Membranes Flashcards
what are the functions of cellular membranes
Permeability barrier/compartmentalization
- Communication
- Energy conversion
- Surface recognition
what is the structure of cell membranes
Composition of typical plasma membrane: ~ 45% lipid ~ 50% protein ~ 5% carbohydrate
- Membrane lipids are amphipathic (hydrophobic and hydrophilic)
e. g. phospholipids, cholesterol, glycolipids
what are phosphoglycerides
- glycerophospholipids
- Backbone = glycerol
- 2 fatty acids in an ester link
- Headgroup derived from alcohol
- Most lipids are phospholipids
whar ate the different headgroups on Phosphoglycerides
Phosphatidic acid: just an -H (net charge -2)
- phosphatidylethanolamine: ethanolamine attachment (net charge 0)
- phosphatidylcholine: choline attached (net charge 0)
- phosphatidyl serine: serine attachment (net charge -1)
dw about just know that headgroups vary
what are sphingolipids
- very common
- backbond = sphingosine (not a glycerol backbone)
- 1 fatty acids in an amide link
how are sphingolipids similar to glycerolipids
structurally similar
- Sphingolipids are similar in shape to glycerolipids
- The sphingosine mimics the glycerol plus one fatty acid
-
what are glycerolipids
Some sphingolipids have carbohydrate headgroups
Glycosphingolipids – part of the ABO blood typing
Explains blood compatibility,
what is cholesterol
- major animal cell sterol
- four fused rings
- steoid also has an alkyl side chain
- the polar group is a simple alcohol
how are lipids classified
- storage lipids: triacyl glycerol
- phospholipids: glycerophospholipids and sphingolipids
- glycolipids (sugar attached): sphingolpids and galactolipids (sulfolipids)
* both phospholipids and glycolipids are polar membrane lipids
what dictates membarne composition
- dictated by function
- different membrane types have diff lipids compositions (ir diff membranes have diff functions_
- phosphatidylcholine is a major lipid in all membranes
- cholesterol is a major component of plasma membrane
- minor lipids doesnt mean they are unimportant
what is a lipid bilayer
- non polar tails assoicte in interior, very stable this is the basic matrix of all biological membranes
*impermeable to ions and polar molecules
*note lipid MONOLAYER is never uncommon
what are liposomes
- resemble lipid bilayers, the center is squeous
- artificial liposomes allow the study of membrane transporters
what are the functions of membrane proteins and the two types
functions: transporters and channels, receptors, structural components, adhesion proteins, surface antigens
Types: peripheral membrane and integral membrane
*integral membrane prot may be covalently attached lipid anchor or transmembrane domain
what is the fluid mosaic model of membrane structure
- prot attaching to inner leaflet then those on other, paly different roles
- may have transmembrane protein, the one on cytoplasmic surface and extracellular will have diff domains bc performing diff functions
- have lipid anchroned (GPI= glycosylphosphatidylinosital)
how are mmembrane proteins removed from the memrabne
- if pH change or chelator (reomved stabilizing Ca2+) releases a membrane protein: peripheral protein
(pH changes how it can interact with protin intermedate or polar heads on membrane)
- if a detergent is needed to remove a memrbane protein: integral protein, hard to release bc alpha helix is hydrophobic so need to add detergent
- lipid anchored memrbane prot is released by phospholipase,
how are pheripheral membrane proteins attached to memrbae
- interactions between peripheral proteins and polar head groups of membrane lipids: electrostatic interations and hydrogen bonds
*loser, easy to disrupt
how are lipid anchored membrane proteins attached to membrane
can have GIP achor:
- binding to pro at carboxy term via carbocy terminous
- sugar containing lipid
palmitoyl group
- this anchoring typically happens on an internal Cys or Ser
- linkage via sulfur
Myristol gorup
- anchors via amino terminous on Gly
- linkage NOT via sulfur
Farnesyl (or geranylgeranyl) group
anchoring of gorup on carboxy terminims on a cys
- linkage via sulfur
*
how are membrane spanning proteins linked to embrane
- glycophorin A is a single spanning transmembrane prot
*most will ahve more than one transmembrane domain
- highly hydrophobic rep with yellow and less hydrophobic rep in blue
- amino term not ALWAYS on outside and carboxy in inside
Diversity of integral membrane protein organizations
type 1: C term inside and N term outside
type 2: N term in and C term outside
Type III: multiple transmembrane domains, but its one single protein
type IV: lipid anchored
type V: macromolecules, protein complex with multiple subunits (could be all the same gene product, ie same copy or could have diff subunits from diff genes) multiple polypeptides that come together (most are this category)
Type VI: have a combination of lipid anchored and a transmemrbane domain, some long terminous that is lipid anchored
what are hydropathy plots
- can be used to predict transmembrane helices
- Hydropathy plots = mean hydrophobicity of a protein segment
*used to predict hydrophobic alpha helices, looking at each individual amino acid and plotting its hydrophobility
If > 20 successive residues have a high hydropathy index, possibly a transmembrane segment
what is bacteriorhodopsin
- multispanning transmembrane protein
- 7 transmembrane segments, short extracellular loops, member of the G protein couple receptor family
what are annular lipids
- Crystal structures of membrane proteins often show a layer of well ordered lipids
- Head groups of these “annular” lipids interact with the hydrophilic extracellular loops
Fatty acids tails interact with the transmembrane helices
- Lipids look similar to bilayer fatty acid
- asocaite with integral emmebrane prot at endoplasmic reticulum
what are β-barrel integral membrane proteins
- Bacterial and mitochondrial outer membrane proteins are β-barrels
- Backbone hydrogen bonds are between strands
- Strands can be seven residues → don’t show up on hydropathy plots
(typically in eukaryotes transmambrane prot are alpha helicies, but beta barrels which are transmembrane segments also can happen
explain resides at the membrane interface
- Tyrosine and tryptophan residues are concentrated where the polar head groups meet the acyl chains
- Charged residues (Arg, Lys, Glu, Asp) are found almost exclusively in the aqueous phase
* in picutre all are alpha helicies but not meltoporin
what are glycoprotins
any of a class of proteins that have carbohydrate groups attached to the polypeptide chain
N-linked carbohydrate chain
- Asn side chain (-CO-NH2)
- N-acetylglucosamine (GlcNAc)
O-linked carbohydrate chain
- Ser or Thr side chain (-OH)
- N-acetylgalactosamine (GalNAc)
Sugar groups of glycoproteins and glycolipids: Contribute to cell surface recognition Function as receptors
how ar emembranes dynamic
- change shape without loss of integrity or becoming leaky
- fluid-mosic model: allows for lateral movement and flipping/flopping/scrambling
- membranes usually bounce off ech toher but can fuse
explain lipid bilayer state changes
Gel phase (cold)
- all motion of bilayer is constrained
- lipids ordered in paracrystalline state
*doenst really happen in physiologcal conditions
Liquid-ordered state (physiological)
- intermediate thermal motion of acyl chains and atoms
- lateral movemnt in plane of bilayer is allowed
liquid-disordered state (fluid state)
- hydrocarbon chains are in constant motion, no regualt organization
how does membrane composition affect fluidity
at physiolgical temp:
- Long chain fatty acids (C16:0, C18:0) pack well into liquid-ordered state, Unsaturated and shorter chain fatty acids favour liquid-disordered state and Sterols (i.e., cholesterol) reduce fluidity
- cells regulate lipid composition to achieve a constant membrane fluidity
ex: bacteria synthesize more unsaturated fatty acids and fewer saturated ones when cultured at low temperatues
explain the behaviour of cholesterol
Effect on membrane dynamics depends on lipid composition and temperature:
Long, saturated fatty acids: increases fluidity
→ cholesterol interferes with acyl chains from interacting
Unsaturated, cis fatty acids: decreases fluidity
→ cholesterol allows efficient packing of kinked chains
High temperatures: decreases fluidity
→ rigid cholesterol interacts with flexible acyl chains
Low temperatures: increases fluidity
→ cholesterol prevents acyl chains from interacting
what are flipases, floppases and scamblases
flipase: moves from outer to cytosolic leaflet, uses ATP
Floppase: remove phospholipids from cysosolic leaflet to the outer leaflet
Scramblase: moves lipids in either direction towards equilibrium (has properties of lip and flopase)
EXAM deff between flip and flop
explain diffusion in the membrane
At physiological temperatures, transbilayer motion or “flip-flop” occurs very slowly
Requires that the polar or charged head group leave its aqueous environment and move through the hydrophobic interior of bilayer
explain lateral diffusion by hops
- single particle tracking-follows a single lipid molecule on a short time scale (μs)
- lipids diffuse rapidly and freely within a restricted region
- More rarely, they “hop” into a new region
- Lipids behave as though they are corralled by fences, which they occasionally escape
*figure loking at same lipid, watching it move around
what may be credited for restriction of lipids and protein motion
spectrin
*‘fences” are created from cytoskeleton
Some proteins are tethered in larger aggregates or “patches”
Spectrin is part of the cytoskeleton: links to membrane proteins through ankyrin and may act as the corral, keeping lipids from diffusing freely
what are microdomains in plasma memrbanes
- lipid rafts
- protions of bilayer that are unique fomr toher protions
- diff head groups (blue adn green) mean theres diff integral memrbane prot in the microdomain
Note: lipids are not randomly distributed, even within a leaflet
