Lipoprotein Membranes, Barriers and Gates

1
Lipoprotein Membranes, Barriers and Gates

• MSc Toxicology Cell Biology 2003

2
The common phospholipids

• The membranes in and surrounding cells consists
  principally of phospholipids and proteins
• Phospholipids are detergents and naturally
  orientate themselves as micelles or as bilayers
  depending on the shape of the lipid. This lipid
  bilayer is the basis of the membrane structure

3
Continued

• The common phospolipids are based on glycerol.
  The 1 and 2 hydroxyls are esterified with fatty
  acids in the range C12 to C24 with C16 and C18
  being most common in warm blooded animals. A
  phosphate group is attached to the 3 position and
  to that may be attached either a choline, serine,
  ethanolamine or inositol residue.

4
Assymetry

• Phospholipid bilayers separate distinct spaces
  (eg the cytosol and the outside world, the
  cytosol and a cell organelle etc.)
• The two sides of a lipid bilayer may be quite
  different in lipid composition, for example in
  the plasma membrane the outer leaflet is
  primarily phosphatidyl choline, the inner leaflet
  phosphatidyl ethanolamine and serine
• Flippases maintain the difference
• And this is important. The presence of serine on
  the outer surface of the membrane signals that
  the cell is dying by apoptosis and should be
  removed.

5
And proteins

• Proteins may associate with the lipid bilayer in
  a number of ways
• They may bind to the surface of the bilayer on
  one side or the other.
• They may pass through the bilayer and they may do
  so in either direction (remember a lipid bilayer
  is separating two distinct spaces eg cytosol and
  the lumen of the endoplasmic reticulum).
• They may pass repeatedly through the membrane.
  This is seen especially with enzymes which are
  acting on hydrophobic molecules which will be
  dissolved in the bilayer

6
Continued

• Other proteins are attached to the membrane in
  ways in which, in principle, they may readily be
  released. The two principle ways in which
  attachment concerns are-
• Links with inositol residues either directly or
  via an short chain of carbohydrates
• Links to hydrophobic molecules which act as
  membrane anchors. This is extremely important in
  the action of the small GTP binding proteins
  which play a major role in controlling cell
  function

7
Melting Membranes and fatty acid composition

• The phospholipid bilayer may organise itself in
  two distinct ways.
• At low temperatures the fatty acid side chains
  arrange themselves as a liquid crystal which
  will limit diffusion of proteins in the membrane
• As the temperature is increased the membrane
  melts allowing free diffusion of the fatty
  acid.
• The transition temperature between these forms
  is governed by the ease with which fatty acids
  pack. Long fatty acids and those with multiple
  double bonds pack poorly and have low transition
  temperature, useful for fish

8
Cholesterol

• Steroids are the second most important lipid in
  the cell membranes. The concentration of
  cholesterol (the one used in animal cells) varies
  markedly between different membrane systems,
  being highest in the plasma membrane.
  Cholesterol thickens the membranes and smears
  the transition from liquid crystal to fluid mosaic

9
Sphingolipids and lipid rafts

• While a part of the cholesterol is associated
  with the normal phospholipids another part is
  associated with the complex phospholipid
  sphingomyelin and forms rafts or microdomains
  which are especially concentrated in the plasma
  membrane. These lipid rafts seem to play a role
  in endo and exocytosis (eg HIV will only bud from
  a such a raft). They are both highly fashionable
  and poorly understood.

The figure shows a atomic force micrograph of
sphingomyelin rafts in a phosphlipid sea, The
yellow spikes are of a phosphtidyl
inositol-linked glycoprotrin
10
Other membrane lipids

• The mitochondrial inner membrane is rich in
  cardiolipin which makes up about 20 if the
  membrane lipid.
• Glycolipids are found only on thee outer surface
  of cells. In animals these largely are largely
  formed from sphingosine rather than from
  glycerol. Two major classes are the gangliosides
  and cerebrosides and, as these names indicate
  these are especially important in the central
  nervous system.

11
Passage across membranes

• Hydrophobic molecules will dissolve in the
  membrane and move across the membrane readily.
  Binding proteins in the cytosol will extract
  them from the membranes
• Fat soluble materials pass freely across
  membranes
• Water passes freely across the membrane
• Hydrophilic compounds pass very slowly across the
  membrane as illustrated by the difference in
  concentrations of Na and K ions between blood
  and cytosol. Transfer of such molecules requires
  permiases (only transport down a concentration or
  active transport (capable of transporting up a
  concentrationgradient

12
Assisted Passage

• The simplest way in which molecules can move
  across membranes is as a result of the action of
  permeases. These bind the material to be
  carried, flip in the membrane and release on the
  other side. They can only transport down a
  concentration gradient but if the compound is
  removed readily once in the cell (eg glucose is
  converted to glucose-6-phosphate) then there will
  be net movement
• Related are exchange transporters. These are
  especially important in mitochondria where they
  mediate exchange of ADP for ATP and carry di and
  tricarboxylate acids

13
Transfer up concentration gradients

• This requires energy. This may be supplied by
• By ATP directly. Examples here are transfer of
  Na and K across the plasma membrane. This
  carrier is especially interesting in that three
  positively charged ions are moved out of the cell
  for every two pumped in. The result is to set up
  a potential gradient across the plasma membrane.
  Other important pumps are the H ion pump that
  acidifies the endosomal compartment and lowers
  the pH in the stomach and the Ca ion pump in
  the e.r.

14
Continued

• 2) A second type of transfer up a concentration
  gradient is co-transportation. Here passage of
  one molecule up its concentration gradient is
  balanced by passage of another molecule down its
  gradient. Thus glucose is absorbed in the
  intestine by a co-transporter in which Na ions
  pass into the cell along with the glucose. The
  energy is provided indirectly by the Na/K pump.
  The glucose absorbed at the apical face is then
  released from the basolateral face by a permease

15
Movement of large molecules across membranes

• Passage of proteins across membranes requires
  specific recognition the protein carries on it
  a ticket which specifies its final location.
• The mechanisms will be described in later
  lectures when we discuss the cell organelles

16
Passage of messages across membranes

• Peptide hormones and growth factors act by
  binding to receptors on the cell surface
• The receptors are proteins which span the plasma
  membrane. The binding of a ligand to its
  receptor will cause a change in folding of the
  extracellular domaine of the receptor. This
  will, in turn result in changes both in the
  transmembrane domaine and then in the
  intracellular domaine.
• The change in the intracellular domaine will
  activate enzyme activity, especially protein
  kinases, which will trigger a signal cascade
  within the cell

17
Membrane domains

• We would expect that proteins should be able to
  diffuse freely in a membrane but in some cell
  types we find that they are restricted to
  particular areas. In some cases, such a
  epithelial cells the cells in a tissue are joined
  together by tight junctions in other cases such
  as spermatozoa and nerve axons the barrier is
  less clear

18
Membranes and plastic bags

• Because lipid bilayers are impermeable to
  hydrophilic we tend to think that they must
  resemble something like cling film. This is not
  the case. We can use laser tweezers to move
  the membrane around. In addition if we look at a
  living cell we can see that membranes are
  continually breaking apart and rejoining the
  nearest analogy being an oil slick. However it
  should also be noted that, as we will see in the
  next lecture, the plasma membrane, in particular,
  is associated with filamentous elements which
  allows the cells to deform then spring back into
  shape

19
To Summarise

• The core of the cell membranes is a phospholipid
  bilayer.
• There are considerable differences between the
  concentrations of other lipids both within and
  between cells
• Because the lipid bilayer is impermeable to
  hydrophilic materials special transport systems
  have evolved.
• Because of the vital role of membranes and
  membrane-associated proteins in cell function
  agents which damage the membrane will usually
  show marked cytotoxicity