This summary contains information about the following subjects: membranes, sphingolipids, glycolipids, phospholipids, bilayers, membrane proteins, integral membrane proteins, peripheral membrane proteins, folding of membrane proteins, beta-barrels, glycosylation of membranes, detergents, nanodiscs,...
Experimental Cell Biology: Membranes and Borderlines
Different Glycolipids
types of lipid Cell membranes contain glycolipids and cholesterol.
molecules Glycolipids have a phosphate head group instead of
found in cell sugar attached. Eukaryotic plasma membranes
membranes. contain large amounts of cholesterol (1 per
All of the phospholipid). It contains a ring structure to which
lipid molecules a polar hydroxyl group + nonpolar hydrocarbon
in the cell chain is attached. Cholesterol can make the plasma
membrane are amphiphilic. That means they membrane stiffer. When the temperature is high,
have a hydrophilic or polar end and a the membranes are more fluid. The fewer
hydrophobic or nonpolar end. Phospholipids unsaturated fats the membrane contains, the more
have a polar headgroup containing a fluid the membranes are. This is because the kink
phosphate group and two hydrophobic allows the chains to be further apart from each
hydrocarbon tails. One of the tails has one or other.
more cis-double bonds and is unsaturated,
while the other tail does not and is saturated. But why are bacterial membranes rigid if they do
Each cis-double blond creates a kink in the not contain cholesterol?
tail. The differences in length and saturation Glycon molecules are attached to the bacterial
of the tails influence how phospholipid membrane.
molecules pack against one another, thereby
influencing the fluidity of the membrane. The Phospholipids
main phospholipids in animal cell The amphiphilic nature of the phospholipid causes
membranes are the phosphoglycerides, these them to form bilayers spontaneously in aqueous
have a three-carbon glycerol backbone and environments. Hydrophilic molecules dissolve in
two long fatty acid chains that are linked water because they contain charged groups or
through ester bonds to uncharged polar groups that can form either
adjacent carbon atoms of the glycerol. The electrostatic interactions or hydrogen bonds with
the third carbon atom of the glycerol is water molecules. Hydrophobic molecules are
attached to a phosphate group, which is in insoluble in water because all of their atoms are
turn linked to one of the several types of uncharged and nonpolar and therefore cannot form
headgroups. Certain phospholipids are interactions with water molecules. When
confined to only one side of the membrane. amphiphilic molecules are exposed to an aqueous
The assembly of membranes begins in the ER. environment they spontaneously bury their
hydrophobic tails in the interior where they are
Sphingolipids shielded from the water and expose their
These are built from sphingosine rather than hydrophilic heads to the water. This way they can
glycerol. Sphingosine is a long acyl chain with form spherical micelles or bilayers (hydrophobic
an amino group NH2 and two hydroxyl tails sandwiched between hydrophilic head
groups OH at one end. The most common groups).
sphingolipid is sphingomyelin where a fatty The same forces that drive phospho-
acid tail is attached to the amino group and a lipids to form bilayers also provide
phosphocholine group is attached to the a self-sealing property. A small tear
hydroxyl group. Sphingomyelin contains half in the bilayer happens and the
the mass of the lipids in most mammalian cell edges of the lipids get exposed to
membranes. the water, the lipids tend to
rearrange spontaneously. They can
also form a sealed compartment
which can function as a vesicle!
, The bilayer Membrane proteins
The bilayer is fluid and the lipids and proteins Membrane proteins are associated with the lipid
can move. They can diffuse laterally or flip-flop bilayer in different ways. A polypeptide chain
between leaflets (this is very rare and there are usually crosses the lipid bilayer as an α-helix.
specific phospholipid translocators needed for Membrane proteins can be solubilized in
this to happen). detergents. The plasma membrane is reinforced
by the underlying cell cortex. A cell can restrict
the movement of its membrane proteins. The cell
surface is coated with carbohydrates.
Examples of membrane proteins
Lipid bilayers are not homogeneous, but may
form rafts. This means that the lipids are not Different classes of membrane proteins
evenly distributed through the cell. This could be
relevant for transportation (packing in vesicles).
Rafts are usually thick due to the presence of
sphingolipids with longer acyl chains. Rafts
make subcompartments in the membrane.
1) Single-pass integral. Is anchored by an α-helix
2) Multipass integral. Has more segments that
can connect to the membrane
3) Beta-barrel integral. Has β-strands. On the
The bilayer itself is asymmetric in composition bottom side, this membrane protein has amino
which is important for membrane function. The acid chains that are hydrophilic.
binding of specific proteins to lipids is often 4) Amphiphilic helix in one leaflet integral. The
important for signalling. For example, a part that’s in the membrane is hydrophobic and
phosphatidylinositol binds to a cytosolic protein the part outside of the membrane is polar/
that in turn activates phosphorylation. hydrophilic.
Phosphatidylserine (neg) is located at the 5) Lipid-anchored (lipoprotein) integral.
cytosolic side and glycoproteins are exclusively 6) GPI-anchored integral. An oligosaccharide
located in the outer layer and protect the group on the protein is bound to
membrane, generate electrical effects and are phosphatidylinositol. It is an lipoprotein.
involved in cell recognition. Some bacterial 7 + 8) Non-covalent interactions with other
toxins can also bind to these proteins and enter membrane proteins peripheral. This is not an
the cells. integral, cause it’s not in the membrane. It only
has connections with the proteins that are in the
membrane.
Why are membrane proteins so important?
• They are involved in cellular processes such as
signal transduction, communication and
transport
• 70% of drug targets are membrane proteins
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