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Molecular, Cellular and Developmental Neuroscience
January 10, 2008 9-10:50 am
Membranes and Membrane Proteins
Lecturer: Professor Eileen M. Lafer
Contact Info.: 415B, 567-3764, [email protected]
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 Hydrophilic
Tail
Head
HOW DO AMPHIPATHIC
MOLECULES ARRANGE
THEMSELVES IN AQUEOUS
SOLUTIONS?
1. Micelles:
limited structures
microscopic
dimensions:
<20 nm in diameter
MOVIE: MICELLE FORMATION
MemSnD1.avi
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
a-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 a
membrane 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 of
higher concentration to a region of lower
concentration.
For any given molecule, the greater the
concentration difference the greater the rate of
diffusion.
THE DIRECTION OF DIFFUSION
FOLLOWS THE CONCENTRATION
GRADIENT
OUT
IN
10 mM
1 mM
5 mM
1 mM
no diffusion
5 mM
10 mM
DIFFUSION RATES
The overall rate of diffusion is determined by
multiplying the diffusion coefficient and the
magnitude of the concentration gradient:
Rate ~ D x (Cside1/Cside2)
SMALL LIPOPHILIC MOLECULES
DIFFUSE ACROSS MEMBRANES BY
SIMPLE 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
TRANSLOCATION
SYSTEMS
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 potassium
against their concentration gradients utilizing
the energy of ATP hydrolysis.
MOVIE:
Establishes the concentration gradients
of sodium and potassium essential for
synaptic transmission.
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
Channel
(translocates ~107 ions per second)
Passive Transport
1. Voltage Gated
2. Ligand Gated
3. cAMP Regulated
4. Other
EXAMPLE
Na+ channel
AChR
Cl- channel
Pressure Sensitive
Transporter (translocates ~102-103 molecules per second)
Passive Transport
Glucose transporter
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
1o
2o
Utilizes the energy of ATP Utilizes the downhill flow
hydrolysis to transport a of one gradient to power
molecule against its
the formation of another
concentration gradient.
gradient.
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
TransportTransports
Ca++ against its
concentration gradient
utilizing the energy of
ATP hydrolysis
Sarcoplasmic Reticulum
Ca++ ATPase
MECHANISM of SR Ca++ ATPase
EXAMPLE: Symporter
Secondary 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+/K+
Pump
Na+-Glucose Symporter
EXAMPLE:
Uniporter
Passive Transport
Voltage 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