Download Membrane Proteins: Transport

Membrane Proteins: Transport
Membrane Structure
• All cells, as well as the organelles,
are surrounded by membrane.
• Membranes are made primarily of
phospholipids arranged in a
“bilayer”, that is the membrane is
two molecules thick (8000
membranes stacked would be
about the thickness of a sheet of
paper)
• Proteins “float” around in the
phospholipid bilayer, this
arrangement is called the “fluid
mosaic” model
RBC Protein Arrangement
• “Cell and Molecular Biology” is an attempt to elucidate
the way in which cells work.
Some Proteins are
Complex
Structures
• The photosynthetic
reaction center of
Rhodopseudomonas
vividis contains 4
different subunits:
two are multipass
integral membrane
peptides, one is an
integral protein facing
the cytosol, and one is
a peripheral peptide
facing the exterior of
the cell
Membrane Proteins
Membranes are Selectively
Permeable
• Some molecules can diffuse
through membranes.
• O2, CO2, and H2O can enter
and leave a cell by diffusing
through the membrane.
• Salts (ions) and many other
molecules such as sugars and
amino acids cannot diffuse
through membranes
Diffusion is the movement of solute particles from a region of higher
concentration to a region of lower concentration. (Actually from higher
free energy to lower free energy, charge also has an effect here.)
Osmosis
• If a membrane separates two
solutions of different
concentrations, and the solute
is not able to diffuse through
the membrane, the H2O will
move through the membrane
to equalize the concentrations
on each side.
• This process is called osmosis
– The higher concentration
solution is said to be hypertonic
– The lower concentration
solution is hypotonic
– Two solutions of equal
concentration are isotonic
Cells are Affected by the Osmotic Conditions of their
Surroundings
• Hypotonic conditions cause
a cell to swell as water
moves in.
• Hypertonic conditions cause
a cell to shrivel, as water
moves out. Plant cells
undergo a change called
plasmolysis as the cell
membrane pulls away from
the cell wall
• Animal cells will become
“leaky”, and may lyse, if
exposed to hypotonic
solutions.
• Plant cells are protected from
excessive swelling by the
cell wall.
Transport Across the Membrane
• Some molecules enter and leave the cell by passive
transport: they diffuse through the membrane
• Many molecules are actively moved across the
membrane by transport proteins in the membrane
• Some molecules move through protein pores or
channels in the membrane, moving from high
concentration to low concentration. This process is
called facilitated diffusion
• Active transport can move molecules from low
concentration to high concentration and requires
energy (often ATP)
Movement into Cells vs. Lipid
Solubility and Size
• Different molecules
diffuse into cells at
different rates
• Lipid molecules tend to
diffuse more readily
• Small molecules tend to
enter more easily
• Polar and charged
molecules do not diffuse
across membranes easily
Transport Kinetics
• How do you distinguish
transport from diffusion?
• If a protein is involved in
transporting a molecule
across a membrane,
saturation kinetics will be
seen
Transport or Diffusion?
• Generally several orders of magnitude faster
than diffusion
• Saturation kinetics observed (like MichaelisMenton); diffusion rate is directly
proportional to concentration of solute
• Simple diffusion: similar molecules enter at
similar rates; transport, specificity for
particular molecules
• Inhibition is possible for transport
Types of Transporters
• Different types of transport proteins can move : one
solute (uniports), two solutes in the same direction
(symports), or two solutes in opposite directions
(antiports).
Identifying Transport Proteins
• Transport proteins are tentatively identified by
isolating proteins bound to transport inhibitors
• Definitive identification requires reconstituting
membranes with the protein and demonstrating
transport activity
Ionophores: Permit Ions to Cross
Membranes
• Ionophores are common growth enhancers
in livestock feed, and are used in
veterinary medicine as a coccidiostat in
poultry
Determining the
Type of
Ionophore
• Movement of a carrier
will slow in a gel
compared to a liquid
• A channel will not be
effected as much by te
membrane phase
change
Transport Proteins show Specificity
• Many properties of
transport proteins
are similar to
properties of
enzymes
Models of Transport
• Movement of solute across the membrane is accomplished by
changes in the protein’s conformation.
Band 3 Anion
Antiport
• Anion (negative
ion) transporters
in the RBC are
responsible for
the ability of the
body to rid itself
of CO2
Types of Movement Across
Membranes
Properties of solute transport can be used to
distinguish between simple diffusion, facilitated
diffusion, and active transport.
Ion Channels
• Facilitated diffusion of small ions
• Some protein channels are large non-specific
channels
• But, separate channels exist for Na+, K+, Ca2+,
Cl• These are called ion channels
• They are not “just openings” in the membrane
Controlling Ion Channels
• Controlled ion channels are referred to as
“gates”
• These can be controlled by:
• signal molecules; Ligand gated Channels
• Electrical differences; voltage gated
channels
• Mechanical effects; receptors
Importance of Ion Channels
• Changes in Na+ and K+ allow nerve impulses to
be transmitted down a nerve cell
• Many toxins and anesthetics interfere with ion
channels
• Japanese puffer fish toxin (Tetrodotoxin) blocks
Na+ channels: a few micrograms can cause
paralysis and death if eaten
• Cystic fibrosis is caused by a defect in Clchannels; decreased fluid secretion causes thick
mucus which clogs airways and other passages