BIOC 503 - Membrane Composition & Structure

micelle

single layer of lipid molecules, spherical structure with head group on the surface to be compatible with surrounding water, hydrophobic core

• lipid molecule are lysophospholipids = individual units are wedge-shaped (cross section of head greater than the one of the chain)

lysophospholipids

individual units that are cone-shaped (cross section of head greater than the one of the chain) which favors the formation of micelles

• each cone-shape subunit of micelle has 1 tail ONLY

bilayer

2 lipid monolayers assembled using hydrophobic interactions at the end of fatty acid tails and along the side of the chain so that the hydrophobic part aggregate and try to shield themselves from the water

• sides of ___ structure still exposed to water

• individual subunit is CYLINDRICAL (cross section of head and tails are about equal) & has 2 TAILS

liposome

closed bilayer with hydrophilic cross section and an inside aqueous cavity formed by the original “bottom layer” when it closed

• cylindrical subunits (as it formed from bilayer)

• important for drug delivery (aqueous solution on the inside of cavity in liposome good transport, delivered when liposome makes contact with cell membrane

Tendency of different lipids to form bilayer

• triacylglycerols = NO because they do not have polar head group, they are fully hydrophobic

• glycerophospholipids = YES because they have a phosphate head group

• galactolipids = YES because they have a polar head group which is a sugar unit

• ceramides = YES because they are relatively polar with their 2 -OH groups and their amino group

• sphingophospholipids = YES because polar head group with phosphate attached to one of the original -OH group

• glycosphingolipids = YES polar sugar molecule head group

• gangliosides = YES because type of glycosphingolipids

• cholesterol = NO due to large hydrophobic body made of 4 fused ring

AKA only cholesterol and TAGs do NOT form bilayer because 1 head group and 2 tails are necessary for its formation

fluid-mosaic model

model proposed by Singer & Nicolson in 1972

membrane is a fluid, changing bilayer that contains proteins and cholesterol

cholesterol

main component of plasma membrane but NOT of the organelles’ membranes EXCEPT for lysosomes as those organelles are created by endocytosis from plasma membrane so composition is similar.

cardiolipin

mainly found in inner membrane of mitochondria as it is required for electron transport chain (ETC)

outer

composition of ___ monolayer

• PC (phosphatidylcholine)

• sphingomyelin

inner

composition of __ monolayer of membrane

• PE (phosphatidylethanolamine)

• PS (phosphatidylserine)

• PI (phosphatidylinositol)

• PA (phosphatidic acid)

apoptosis

PS and PE are usually in the INNER monolayer of the cell membrane, but during ___ they are translocated to the OUTER layer so cell can be recognized and cleared by phagocytes

• occurs during early stages

• NOT the cause of the phenomenon

phosphatidylinositol (PI)

complex lipids located on the INNER monolayer of cell membrane as a precursor to signaling molecules IP3, DAG, and PIP3

• only make up 5% of membrane but extremely important

PIP kinase

PI is phosphorylated by the ___ enzyme to make PIP

PIP2

PIP is phosphorylated by PIP kinase to make ___ which is the precursor for cytosolic IP3 and DAG signaling molecules

phospholipase C (PLC)

PIP2 is broken down/hydrolyzed into DAG and IP3 by the ____ enzyme

PI3 kinase

PIP2 is phosphorylated by the ___ enzyme to make PIP3 which is a signaling molecule that stays attached to the inner layer of cell membrane

PIP3 mechanism

1. PI3 kinase phosphorylate PIP2 to make PIP3

2. PIP3 activates PDK

3. PDK activates AKT which PREVENTS apoptosis

PTEN

levels of PIP3 in a cell are regulated by the ratio of PI3 kinase and ___

• ___ is a phosphatase that dephosphorylate PIP3 into PIP2

lipid rafts

special domains of the plasma membrane containing saturated, less fluid lipids

• can be large or small

• are mobile

• more cholesterol and sphingolipids (which are more saturated) = more rigid structure

• important in EGFR signaling (growth factor receptor signaling) as they are proteins working BETTER inside a lipid raft

• some proteins work better outside of the lipid raft, and some work better inside of it, all of it is based on how efficient the transduction of the signal is.

peripheral proteins

protein attached to ONLY one side of the membrane & can be removed easily 

integral protein

protein imbedded deep through the membrane with transmembrane domains and covalent linkages making it hard to remove from the membrane (unless strong detergent, chelating agent, pr change in pH used to disrupt the membrane itself)

amphitropic proteins

membrane proteins that are both hydrophilic and hydrophobic & can be attached covalently or non-covalently to the lipids or carbohydrates connected to the lipids

• biological regulation determines attachment or detachment to/from membrane

6

there are __ types of INTEGRAL membrane proteins

• all of them are COVALENTLY aka strongly bound to membrane (and thus require strong detergent, change of pH, chelating agent to disrupt membrane and separate from it)

type I

amino terminus OUTSIDE

carboxy terminus INSIDE

type II

amino terminal INSIDE

carboxy terminal OUTSIDE

type III

integral membrane continuous protein with linked transmembrane domains 

• GPCRs

Type IV

several different transmembrane proteins next to each other to form a bigger structure

• multisubunit receptors, ion channels

Type V

protein only on the INSIDE layer anchored to the membrane by fatty acids

type VI

single transmembrane protein with outside domain anchored to membrane by fatty-acid-like elements

fatty acids, glycosyl PI

___ & ___ (GPI) can anchor proteins into the cell membrane) aka act as lipid anchors

hydropathy index

test to show the hydrophobic regions o fa protein tested

• positive = hydrophobic

• negative = hydrophillic

diversity

there is ___ in lipid composition of cell membranes based on organisms and among cell types, even among different membranes of the same cell

asymmetric

there is an __ distribution of lipids between the inner and outer monolayers of cell membrane

• __ lateral distribution reason for lipid rafts too