used to predict of a protein is an integral membrane protein

• integral membrane proteins have large spans of np AAs in a sequence where they are in the membrane

• DOES NOT predict which way it goes in the membrane, just that it is likely integral

• only works for alpha helices

• plot has upward peaks where np regions are and downward peaks for polar regions of the sequence

hydropathy plot showing equal amounts of np and polar AAs

• so it could be a beta barrel, which has polar and np regions

Beta barrels

integral membrane proteins that have a polar interior and np exterior where it comes into contact with the bilayer

• interior is polar so aqueous solutes can pass through it

• due to its np and polar aspects, it cannot be read clearly on a hydropathy plot

What is on the extracellular side of the bilayer?

sugars, which help cells recognize each other and help in signaling processes

• if they were on the inside of the cell, they would be eaten

cytosolic domains of the bilayer/transmembrane

can have disulfide bonds, which are ruptured inside the cell

• bonds inside the cell are reduced (so disulfide bonds break on the cytosolic side)

detergent

• amphipathic molecules that are used to isolate lipids

• can be ionic (fully charged) or non-ionic (polar)

• remove phospholipids from the membrane’s bilayer

• cause lipids to form micelles

types of detergents

SDS: disrupts the covalent bonds during gel electrophoresis

others: Triton X-100 and Beta-octylglucoside

CMC

the concentration of detergent you must add to start forming micelles form lipids

• critical micelle concentration

How to remove peripheral membrane and lipid-anchored proteins

• with some detergent, but not much, since they are already mostly polar (which makes them more hydrophillic)

How to remove integral membrane proteins from the lipid bilayer

• requires a lot of detergent since integral proteins are mostly np

• requires an amount of detergent >/= the CMC

• then micelles are formed

FRAP for studying the lipid bilayer

• can measure the fluidity of a membrane

• 1) add fluorescence to phospholipids on the membrane

• 2) bleach a certain area of the phospholipids with a laser

• 3) see if fluorescence appears in the bleached area, and how quickly, to see the speed at which the membrane moves (measures membrane fluidity)

• the faster the fluorescence moves= a more fluid membrane

proteins that distort the plasma membrane

cell cortex

spectrin

cell cortex

• a thin layer of proteins under animal cell membranes that helps the cell retain shape

• an elaborate and dynamic network of cytoskeleton just under the plasma membrane

spectrin

• protein that prevents cells, especially RBCs from collapsing and helps them retain structure

• a filamentous protein in the cortex that forms tetramers to strengthen RBCs

• forms pentagonal/hexagonal arrangements