unit 15: Cell membranes Flashcards
what are the functions of lipids in animals?
provide structure in cell membranes
energy storage
metabolites and messengers
enzyme cofactors
pigments (light retinal pigments, photosynthetic pigments)
what are the three main types of animal membrane lipids and their components?
- phospholipids (glycerophospholipids, sphingophospholipids)
- glycolipids (glycosphingolipids)
- sterols (cholesterol)
what are the general features of fatty acids
- can have varying lengths of hydrocarbon chains w/ terminal COOH group
- usually even number of carbon atoms in cellular fatty acids
- can be saturated or unsaturated. and varying degrees of saturation
describe the general features of sterols
- a 4 ring hydrocarbon structure w/ different hydrocarbon tails
- has a OH- group (equivalent to a polar head group)
- is only produced by eukaryotes
what are the main functions of membranes
- enclose cell (plasma membrane)
- intracellular compartmentalization (internal membranes in eukaryotes)
- controls flow of molecules. between inside and outside of cell and intracellular compartments
- ATP synthesis: occurs in plasma membrane of prokaryotes, inner mitochondrial membrane/chloroplast thylakoid membrane of eukaryotes
- cell adhesion
- cell communication/signaling
composition and fundamental architecture of cellular membranes
- double-layered sheet like structures (leaflets) about 5-10 nm thick
- composed mainly of lipids and proteins.
- some carbohydrates can be attached
- 1:4 to 4:1 lipid: protein ratio
- non covalent assemblies of phospholipids stabilized by non covalent forces
- asymmetric composition
- electrically polarized. usually negative inside. the membrane potential is about 60mV
- fluid mosaic model
- dynamic
how do lipids interact with water and the forces contributing to formation of membranes
- lipid bilayers form through self assembly of phospholipids via non covalent forces
- driven by hydrophobic effect and van Der Waals forces b/w acyl chains
- stabilized by interaction of hydrophilic head groups w/water. this is electrostatic interactions and hydrogen bonding
- cooperative structures can be extensive
- self healing structures: energetically unfavorable when planar. becomes energetically favorable when membranes fuse.
rank the permeability of gases, small uncharged polar molecules, large uncharged polar molecules, ions, charged polar molecules
gases - permeable
small uncharged polar molecules- slightly permeable
large uncharged polar molecules- impermeable
Ions- impermeable
charged polar molecules- impermeable
what are the categories of membrane proteins
peripheral membrane proteins
lipid-anchored proteins
integral membrane proteins
describe peripheral membrane proteins
- electrostatic interaction and H-bonding w/polar head groups and other proteins
- easily dissociated by ph, salt
describe integral membrane proteins
- usually span membrane
- hydrophobic domains interact w/hydrophobic core
- hydrophilic domains interact w/surface
- require detergents for disociation
describe lipid-anchored proteins
- covalently attached hydrophobic chain provides membrane anchor
what are the different motifs found in membrane-spanning domains of integral membrane proteins
- single pass transmembrane protein
- multi pass transmembrane proteins
- beta strand transmembrane proteins
describe single pass transmembrane protein
a-helix
oligomerization by coiled-coil formation
hydrophobic amino acid side chains protrude from helix:
van der Waals interactions w/ lipids
peptide bonds toward interior of helix
describe multi pass transmembrane proteins
- several a-helices
- bacteriorhodopsin: proton pump, similar to eukaryotic G-coupled protein family, 7 membrane spanning helices connected by loops
- eukaryotic aquaporins: facilitate diffusion of small hydrophilic molecules, half-helices form channel, tetramers w/ several membrane-spanning domains
describe b-strand transmembrane proteins
b-barrel motif formed by H-bonds between neighboring antiparallel beta-strands
- non polar residues outside
- polar residues line interior
describe a method for prediction of certain motifs
hydropathy plot:
- identifies segments w/hydrophobic amino acids from primary seq.
- plots free energy change for transferring 20 amino acid segments from hydrophobic to aqueous environment
- useful for predicting a-helical transmembrane domains
what effect do ionic detergents have on membrane proteins
- bind to hydrophobic regions of proteins (membrane or soluble proteins)
- denature proteins at high concentration
what effect do non-ionic detergents have on membrane proteins
- milder non-denaturing
- solubilize membrane proteins at low concentration
- solubilize membranes by micelle formation at high concentration
define lipide/lipodomics
Complement of lipid-containing molecules of a cell/organ/tissue and its analysis
describe unsaturated fatty acids
- cis configuration of double bonds in natural FA
- double bonds are usually separated by one or more methylene groups in polyunsaturated fatty acids
- have a lower melting point compared to saturated FA (ex. steric acid has a higher mp than oleic acid)
what are methylene groups
CH2
why are fatty acids a good source of energy?
they are highly reduced, more than carbs. so they are electron rich
- this means they can provide a lot more energy than carbs
what do membrane lipids contain
polar head group and non polar tail
membrane lipids are___
amphiphilic, meaning they contain both polar and non polar portions
contrast phospholipids and glycolipids
phospholipids:
- contain a phosphate group
- can have a glycerophospholipids/phosphoglycerides or sphingolipid/ sphingophospholipid
glycolipids:
- contain a carbohydrate
- can have sphingolipid or galactolipid
describe glycerophospholipids
glycerol attached to
- 2 FA
- Phosphate - alchohol
describe sphingolipids
phospholipid:
sphingosine backbone
- 1 FA
- Phosphate - alcohol
glycolipid:
sphingosine backbone
- 1 FA
- carbohydrate portion
galactolipids
glycerol backbone
- 2 FA
- carbohydrate
describe phosphatidic acid
- precursor glycerophospholipids/ Phosphogylcerides
- is just the phosphate group, no alcohol bound
- small amount found in membranes
- glycerol backbone; C1 and C2 are ester to FA one usually has a cis double-bond. C3 ester bound to the phosphate group
in phosphoglycerides the phosphatidate is ester bound to an alcohol
what are common alcohols in glycerophospholipids and their charges once glycerophospholipids
serine (PS)- (-)
ethanolamine (PE)- 0, zwitterion
choline (PC) - 0, zwitterion
glycerol (Cardiolipin) - (-)
inositol (PI)- (-)
- alcohol head group determines glycerophospholipid charge
where are PC, PE, and cardiolipin found
PE, PC- most abundant in eukaryotic membranes
Cardiolipin- mitochondrial inner membrane, bacterial membrane
describe plasmogens
C1= ether bond to FA, and Ca double bonded to Cb
C2= ester linkage
fights against reactive oxygen species (ROS)
increased chemical stability
functional significance not well understood
abundant in nervous tissue
least abundant in liver
describe ceramide
- the sphingosine backbone
- C18 amino alcohol molecule, with long unsaturated chain
- FA linked by amide bond to C-2
C1 in sphingolipids can be bond to?
phosphodiester bond to phosphate-alcohol= sphingophospholipids
- can be Phosphocholine or phosphoethanolamine
glycosidic bond to sugar= glycosphingolipids
what are the subclasses of sphingolipids
- sphingomyelins
- glycosphingolipids
- gangliosides
describe sphingomyelins/phosphosphingolipids
- head group is phosphocholine or phosphoethanolamine
- found in plasma membranes of animal cells (eukaryotes)
- lots in myelin
describe glycosphingolipids / sphingosine derivatives (cerebrosides + Globosides)
- head group is one or more sugars
- sugar is always extracellular
- used for cellular recognition (e.g blood groups)
- neutral
- cerebrosides: 1 sugar (can be glucose or galactose).
- Globosides: 2 or more sugars.
describe gangliosides
- sphingosine derivative
- several sugars with sialic acid
- negative charged
describe the clinical correlation of gangliosides with botulinum toxin
- toxin binds to protein receptor and ganglioside for uptake
- interferes with vesicle trafficking by blocking release of acetyl choline from motor neurons into synapsis
- causes flaccid paralysis
what do Amphiphilic/ amphipathic lipids do
- Determine physical features of membrane (thickness, fluidity)
- which determines barrier function
(regarding molecules like polar and charged molecules)
what do Membrane proteins do
- give specific functions to membrane that determine transport and communication)
what do free fatty acids and detergents form in water
micelles
what do phospholipids and sphingolipids form in water
bilayers or liposomes
what are the biological consequences of membranes
- Closed compartments
- Internal and external
faces/leaflets
cytosolic leaflet , exoplasmic leaflet
define cytosolic and exoplasmic leaflet leaflet
cytosolic: it is found on the internal side of the plasma membrane, but the external side of organelles
exoplasmic: it is found the external side of the plasma membrane, but the internal side of organelles
what happens to cytosolic and exoplasmic leaflets during budding and fusion (endocytosis and exocytosis)
they are persevered
ways to study membranes
- generate liposomes or planar
bilayers with different lipid composition- Can extract cellular membrane lipids with organic solvents
- create synthetic lipids
- study Liposomes
- study membrane features
- include other molecules for use of transfection and drug delivery
- Planarbilayertostudymembrane
permability
rank according to increasing permeability
K+
Na+
Glucose
Cl-
indole
H20
Glycerol
Tryptophan
urea
Na+
K+
Cl-
Glucose
Tryptophan
urea and glycerol
indole
H20
for small molecules, permeability depends on?
the solubility of the molecule in nonpolar solvent relative to its solubility in water
what does Membrane crossing involve?
shedding water solvation shell
dissolving in hydrophobic core
diffusing through core
and becoming resolvated by water
what are the motions of lipids within membranes
- Rapid lateral diffusion
- rotational motion
- depend on fluidity
-Very slow transverse lipid diffusion (flip-flopping) *very unlikely if uncatalyzed
– Helps to preserve membrane asymmetry
– Catalyzed by translocases (flippase, floppase, scramblase)
effect of heat on membrane fluidity
- heat increases disorder
- goes from gel-like or rigid to fluid like
what are membranes at physiological ph
fluid like (disordered)
how to quantify lateral motion of membrane components
- use Fluorescence recovery after photobleaching (FRAP)
- bleach area with laser beam
- then measure the time it takes for fluorescence to recover
- the bleached area is replaced with unbleached components by lateral diffusion
- can be restricted b/c of tethering to other molecules
factors that affect membrane fluidity
- temperature
phase-transition temp (melting temp) when membrane goes from solid like to fluid like - membrane composition
fatty acid length
fatty acid saturation
cholesterol content
what does fatty acid length do to membrane fluidity
- longer FA decrease fluidity b/c more interactions
- cis double bonds increase fluidity b/c less interactions
how does cholesterol affect fluidity
- more unsaturated FA. becomes stiffer. decreases fluidity
- prevents crystallization at lower temps
- in biological membranes with alot sphingolipids
cholesterols correlation with spur cell anemia
-reduced fluidity of RBC b/c of high cholesterol content
- produces RBC with shortened life spans (acanthocytes)
- results in spur cell anemia
- caused by chronic liver disease, alcoholic cirrhosis
how is membrane thickness affected
- saturated FA increase thickness
- Cholesterol increases thickness of membrane regions with a lot of phosphoglycerides
- Cholesterol has no effect on thickness of membrane regions with a lot sphingomyelin (SM) or phospho- sphingolipids
how is membrane curvature affected
- by lipid shape
- More curvature with cone-shaped lipids (PE) vs. cylinder-shaped (PC)
- inserted proteins
why is membrane curvature important
- for membrane budding and vesicle formation
describe membrane lipid asymmetry
exotoplasmic side
- Glycolipids
- Cholin-containing lipids (PC, SM)
cytosolic side
- NH2-group-containing lipids (PS, PE), and PI (negative charge)
Cholesterol evenly distributed in both leaflets
what are the consequences of membrane asymmetry
Affects membrane curvature
– Less fluid outer leaflet with SM, PC
– More fluid inner leaflet with PI, PE, PS
– natural curvature
Mediates cell signaling
– Phospholipid cleavage by
Phospholipase C
– PI Phosphorylation by PI Kinases
– Platelet aggregation (PS)
– Detecting apoptotic cells by externalized PS
describe membrane raft domains
- Microdomains with a lot of SM, cholesterol and certain glycolipids
- thicker and less fluid than sea domains
- a lot in certain membrane proteins (lipid- or GPI-anchored)
- Identified by resistance to detergents
do membrane protein content vary in different membranes? most abundant?
yes; in mitochondria
protein composition is…
cell-type specific
proteins mediate all membrane functions except?
the barrier
* mediate transport and signal transduction/communication
what are the types of lipid-anchored proteins
- Long-chain Fatty Acids
- Isoprenoids
- GPI-anchor
describe lipid-anchored protein: long-chain fatty acids
Palmitate, myristoate
* Linked by amide bond to N-terminal gly or thioester bond to internal cys
describe lipid-anchored protein: isoprenoids
- Prenyl group
- Linked by thioether bond to C-terminal cysteine
- Often second group linked to internal cysteine
describe lipid-anchored protein: GPI- anchor
- Glycolipid with PI linked to C-terminus
- Always extracellular
describe membrane carbs
Bound to proteins and membrane lipids
- most transmembrane proteins are glycosylated
- on the exoplasmic side
- Creates protective coating on
cells (glycocalix) - Prevents inappropriate cell-cell interaction
how to solubilize lipids and membrane proteins
- detergents (for lipids and integral membrane proteins)
detergents are what type of molecules
amphiphatic
describe detergents
- can be Ionic and non-ionic detergents
– Can get into bilayer and disrupt forces
– Water-soluble at low concentration
– Form micelles at higher concentration
which of the following are ionic detergents and which are non ionic:
A. sodium deoxycholate
B. Sodium dodecylsulfate (SDS)
C. Triton X-100
D. Octylglucoside
ionic: A,B
non ionic: C,D
What do ionic detergents do vs. non ionic detergents
ionic detergents:
– Bind to hydrophobic regions of proteins (membrane or soluble proteins)
– Denature proteins at high concentration
non ionic:
– Milder, i.e. non-denaturing
– Solubilize membrane proteins at low concentration
– Solubilize membranes by micelle formation at high concentration
