Chapter10 - Membrane structure - 092408.PPT

8/10/2019 Chapter10 - Membrane structure - 092408.PPT

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Chapter 10

Membrane Structure


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All biological membranes have a common structure each is a very thinfilm of lipid and protein molecules held together by noncovalent interactions


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The Lipid Bilayer

Membrane lipids are amphipathic molecules, most of which

spontaneously form bilayers


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The most abundant membrane lipids are the phospholipids

Phospholipids have a polar head group and

two hydrophobic hydrocarbon tails


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The tails are usually fatty acids, and

they can differ in length. One tail

usually has one or more cis-double

bonds, while the other does not.


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The shape and amphipathic nature of the

lipid molecules cause them to form bilayers

spontaneouly in aqueous environments.


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Hydrophilic and hydrophobic molecules interact differently with water


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Packing arrangements of lipid molecules in an aqueous environment

Lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior

and expose their hydrophilic heads to water. Being cylindrical, phospholipid

molecules spontaneously form bilayers in aqueous environments


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Liposomes

The lipid bilayer is a two-dimensional fluid

Individual lipid molecules are able to diffuse freely within lipid bilayers.

Demonstrated using synthetic lipid bilayers andelectron spin resonance (ESR) spectroscopy


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A black membrane


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Phospholipid mobility


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The fluidity of a lipid bilayer depends on its composition and its temperature

The membrane becomes more difficult to freeze if the hydrocarbon chains are short

or have double bonds, so that the membrane remains fluid at lower temperatures


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The lipid bilayer of many cell membranes is not composed exclusively of

phospholipids, it often also contains cholesterol and glycolipids


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Eucaryotic plasma membranes contain large amounts of cholesterol


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Lipid bilayers can form domains of different compositions

Lipid phase separation in artificial lipid bilayers

(A) 1:1 mixture of phosphatidylcholine and sphingomyelin

(B) 1:1:1 mixture of phosphatidylcholine, sphingomyelin, and cholesterol


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Plasma membrane contains lipid rafts that are enriched in sphingolipids,

cholesterol, and some membrane proteins

Lipid rafts are small specialized areas in membranes where some lipids (primarily

sphingolipids and cholesterol) and proteins are concentrated


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Lipid droplets are surrounded by a phospholipid monolayer


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The asymmetry of the lipid bilayer is functionally important

The lipid bilayer of human red blood cells


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Signaling functions of inositol phospholipids in the cytosolic leaflet of the plasma membrane


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Glycolipids are found on the surface of all plasma membranes


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Membrane Proteins


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Membrane proteins can be associated with the lipid bilayer in various ways


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Covalent attachment of a protein to the membrane by a

fatty acid chain or a prenyl group


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In most transmembrane proteins the polypeptide chain crosses the lipid bilayer

in an -helical conformation


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Membrane regions and preferred amino-acid locations. A snapshot of a lipid bilayer membrane

and its three major regions. Grey, carbon atoms; red, oxygen; white, hydrogen bound to oxygen;

orange, phosphorus. In an -helix, 20 amino acids (blue) can span the hydrocarbon core, and 10

amino acids (green) can span the interfacial region. Arrows indicate where most of each amino

acid (denoted by its three-letter symbol) would be found at equilibrium based on transfer free-

energy measurements (Nature, 2005, Vol 433, 367-369)


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A possible scheme for the membrane-insertion decision, as proposed by Hessa et al. A top

view of the membrane and the translocon pore that crosses it. Inside the pore is a peptidehelix surrounded by water. The pore opens sideways into the membrane, allowing the helix

to interact with the membrane lipids. If the peptide is more compatible with lipid than with

water, it will transfer into the membrane; otherwise, it will continue to be moved through

the pore. The figure is only intended to convey the basic principle, and omits many

mechanistic and structural issues.

(Nature, 2005, Vol 433, 367-369)


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Hydropathy plots identify potential -helical segments in a polypeptide chain


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Converting a single-chain multipass protein into a two-chain multipass protein


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Steps in the folding of a multipass transmembrane protein


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An alternative way for peptide bonds in the lipid bilayer to satisfy their

hydrogen-binding requirements is for multiple transmembrane strands

of polypeptide chains to be arranged as sheet in the form of a closed barrel

M b i l l d d h i h i i h i


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Many membrane proteins are glycosylated and have intrachain or interchain

disulfide bonds


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The cell coat, or glycocalyx is the carbohydrate-rich zone on the cell surface

Likely functions

protect cells against mechanical and chemical damage

keep foreign objects and other cells at a distance


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A simplified diagram of the cell coat (glycocalyx)


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Structure and function of detergent micelles


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The use of mild detergents for solubilizing purifying and reconstituting


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The use of mild detergents for solubilizing, purifying, and reconstituting

functional membrane systems

Bacteriorhodopsin is a proton pump that traverses the lipid bilayer as seven helices


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The archaeanHalobacterium salinarumshowing patches of purple membrane

that contain bacteriorhodopsin molecules

Bacteriorhodopsin is a proton pump that traverses the lipid bilayer as seven helices

The three dimensional structure of a bacteriorhodopsin molecule


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The three-dimensional structure of a bacteriorhodopsin molecule

Membrane proteins often function as large complexes


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Membrane proteins often function as large complexes

The three-dimensional structure of the photosynthetic reaction center of the bacterium

Rhodopseudomonas viridis

Many membrane proteins diffuse in the plane of the membrane


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y p p


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Fluorescence recovery after photobleaching


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Three domains in the plasma membrane of a sperm cell


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Four ways of restricting the lateral mobility of specific plasma

membrane proteins

The cytosolic side of plasma membrane proteins can be studied in red blood


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The cytosolic side of plasma membrane proteins can be studied in red blood

cell ghosts

The spectrin based cytoskeleton on the cytosolic side of the human RBC membrane


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The spectrin-based cytoskeleton on the cytosolic side of the human RBC membrane


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Chapter10 - Membrane structure - 092408.PPT