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Blueprint Series

Complete Solution
Chapter 7

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BIOMEMBRANE
STRUCTURE

REVIEW THE CONCEPTS
1
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Each layer is arranged so that the polar
head groups of the phospholipids are exposed to the aqueous environment on one side
of the bilayer and the hydrocarbon tails associate with the tails of the other layer to
create a hydrophobic core
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When stained with osmium tetroxide, which binds strongly to polar head groups, and viewed in cross section, the bilayer
looks like a railroad track with a light center bounded on each side by a thin dark line
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The amphipathic nature of phospholipid molecules (a hydrophilic head and hydrophobic tail) allows these molecules to self-assemble into closed bilayer structures when
in an aqueous environment
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The different types of proteins present on the two
faces of the bilayer contribute to the distinctive functions of each membrane, and control the movement of selected hydrophilic molecules and macromolecules across it
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The three main types of lipid molecules in biomembranes are phosphoglycerides,
sphingolipids, and steroids
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CHAPTER 7: Biomembrane Structure

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Such movements are driven by thermal energy,
and may be quantified by measuring fluorescence recovery after photobleaching,
the FRAP technique
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The extent and rate at which fluorescence recovers in the bleached area, as fluorescent molecules diffuse back into
the bleach zone and bleached molecules diffuse outward, can be measured
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The
degree of fluidity depends on factors such as temperature, the length and saturation of the fatty acid chain portion of phospholipids, and the presence/absence of
specific lipids such as cholesterol
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Water-soluble substances are hydrophilic; they are therefore repelled by the
hydrophobic core of the bilayer, which is composed of non-polar hydrocarbon
tails of the phospholipids
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The proteins fold such that their non-polar residues are in
contact with the phospholipid bilayer and their polar residues line the channel
through which the hydrophilic substances travel from one side of the cell membrane to the other
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Membrane-associated proteins may be classified as integral membrane proteins,
lipid-anchored membrane proteins, or peripheral membrane proteins
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Lipid-anchored membrane proteins have one
or more covalently attached lipid molecule, which embeds in one leaflet of the
membrane and thereby anchors the protein to one face of the bilayer
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7
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aquaporins
b
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Cytosolic proteins are anchored to the plasma membrane by acylation or prenylation
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Prenylation occurs when the –SH group on a cysteine residue at or near
the C-terminus of the protein is bound through a thioether bond to either a farnesyl or a geranylgeranyl (prenyl) group
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CHAPTER 7: Biomembrane Structure

9
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Each face therefore interacts with different
environments and performs different functions
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For example, different types of phospholipids and lipid-anchored membrane
proteins are typically present on the two faces
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Finally, in the case of
the plasma membrane, the lipids and proteins of the exoplasmic face are often
modified with carbohydrates
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Detergents are amphipathic molecules
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This breaks up the organization of the lipid bilayer, ultimately leading to
formation of micelle droplets, composed of a single phospholipid layer with the
polar heads in contact with water and a hydrophobic core excluding water
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Because of their charge, they can also disrupt the ionic and hydrogen
bonds holding together the secondary and tertiary structure of a protein and are
thus useful for completely denaturing a protein
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At concentrations below the critical micelle concentration, they also prevent the
hydrophobic regions of proteins that have been extracted from the cell membrane from interacting with each other and forming insoluble aggregates
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a
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lipid anchored
c
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No, a strong ionic detergent like SDS will
denature the protein
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Lipid raft
13
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Membrane phospholipids are synthesized at the interface between the
cytosolic leaflet of the endoplasmic reticulum (ER) and the cytosol
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Membrane-bound enzymes of the ER then link these small molecules to
create larger, hydrophobic membrane phospholipids
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Membrane phospholipids can be flipped from the cytosolic leaflet of the
ER membrane to the exoplasmic leaflet
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c
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g
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Some of this transport is by vesicles
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Small, soluble lipid-transfer proteins also mediate transfer
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CHAPTER 7: Biomembrane Structure

14
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These fatty acids differ in carbon atom number by multiples of 2 because
they are elongated by the addition of 2 carbon units
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15
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Therefore, they associate with fatty-acid binding proteins
(FABPs),which are cytosolic proteins that contain a hydrophobic pocket
or barrel, lined by b sheets
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16
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This enzyme catalyzes the
rate-controlling stepin cholesterol biosynthesis
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In fact, the cholesterol
biosynthetic pathway was the first biosynthetic pathway shown to
exhibit this type of end-product regulation
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The
expression and enzymatic activity of HMG-CoA reductase is
suppressed
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Sterol sensing by
this domain triggers the rapid, ubiquitin-dependent proteasomal
degradation of HMG-CoAreductase
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17
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One line of
evidence for thisis the effect of chemical and mutational inhibition of
the classical secretory pathway
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Membrane lipids produced in the ER cannot move
to the mitochondria by classic secretory transport vesicles
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Other
mechanisms are thought to exist
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They include direct membrane-membrane contact and small,
soluble lipid-transfer proteins
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Statins block the conversion b-hydroxy-b-methylglutaryl linked to CoA
(HMG-CoA) to mevalonate (an important intermediate in cholesterol
synthesis) by competitively binding the enzyme necessary for this
conversion (HMG-CoAreductase)

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