[2] Class 2:membrane Flashcards
Sheet like structures that form closed boundaries between different compartments:
Biological membranes
Biological membranes mainly consist of _____ and _____. ALso, _____ are linked to lipids and proteins.
Lipids,
Proteins,
Carbohydrates.
MEbranes have both a _____ and a ______ unit.
hydrophilic and hydrophobic
These are embedded in lipid bilayers and mediate distinct functions
Specific proteins.
Biological membranes form ________ assemblies.
Non-covalent
Biological membranes are symmetric or asymmetric?
Asymmetric
3 common features of biological membranes:
Asymmetric
Fluid structure: lipids and proteins can diffuse rapidly in the plane but not across the membrane.
Electrically polarized: (-) inside
Membranes are _________ b/c they contain a hydrophilic unit [polar head] and hydrophobic tail [straight or wavy lines]
Amphipathic
Why does oil form a monolayer when placed in water?
Polar heads are in contact w/ the water and the nonpolar hydrophobic lipid tails project into air
What is a consequence of the amphipathic nature of the membrane lipids?
Membrane formation
What are the 2 possible membrane formations of amphipathic membrane lipids?
Micelle
Lipid bilayer
THis is a limited structure
Micelle
This is favored structure for phospholipids and glycolipids:
10^6 nm =1mm
Lipid bilayer [bimolecular sheet]
Formation of lipid bilayers is a _______ process
Self-assembly
3 MAjor driving forces for the assembly of lipid bilayer [bimolecular sheet]:
Hydrophobic interactions- major force
Van der waals
Electrostatic and hydrogen-bonding
Attractive forces btw hydrocarbon tails
Van der waals
Attractions btw polar head groups and water molecules:
Electrostatic and hydrogen-bonding
What predominantly holds the lipid bilayer together?
Hydrophobic interactions
Hydrophobic interactions have 3 consequences of lipid bilayer formation:
Inherent tendency to be extensive
Tend to close on themselves leaving no edges exposed w/ hydrocarbon chains-forms compartments
Self-sealing- energetically favorable
Lipid bilayer low permeability for
Ions and most polar molecules
How difficult is it for water to traverse the membrane?
Easy- it is small, high [ ] and lack of a complete charge
Are all biological membranes asymmetric?
Yes
Is there even distribution of lipid molecules in lipid bilayers?
No, asymmetrically distributed.
Inner and outer leaf =different
Membrane ________ have unique orientations.
Proteins
Where is cholesterol present?
Large amounts of cholesterol are present in both leaflets- thought to be about equally distributed in both mono layers
Hydroxyl group of cholesterol is positioned wherein the membrane?
Near the polar heads of the phospholipid.
Rapid diffusion:
Lateral diffusion
-generally not stationary…fluid mosaic
Very slow diffusion:
Tranverse or flip-flop diffusion…Req’s more energy
Do proteins move in the fluid mosaic model?
What diffusion can they perform?
Yes, laterally they do not flip-flop or tranverse diffuse through membrane.
How long does it take a phospholipid molecules to flip-flop?
Once in a several hours.
During the tranverse and lateral diffusion, can asymmetry be preserved?
Yes
Membrane fluidity is controlled by:
FA composition
And cholesterol content
Fatty acid composition affects membrane fluidity how?
Length of FA chains;
Saturation levels;
Position of double bonds [cis]-provides more fluidity.
How does cholesterol content affect membrane fluidity?
Bidirectional…stabilizes at high Temp. And more flexibility at low temps.
Provides an environment where signal transduction and molecule transportation take place:
Cholesterol content
How do you classify difference in membrane proteins?
Based on the difference in dissociability
Interact extensively w/ hydrocarbon chains of membrane lipids.
Can be released ONLY by agents that compete for these nonpolar interaction-organic solvents and detergents.
Spans the entire lipid bilayer:
Integral membrane proteins
What aspect of the bimolecular sheet [lipid bilayer-membrane] is responsible for most of the dynamic processes?
Membrane proteins
How do different membranes differ?
Protein content
HOw can one view proteins in a membrane?
Sodiumdodecylsulfate polydactyl amine gel electrophoresis (SDS/PAGE)
A light-driven proton pump converts the energy of light into trans membrane proton gradient that is used to synthesize ATP:
Bacteriorhodopsin (BR)
Bacteriorhodopsin is built almost entirely of ______ b/c most amino acids are nonpolar and arranged ________ to the bilayer plane.
Alpha helices
Perpendicularly
The most common structure motif in membrane proteins:
Membrane-spanning alpha helices
Ex of integral protein that binds to luminal leaflet of the ER.
It is a homodimer consisting of primarily Alpha helices.
It is NOT a membrane-spanning protein
Cyclooxygenase [COX] 1
ALso called
Prostaglandin H2 synthase [PGHS] 2
These are bound to membranes primarily by electrostatic and hydrogen-bond interactions:
Peripheral membrane proteins
Peripheral membrane proteins dissociate from membrane by adding:
Salt or pH changes
Peripheral membrane proteins are bound to surfaces of:
Integral proteins bound on either:
Cytosolic or extracellular side of membrane
OR
Anchored to lipid bilayer by COVALENTLY attached hydrophobic chain; i.e. FA
How are lipids asso. W/ proteins?
Covalently
Lipids anchor their attached proteins to membrane and mediate protein-protein interactions.
3 kinds of modification:
Palmitoylation
Farnesylation
Glycosylphosphosphatidylinositol
Modification Of cysteine residues by a thioester bond.
Palmitoylation
Modification of cysteine residues at the C-terminus
Farnesylation
modification link to the carboxyl terminus
Glycosylphosphatidylinositol
Covalent attachment of a farnesyl (c15) unit to C-terminal tetrapeptide CAAX in which cys is followed by 2 aliphatic residues.
After farnesyl group is appended to protein in thioETHER linkage w/ cys residue, the AAX trip eptide is hydrolytic ally cleaved away.
Farnesylation at the C-terminus
Functions of farnesylation at the C-terminus :
Anchoring the protein to the membrane and facilitating the protein-protein interaction.
Thioester bond
Palmitoylation
Thioether bond
Farnesylation
Looks like O-glycosidic linkage:
GPI anchor
GPI anchor:
Links proteins to OUTER leaflet
Many Cell-surface hydrolytic enzymes and adhesions are tethered by GPI unit
Mitochondria 2 membrane systems:
Outer membrane and inner membrane
Internal ridges and cristae
Inner membrane mito.
2 compartments in mito.
Intermembrane space
Matrix
Oxidative phosphorylation takes place in the:
Inner-mitochondrial membrane
The site of most of the TCA cycle and FA oxidation:
Matrix
This membrane is permeable to most small molecules and ions
Outer membrane
Impermeable to nearly all ions and polar molecules:
Inner membrane
VDAC
Mitochondrial porin
