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Molecular, Cellular and Developmental Neuroscience
January 10, 2008 9-10:50 am
Membranes and Membrane Proteins
Lecturer: Professor Eileen M. LaferContact Info.: 415B, 567-3764, Lafer@biochem.uthscsa.edu
Recommended Reading: Stryer Edition 5: Chapters 12 and 13, pp. 319-369
or Stryer Edition 6: Chapters 12 and 13, pp. 326-372
BIOLOGICAL MEMBRANES
1. The boundaries of cells are formed by biological membranes.
2. The boundaries of organelles are also formed by biological membranes.
3. Membranes define inside and outside of a cell or organelle.
4. Membranes confer cells and organelles with selective permeability.
CHEMICAL SYNAPSE
MACROMOLECULAR CONSTITUENTS OF MEMBRANES
1. LIPIDS:cholesterol
sphingolipids: sphingomyelin (SP); gangliosides glyceryl phospholipids: phosphatidylcholine (PC),
phosphatidlyethanolamine (PE), phosphatidylglycerol (PG),
phosphatidlyserine (PS), phosphatidlyinositol (PI), cardiolipin (CL).
2. PROTEIN: integral and peripheral.
3. CARBOHYDRATE: in the form of glycoprotein and glycolipid, never free.
BIOLOGICAL
LIPIDS ARE
AMPHIPATHIC
Hydrophobic Tail
Hydrophilic Head
HOW DO AMPHIPATHIC MOLECULES ARRANGE
THEMSELVES IN AQUEOUS SOLUTIONS?
1. Micelles:
limited structures
microscopic dimensions:
<20 nm in diameter
MemSnD1.avi
MOVIE: MICELLE FORMATION
2. Lipid Bilayers:
bimolecular sheet
macroscopic dimensions:1 mm = 106 nm
MOVIE: BILAYER FORMATION
MemSnD2.avi
Both of these arrangements allow the hyrdrophobic regions to be shielded from the aqueous environment, while
the hydrophilic regions are in contact with the aqueous environment.
Which arrangement is favored by biological lipids?
BILAYERS
The two fatty acyl chains of a phospholipid or glycolipid are too bulky
too fit in the interior of a micelle.
LIPID BILAYERS FORM BY SELF-ASSEMBLY
1. The structure of a bimolecular sheet is inherent in the structure of the constituent lipid molecules.
2. The growth of lipid bilayers from phospholipids is a rapid and spontaneous process in aqueous solution.
LIPID BILAYERS ARE COOPERATIVE STRUCTURES
1. They are held together by many reinforcing non-covalent interactions, which makes them extensive.
2. They close on themselves so there are no edges with hydrocarbon chains exposed to water, which favors compartmentalization.
3. They are self-sealing because a hole is energetically unfavorable.
CHEMICAL FORCES THAT STABILIZE LIPID BILAYERS
1. Hydrophobic interactions are the primary force. These occur between the extensive hydrophobic lipid tails that are stacked in the sheet.
2. van der Waals attractive forces between the hydrocarbon tails favor their close packing.
3. Electrostatic interactions lead to hydrogen bond formation between the polar head groups and water molecules in the solution.
THEREFORE THE SAME CHEMICAL FORCES THAT STABILIZE
PROTEIN STRUCTURES STABILIZE LIPID BILAYERS
MEMBRANES THAT PERFORM DIFFERENT FUNCTIONS CONTAIN DIFFERENT SETS OF PROTEINS
A. Plasma membrane of an erythrocyte.
B. Photoreceptor membrane of a retinal rod cell.
C. Sarcoplasmic reticulum membrane of a muscle cell.
PROTEINS ASSOCIATE WITH THE
LIPID BILAYER IN MANY WAYS
1. A,B,C: Integral membrane proteins-Interact extensively with the bilayer. -Require a detergent or organic solvent to solubilize.
2. D,E: Peripheral membrane proteins-Loosely associate with the membrane,
either by interacting with integral membrane proteins or with the polar head groups of the lipids.
-Can be solubilized by mild conditions such as high ionic strength.
PROTEINS CAN SPAN THE MEMBRANE WITH ALPHA HELICES
Structure of bacteriorhodopsin.MOST COMMON STRUCTURAL MOTIF
-helices are composed of hydrophobic amino acids.
Cytoplasmic loops and extracellular loops are composed of hydrophilic amino acids.
A CHANNEL PROTEIN CAN BE FORMED BY BETA SHEETS
Structure of bacterial porin.
Hydrophobic amino acids are found on the outside of the pore.
Hydrophilic amino acids line the aqueous central pore.
INTEGRAL MEMBRANE PROTEINS DO NOT HAVE TO SPAN THE
ENTIRE LIPID BILAYER
Prostaglandin H2 synthase-1(one monomer of dimeric enzyme is shown)
PROTEIN DIMERIZATION LEADS TO THE FORMATION OF A HYDROPHOBIC
CHANNEL IN THE MEMBRANE
PERIPHERAL MEMBRANE PROTEINS CAN ASSOCIATE WITH MEMBRANES THROUGH COVALENTLY ATTACHED HYDROPHOBIC
GROUPS
FLUID MOSAIC MODEL ALLOWS LATERAL MOVEMENT BUT NOT
ROTATION THROUGH THE MEMBRANE
LIPID MOVEMENT IN MEMBRANES
LIPIDS UNDERGO A PHASE TRANSITION WHICH FACILITATES THE LATERAL DIFFUSION
OF PROTEINS
MOVIE: LIPID DYNAMICS
MemSnD3.avi
MOVIE: FLUID MOSAIC MODEL
MemSnD4.avi
MEMBRANE FLUIDITY IS CONTROLLED BY FATTY ACID
COMPOSITION AND CHOLESTEROL CONTENT
DIFFUSION ACROSS A MEMBRANE
The rate of diffusion of any molecule across amembrane is proportional to BOTH the molecule's diffusion coefficient and its
concentration gradient.
1. The diffusion coefficient (D) is mainly a function of the lipid solubility of the molecule.
Hydrophilic molecules (water soluble, e.g. sugars, charged ions) diffuse more slowly.
Hydrophobic molecules (e.g. steroids, fatty acids) diffuse more rapidly.
2. The concentration gradient (Cside1/Cside2) is
the difference in concentration across the membrane.
Diffusion always occurs from a region ofhigher concentration to a region of lower concentration.
For any given molecule, the greater the concentration difference the greater the rate of diffusion.
OUT IN
10 mM 1 mM
5 mM 5 mM
1 mM 10 mM
no diffusion
THE DIRECTION OF DIFFUSION FOLLOWS THE CONCENTRATION
GRADIENT
The overall rate of diffusion is determined by multiplying the diffusion coefficient and the
magnitude of the concentration gradient:
Rate ~ D x (Cside1/Cside2)
DIFFUSION RATES
SMALL LIPOPHILIC MOLECULES DIFFUSE ACROSS MEMBRANES BY
SIMPLE DIFFUSIONSIMPLE DIFFUSION
1. The small molecule sheds its solvation shell of water.
2. Then it dissolves in the hydrocarbon core of the membrane.
3. Then it diffuses through the core to the other side of the membrane along its concentration gradient.
4. Then it is resolvated by water.
MOVIE: LARGE AND POLAR MOLECULES DO NOT READILY DIFFUSE ACROSS MEMBRANES
BY SIMPLE DIFFUSION
MemIT1.avi
LARGE AND POLAR MOLECULES ARE TRANSPORTED ACROSS
MEMBRANES BY PROTEINACEOUS
MEMBRANE MEMBRANE TRANSLOCATION TRANSLOCATION
SYSTEMSSYSTEMS
1. Passive Transport (also called facilitated diffusion):
The transport goes in the same direction as the concentration gradient. This does not require an input of energy.
2. Active Transport:
The transport goes in the opposite direction as the concentration gradient. This requires an input of energy.
SODIUM-POTASSIUM PUMP
Actively exchanges sodium and potassiumagainst their concentration gradients utilizing
the energy of ATP hydrolysis.
Establishes the concentration gradientsof sodium and potassium essential for
synaptic transmission.
MOVIE:
Movie 12-10.avi
ACETYLCHOLINE RECEPTOR Passively transports sodium and potassium ions along their concentration gradients in response to neuronal signals. Example of a ligand-gated ion channel.
MOVIE: Movement through
an ion channel.
MemIT4.avi
CLASSIFICATION OF MEMBRANE TRANSLOCATION SYSTEMS:
TYPE CLASS EXAMPLE
Channel (translocates ~107 ions per second)
Passive Transport Passive Transport 1. Voltage Gated Na+ channel 2. Ligand Gated AChR 3. cAMP Regulated Cl- channel 4. Other Pressure Sensitive
Transporter (translocates ~102-103 molecules per second)
Passive TransportPassive Transport Glucose transporter
Active Transport Active Transport 1. 1o-ATPase Na+/K+ Pump 2. 1o-redox coupled Respiratory Chain
Linked 3. ATP-binding Multidrug resistance cassette protein transporter 4. 2o Na+-dependent
glucose transport
ACTIVE TRANSPORT MECHANISMS
Utilizes the downhill flow of one gradient to power the formation of another
gradient.
Utilizes the energy of ATPhydrolysis to transport a
molecule against its concentration gradient.
1o 2o
MEMBRANE TRANSLOCATION SYSTEMS DIFFER IN THE NUMBER AND DIRECTIONALITY OF THE
SOLUTES TRANSPORTED
This classification is independent of whether the transport is active or passive.
EXAMPLE: Uniporter
Primary Active Transport-Transports
Ca++ against its concentration gradient utilizing the energy of
ATP hydrolysis
Sarcoplasmic Reticulum Ca++ ATPase
MECHANISM of SR Ca++ ATPase
Na+/K+ Pump
EXAMPLE: SymporterSecondary Active Transport-Na+-Glucose Symporter:Transports glucose against its concentration gradient utilizing the downhill flow of Na+ along its concentration gradient previously set
up by the Na+/K+ pump.
Na+-Glucose Symporter
EXAMPLE: Uniporter
Passive TransportVoltage Gated Ion Channel
K+ Channel: selectively transports K+
PATH THROUGH THE K+ CHANNEL
MECHANISM OF ION SELECTIVITY OF THE K+ CHANNEL
K+ ions interact with the carbonyl groups of the TVGYG sequence in the selectivity filter. Note, Na+ ions are too small to make sufficient productive interactions with the channel.
MECHANISM OF VOLTAGE GATING OF THE K+ CHANNEL
"Ball and Chain Mechanism"
SUMMARY OF TRANSPORT TYPES
Recommended