Please enable JavaScript.
Coggle requires JavaScript to display documents.
Biological Membranes and Transport, A3EB9C5C-B41B-48FD-ABE8-867694BA4977,…
Biological Membranes and Transport
基本組成及知識
Membranes
All cells have a cell membrane, which separates the cell from its surrounding.
Eukaryotic cells have various internal membranes that divide the internal space into compartments (i.e., organelles).
Composed of a variety of lipids and proteins
Complex lipid-based structures that form pliable sheets
Functions of Membranes
Sense external signals and transmit information into the cell
Provide compartmentalization within the cell
– separate energy-producing reactions from energy-consuming ones
– keep proteolytic enzymes away from important cellular proteins
Retain metabolites and ions within the cell
Produce and transmit nerve signals
Allow import and export
– Selective import of nutrients (e.g. lactose)
– Selective export of waste and toxins (e.g. antibiotics)
Store energy as a proton gradient
Define the boundaries of the cell
Support synthesis of ATP
Three major structure
Micelle
Forms in the solution of amphipathic molecules that have larger, more polar head than tail
– fatty acids
– sodium dodecyl sulfate
Aggregation of individual lipids into micelles is concentration dependent.
Micelles are composed of a few dozen to a few thousand lipid molecules.
Vesicle (Liposome)
The central aqueous cavity can enclose dissolved molecules.
They are useful artificial carriers of molecules (e.g., drugs).
Synthetic vesicle membranes can be made in vitro and can contain artificially inserted proteins.
Vesicles fuse readily with cell membranes or with each other.
Small bilayers will spontaneously seal into spherical vesicles in a concentration-dependent manner.
Membrane Bilayer
Hydrophilic head groups interact with water on both sides of the bilayer.
Hydrophobic fatty acid tails are packed inside.
Forms when lipids with polar head groups and more than one lipid tail are in aqueous solution
• phospholipids
• sphingolipids
Consists of two leaflets (e.g., layers) of lipid monolayers
Common Features of Membranes
Form spontaneously in aqueous solution and are stabilized by noncovalent forces, especially hydrophobic effect
Main structure is composed of two leaflets of lipids (bilayer)
– with the exception of archaebacteria: monolayer of bifunctional lipids
Protein molecules span the lipid bilayer
Sheet-like flexible structure, 30–100 Å (3–10 nm) thick
Asymmetric
Some lipids are found more commonly “inside.”
Some lipids are found more commonly “outside.”
Carbohydrate moieties are attached on the outer leaflet.
They can be electrically polarized.
Fluid structures: two-dimensional solution of oriented lipids
Fluid Mosaic Model of Membranes
Lipids form a viscous, two-dimensional solvent into which proteins are inserted and integrated more or less deeply.
Proteins can either be embedded in or associated with the membrane:
Integral proteins are firmly associated with the membrane, often spanning the bilayer.
Peripheral proteins are weakly associated and can be removed easily.
• Some are noncovalently attached
• Some are linked to membrane lipids.
Proposed in 1972 by Singer and Nicholson (UCSD)
The Composition of Membranes
Membrane Composition Is Highly Variable in Different Organisms
Membrane Composition Is Highly Variable in Different Organelles
Ratio of lipid to protein varies
abundance and type of sterols varies
lack of sterols in prokaryotes
type of phospholipid varies
cholesterol predominant in the plasma membrane, virtually absent in mitochondria
galactolipids abundant in plant chloroplasts but almost
absent in animals
Membrane Composition and Asymmetry
Lipid composition of membranes varies by:
tissues
organelles
organisms
Membrane Bilayers Are Asymmetric
The outer leaflet is often more positively charged.
Phosphatidylserine outside has a special meaning:
platelets: activates blood clotting
other cells: marks the cell for destruction
Two leaflets have different lipid compositions.
Archaea Have Multiple Unique Membrane Constituents
Unique linkages
ether linkages in archaea
ester linkages in bacteria
Membrane topology
monolayer in some archaea
Unique fatty acids
branched isoprene chains in archaea
unbranched fatty acid chains in bacteria
Lipid Monolayer
Sulfolobus solfataricus and relatives
volcanic hot springs
temperatures 75–80°C
acidic environment: pH 2–3
Better membrane stability by isoprenoid tetraethers with unique alcohols
Unique glycerol chirality in phospholipids
L-glycerol in archaea
D-glycerol in bacteria
Proteins in Membranes
Integral Membrane Proteins Have Selectively Placed Nonpolar Amino Acids Within the Membrane
Integral Membrane Proteins
Tightly associated with membrane
– Hydrophobic stretches in the protein interact with the hydrophobic regions of the membrane.
Removed by detergents that disrupt the membrane
Have asymmetry like the membrane
– different domains in different compartments
Purified integral membrane proteins still have phospholipids associated with them.
Span the entire membrane
Lipid Anchors
They contain a covalently linked lipid molecule.
long-chain fatty acids
isoprenoids
sterols
glycosylated phosphatidylinositol (GPI)
The lipid part can become part of the membrane.
Some membrane proteins are lipoproteins.
The protein is now anchored to the membrane.
reversible process
allows targeting of proteins
Some, such as GPI anchors are found only on the outer face of plasma membrane.
Farnesylation of Proteins
This reaction is catalyzed by farnesyl transferase.
Nonfarnesylated proteins do not go to the membrane and are inactive.
– promising cancer therapy (onco-Ras)
Farnesylation can be an intermediate in the lipidation of proteins.
Primary sequence of the protein contains
a signature for farnesylation: CaaX.
C is a conserved Cys
“a” is usually an aliphatic amino acid
“X” is Met, Ser, Glu, or Ala
Proteins can be targeted to the inner leaflet of the plasma membrane by farnesylation.
Three Types of Membrane Proteins
Amphitrophic and GPI-linked proteins
Amphitrophic proteins can be conditionally attached to the membrane by covalent interaction with lipids or carbohydrates attached to lipids.
Biological regulation results in attachment to, or cleavage from, lipids.
are linked to the membrane during specific regulatory events and can be reversibly removed.
integral membrane proteins
In the presence of strong detergents, integral membrane proteins can be removed from the membrane.
Peripheral (non-GPI linked) membrane proteins
Relatively loosely associated with membrane
Removed by disrupting ionic interactions either with
high salt or change in pH
Associate with the polar head groups of membranes
Purified peripheral membrane proteins are no longer
associated with any lipids.
can be dissociated from the membrane fairly easily during changes in ionic strength like pH changes.
Amino Acids in Membrane Proteins
Cluster in Distinct Regions
Tyr and Trp cluster at nonpolar/polar interface.
Charged amino acids are only found in aqueous domains.
Transmembrane segments are predominantly hydrophobic.
Functions
Channels, gates, pumps
ions (K-channel)
neurotransmitters (serotonin reuptake protein)
nutrients (maltoporin)
Enzymes
lipid biosynthesis (some acyltransferases)
ATP synthesis (F0F1 ATPase/ATP synthase)
Receptors: detecting signals from outside
hormones (insulin receptor)
neurotransmitters (acetylcholine receptor)
light (opsin)
pheromones (taste and smell receptors)
Hydropathy Plots Can Predict Transmembrane Domains
Membranes上lipids的運動
Membrane Dynamics: Transverse Diffusion
Spontaneous flips from one leaflet to another are rare because the charged head group must transverse the hydrophobic tail region of the membrane.
Membrane Phases
Heating causes phase transition from the gel to fluid.
Under physiological conditions, membranes are more fluid-like than gel-like.
– must be fluid for proper function
Depending on their composition and the temperature,
the lipid bilayer can be in gel or fluid phase
liquid-ordered state (i.e., “gel phase”): individual molecules do not move around
liquid-disordered state (i.e., “fluid phase”): individual molecules can move around
Membrane Dynamics: Lateral Diffusion
Individual lipids undergo fast lateral diffusion within the leaflet.
FRAP
From the rate of return of lipids, the diffusion coefficient of a lipid in the leaflet can be determined.
Rates of lateral diffusion are high (up to 1 µm/sec).
– A lipid can circumnavigate an E.coli cell in one second.
Fluorescence recovery after photobleaching (FRAP) allows us to monitor lateral lipid diffusion by monitoring the rate of fluorescence return.
Membrane Rafts
Lipid distribution in a single leaflet is not random or uniform.
Lipid rafts
contain clusters of glycosphingolipids with longer-than-usual tails
are more ordered
contain specific doubly or triply acylated proteins
allow segregation of proteins in the membrane
Sterols and Hopanols Increase Membrane
Rigidity and Permeability
Cell membranes of many eukaryotes contain sterols.
cholesterol in animals
phytosterols in plants
ergosterol in fungi
Cell membranes of aerobic prokaryotes contain hopanols.
Membrane Diffusion: Flippases
Special enzymes catalyze transverse diffusion.
– Though often referred to by category name “flippase,” there are unique unidirectional and bidirectional enzymes to catalyze lipid movement.
Some flippases use energy of ATP to move lipids against the concentration gradient.
Organisms Can Adjust the Membrane Composition
At higher temperatures, cells need more long, saturated fatty acids.
At lower temperatures, cells need more unsaturated fatty acids.
Membrane fluidity is determined mainly by the fatty acid composition and melting point.
More fluid membranes require shorter
and more unsaturated fatty acids.
Melting temperature decreases as double bonds are added.
Melting temperature increases with length of saturated fatty acids.
Membrane Curvature
Caveolin
Other Modes
Physical Properties of Membranes
Not permeable to large polar solutes and ions
Permeable to small polar solutes and nonpolar compounds
Can exist in various phases and undergo phase transitions
Permeability can be artificially increased by chemical treatment.
– when we want to get DNA into the cell
Dynamic and flexible structures
Examples of Membrane Fusion
Membranes can fuse with each other without exposure of lipids to aqueous solvent.
Fusion can be spontaneous or protein mediated.
Neurotransmitter Release Is Mediated
by SNARE-Type Proteins
V-SNARE assemble on the vesicle membrane.
Q-SNARE (e.g., SNAP-25) are regulatory proteins that are Ca2+ induced.
T-SNARE assemble on the target membrane.
