Membrane Proteins and Lipids

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Membrane Proteins and Lipids

• Membrane Structure

Svetlana Lutsenko, Dept. Biochemistry and Mol.
Bology MRB 624, Ext. 4-6953, lutsenko_at_ohsu.edu
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Biological Membranes Have Multiple Functions

• Serve as barriers separating cell from the
  environment
• Generate intracellular compartments
• Provide nutrient uptake and excretion of
  biosynthetic products
• Mediate cell-cell communications and define cell
  identity
• Receive and transmit various signals

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Major components of cell membranesLipidsProte
insCarbohydrates

• Different membranes have different protein/lipid
  ratio (3.6 for mitochondria, 1.1 for erythrocytes
  plasma membrane, 0.25 for myelin)
• This property can be used to isolate various
  membrane fractions.
• The type of the membranes is identified by the
  presence of specific protein markers. For
  example Na,K-ATPase is a marker of plasma
  membrane, while succinate dehydrogenase is a
  mitochondria marker

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In cell membranes, there is a division of labor
between two major components, lipids and proteins
Lipids are responsible for compartmentalization
of cell metabolites Proteins carry out most of
the biochemical reactions taking place in a cell
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Lipid composition of cell membranes

• Phospholipids (the major lipids, 50-90 of total
  lipid content
• Most phospholipids are derivatives of
  diacyl-glycerol-3-phosphate, phosphatide)
• Sterols (5-25 of membrane content, sterols are
  usually more concentrated in the plasma membrane)
• Glycolipids (the least abundant, usually less
  than 5)

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The Structure of a Phospholipid Molecule
In eucaryots the two variable acyl residues (C14
to C24) are derived from unbranched fatty acids,
for example saturated C16 Palmitate
CH3-(CH2)14-COOH unsaturated C18 Oleate
CH3-(CH2)7-CHCH-(CH2)7-COOH
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Saturated and Unsaturated Acyl Chains have
Different Shapes

• Double bonds are usually in cis configuration.
  This results in a kink in the acyl chain

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Major Phospolipids in Mammalian Cell Membraneare
generated via esterification of the phosphate
group of phosphatidate by hydroxyl group of one
of several alcohols
Common alcohol moieties Choline
OH-CH2-CH2-N(CH3)3 Ethanolamine
OH-CH2-CH2-NH3
Serine OH-CH2-CH
COO- NH3 Glycerol OH-
CH2-CH CH2-OH OH
Inositol
O R1 C O C H2
R2 C O CH O
O
H2C - O - P O -
O
Phosphatidate or Diacylglycerol 3-phosphate is
a key intermediate in the biosynthesis of many
phospholopids
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The simplest glycolipid, cerebroside, contains
only 1 sugar residue. Gangliosides contain a
branched chain of as many as seven sugar residues
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Bile acids (bile salts) are polar derivatives of
cholesterol. They are formed in the liver from
cholesterol, and secreted into the gallbladder.
The bile acids eventually pass via the bile duct
into the intestine, where they aid digestion of
fats and fat-soluble vitamins.
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Formation and properties of lipid bilayer
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Amphipathic properties of phospholipids are
essential for spontaneous formation of the lipid
bilayer
bilayer
bilayer
Why do lipids form a bilayer and not a micelle?
- Two fatty acyl chains are too bulky to fit
into the interior of a micelle
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• Important features of lipid bilayers
• Self-assembly driven by hydrophobic interactions
• Close packing
• Ability to fuse and extend (this property forms a
  basis of exocytosis and endocytosis, fusion of
  egg and sperm cells, cell division)

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Detergents are used to disrupt the structure of
the lipid bilayer and to isolate membrane proteins
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The lipid bilayer is asymmetrical

• lipids are synthesized in the ER, then bud off as
  vesicles, going on to fuse with the plasma
  membrane or organelle
• during synthesis, lipids are segregated into a
  specific layer in the ER

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• in the Golgi, sugars can be modified on
  glycolipids only on the inside-where the enzymes
  are
• the orientation of the two layers is preserved
  during fusion with the destination membrane
• THEREFORE, the two layers of the bilayer have
  different composition (the bilayer is
  asymmetrical)
• the of different types of phospho-, glyco-, and
  sphingolipids is different in the two layers
  (cholesterol is about the same in two layers)

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Membranes are fluid structures
http//www.biochem.arizona.edu/classes/bioc462/462
a/NOTES/LIPIDS/diff.html
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Membrane fluidity is important for

• fusion of membranes, i.e. fusion of vesicles with
  organelles
• diffusion of new lipids and new proteins
  laterally, so they are equally distributed
• diffusion of proteins and other molecules
  laterally across the membrane in
  signaling/reactions
• proper separation of membranes during cell
  division

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Lateral movement (lateral diffusion) Lipids
mobility is about 2 mm per sec Protein mobility
varies from 10-4 to 0.4 mm per sec Rotation
across the bilayer(transverse diffusion,
flip-flop) A phospholipid molecules were shown to
flip-flop once in several hours Proteins do not
rotate
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Fluidity of the membrane also depends on lipid
composition
Factors that affect membrane fluidity Length of
the fatty acyl chains Degree of saturation of the
fatty acyl chains Presence of cholesterol
The rigid cholesterol ring system interferes with
close packing of phospholipid fatty acid tails
and thus inhibits transition from liquid crystal
to crystalline state upon temperature decrease.
At the same time, the rigid cholesterol makes the
membrane somewhat less fluid.
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Membrane Proteins and their Association with the
Lipid Bilayer
Integral (transmembrane) membrane proteins span
the membrane entirely ones or several times and
can only be removed from the membrane by
detergent Peripheral proteins are either
covalently linked to lipids or interact tightly
with transmembrane proteins. These proteins can
be removed from the membrane either by treatment
with enzymes or certain reagents without
disrupting membrane integrity
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a-helix is the major structural element of
membrane-bound proteins
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Examples of transmembrane proteins
Ca2-transporting ATPase
K - channel
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b-barrel
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Pore-forming toxins a-Hemolysin, aerolysin,
anthrax protective antigen, Clostridium septicum
a-toxin all of them are key players in bacterial
pathogenesis
a-Hemolysin a water-soluble 33.2 kDa protein
secreted by Staphylococcus aureus binds to the
blood cells and forms a 232.4 kDa pentameric
structure 100x100 A The water-filled pore has a
diameter of about 14A. Ions, water, and small
molecules (m.w. less than 2 kDa ) permeate the
channel, causing cell death. a-hemolysin has no
obvious hydrophobic sequences in the monomerand
represents the family of b-barrel channels
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The structure of membranes the fluid mosaic
model

• The model was proposed by Jonathan Singer and
  Garth Nicolson in 1972
• The major features of the model
• Membranes are two-dimentional solutions of
  oriented lipids and proteins
• The lipid bilayer has a dual role it forms a
  permeability barrier and interact with
  proteins keeping them soluble and often
  regulating the protein activity
• Membrane proteins are free to diffuse
  laterally, but not rotate from one side of the
  membrane to the other

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The shape of the cell and the mechanical
properties of the plasma membrane are determined
by cell cortex.