Scramblase that transfer
phospholipids in both
directions is activared
sphingomyelin
Ion gradients
Across membrane, activities
of membrane proteins,
synthesize ATP, movement of
selected solute,produce
electrical signals
thin film of lipid and
protein molecules held by
noncovalent interactions
Lipid Bilayer
Fluid structure,
impermeable barrier to
water-solube molecules
Lipid
Molecules
Amphipathics that
have hydrophilic end
and a hydrophobic end
Phospholipids
Have a polar head
group and two
hydrophobic
hydrocarbon tails
phospholipases
activated by
extracellular
signals to
cleave
phospholipid
depends on
composition
and
temperature
Cholesterol
enhance the
permeability-barrier
glycolipids
Liposomes
spherical
vesicles
Black
Membrane
Planar
billayers
Phospholipid
translocators
Membrane-bound
enzymes, catalyze
the rapid flip-flop
of phospholipids
Membrane properties
Glycolipids In the
surface of the
membrane
The glycolipids are lipid
molecules with the most
extreme asymmetry
These molecules are
found in the noncytosolic
monolayer of the lipid
bilayer
The glycolipids tend to
self-associate,
Through hydrogen
bonds between their
sugars
Are in the cell surface
where they have
important roles
Some glycolipids
provide entry points for
certain bacterial toxins
Membrane proteins
Membrane proteins
perform most of the
specific functions of
membranes
Most trans-membrane
proteins are thought to
extend across the bilayer
as a single a helix,
Many proteins are attached
to the membrane only by
noncovalent interactions with
other membrane proteins
Membrane Proteins Can
Be Associated with the
Lipid Bilayer in Various
Ways
Many extend through
the lipid bilayer of the
membrane, with part of
their mass on either side
These transmembrane
proteins are
amphipathic
Having regions that are
hydrophobic and regions that
are hydrophilic
Their hydrophilic
regions are exposed to
water on either side of
the membrane.
Their hydrophobic regions
pass through the membrane
and interact with the
hydrophobic
Only transmembrane
proteins can function on
both sides of the bilayer or
transport molecules across
it.
Proteins bound to the
plasma membrane can be
readily distinguished by the
use of an enzyme called
phosphatidylinositolspecific
Many of the proteins of
this type can be released
from the membrane by
relatively gentle extraction
procedures
Such as exposure
to solutions of very
high or low ionic
strength
Of extreme pH,
which interfere with
protein– protein
interactions but leave
the lipid bilayer intact
In Most
Transmembrane
Proteins the
Polypeptide Chain
Crosses the Lipid
Bilayer in an a-Helical
Conformation
A transmembrane
protein always has
a unique
orientation in the
membrane.
The hydrogen
bonding between
peptide bonds is
maximized if the
polypeptide chain
forms a regular a
helix as it crosses
the bilayer
In single-pass
transmembrane
proteins, the
polypeptide crosses
only once
Whereas in multipass
transmembrane
proteins, the polypeptide
chain crosses multiple
times
Because transmembrane
proteins are notoriously
difficult to crystallize,
relatively few have been
studied in their entirety by
x-ray crystallography.
Some b Barrels Form
Large
Transmembrane
Channels
Multipass
transmembrane proteins
are comparatively rigid
and tend to crystallize
readily.
The number of b strands
varies widely, from as few
as 8 strands to as many
as 22
The b barrel
proteins are
abundant
in the outer
membrane of
mitochondria,
chloroplasts
Many Membrane
Proteins Are
Glycosylated
The great majority of
transmembrane
proteins in animal
cells are glycosylated.
Membrane Proteins
Can Be Solubilized and
Purified in Detergents
The
transmembrane
proteins can be
solubilized only by
agents
That disrupt
hydrophobic
associations and
destroy the lipid
bilayer.
As in glycolipids, the sugar
residues are added in the
lumen of the ER and Golgi
apparatus
In many bacterias
Red Blood Cell
Most protein molecules associated
with the human red blood cell
membrane are peripheral membrane
proteins bound to the cytosolic side of
the lipid bilayer.
Spectrin
Long, thin, flexible rod
about100nm in length.
that constitutes 25 %
of the
membrane-associated
protein mass.
Glycophorin
Small, single-pass transmembrane
glycoprotein with most of its mass on
the external surface of the
membrane.. a Helix
Ankyrin
The binding of spectrin depends on a large
intracellular attachment protein, which
attaches both to spectrin and to the
cytosolic domain of the transmembrane
protein band 3..
Band 3 protein
(anion transporter)
The main function of red blood cells is to carry
O2 from the lungs to the tissues and to help
in carrying CO2 from the tissues to the lungs.
Multipass membrane protein,
traversing the membrane in a
highly folded conformation. The
polypeptide chain is thought to
extend across the bilayer 12 times
Cell surface
Decorated by
carbohydrates, which
coat the surface of all
eucaryotic cells
Membrane Proteins
Membrane proteins do not tumble (flip flop) across the lipid
bilayer, but they do rotate about an axis perpendicular to the
plane of the bilayer
Bacteriorhodopsin
These proteins function as signal transducers rather than
as transporters: each responds to an extracellular signal by
activating another protein inside the cell, which generates
chemical signals in the cytosol,
Cells can confine proteins and
lipids to specific domains within
a membrane