rea lec 11 biomembranes fp (revised)

8/13/2019 Rea Lec 11 Biomembranes FP (Revised)

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CASA Exam 2

Wed (10/9) through Fri (10/11)

Chapters 5, 6, 7, 11 & 12

~ 50 multiple choice questions


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Eukaryotic genes include coding (exons)

and non-coding (introns) segments


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Prokaryotic mRNA is usually polycistronic

Prokaryotic ribosomes can recognize and bind near start codonsin the middle of an mRNA molecule.

Several related proteins are synthesized simultaneously from asingle mRNA


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The Lac Operon

!-galactosidase !-galactoside permease


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Many antibiotics target prokaryotic translation

Weve been able to borrowmuch of this technology from oureukaryotic cousins, the fungi.


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Many fungal antibiotics block translation

Blocked by Streptomycin


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Many fungal antibiotics block translation

Blocked by Tetracycline

Blocked by Chloramphenicol

Blocked by Cycloheximide


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Translation is terminated at stop codons

UAA, UAG and UGA They signal the

binding of release

factors (proteins). A hydrolysis reaction

then frees the

polypeptide. Nascent proteins are

typically met by

chaperone proteins as

they emerge from the

ribosome


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Posttranslational modifications

After biosynthesis, many proteins undergoposttranslational modificationsthat can

substantially alter protein stability and

function.


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Biomembranes are selectively permeable barriers to diffusion


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Biomembranes

most significantmacromolecular structure of cell

protect cell from environmental

chemistry

allow for the development of

concentration gradients

-- Equilibrium is Death --

must be flexible to allow for

changes in cell shape / motility

*


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In eucaryotic cells, the

plasma membrane

represents only 5% of

the total membrane in

the cell

All membranouselements of the cell

share a common

structure

Animal Cell


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Double MembraneNuclear Envelope

Mitochondria

Chloroplasts

Endomembrane System

Nuclear Envelope

Endoplasmic Reticulum

Golgi ApparatusEndosomes

Lysosomes

Transport Vesicles

Plasma Membrane

Animal Cell

*


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The Endomembrane System *


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Biomembranes are primarily

composed ofphospholipids,

amphipathic molecules thatspontaneously form stable bilayers.


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Amphipathic Membrane Components

*


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Figure 11-10 Essential Cell Biology ( Garland Science 2010)

phospholipidtriglyceride

Fat*


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Phosphatidylcholine is the most common

phosphoglyceridein biomembranes

1 saturated FA2 unsaturated FA3 polar head

*


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Membrane Lipids are Highly Diverse

> 500 Different Lipids

Found in Biomembranes

However, Only 5 Lipids Comprise

65 - 85%(gm/100 gm)

Most Biomembranes

*


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These 5 Lipids Comprise 65 - 85 % of Biomembranes *


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Only phosphatidylserine is (negatively) charged at neutral pH

*


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Sphingomyelin does not contain a glycerol backbone

------ phosphoglycerides ----- -- not--

*


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Sphingomyelin does not contain a glycerol backbone

Sphingosine

Cholinemoiety

amide


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Acetone is miscible with water

Acetone disturbs the structure of water, but is not excluded


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Acetone is miscible with water

Acetone disturbs the structure of water, but is notexcluded


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2-methylpropane is notmiscible with water

2-MP cannot hydrogen bond, and is excluded from waterinto an ice-likecage


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Amphipathic molecules form ordered structures in aqueous media

Detergents work by trapping oily contaminants in the

hydrophobic interior of micelles


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1 FA tail

2 FA tails

O

CO-

RO

O=P-O-

Amphipathic molecules form ordered structures in aqueous media

Minimize aqueous contact with hydrophobic moieties

*


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High concentrations of

phospholipids andrelated amphipathic

substances

spontaneously form

spherical compartments


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Liposomes (TEM)


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Liposomes can be used for drug / gene delivery

yourdrug

here

*


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Blackphospholipid membrane


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The lipid bilayer is a 2 dimensional fluid

Fast (~ 2 m/s)

Very Fast

(~ 5,000 rps)

Very Slow~ 1/month

*


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Membrane fluidityis determined by

temperature and membrane composition

longer fatty acid length shorter

less unsaturation more

high cholesterol intermediate

Liquid Crystal

*


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Table 10-1 Molecular Biology of the Cell( Garland Science 2008)


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PC : SM1 : 1

PC : SM: Chol1 : 1 : 1

Lipid Rafts

*


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Figure 10-14a Molecular Biology of the Cell( Garland Science 2008)

AFM image of lipid rafts *


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Atomic Force Microscopy (AFM)

Lifted from Wikipedia


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Figure 10-14b Molecular Biology of the Cell( Garland Science 2008)

Some signaling proteins concentrate at lipid rafts*


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Figure 10-17c Molecular Biology of the Cell( Garland Science 2008)

Phosphatidylinositol

Minor phospholipid

Localized to cytoplasmic

leaflet

Important docking &

signaling molecule

Phosphorylated by specific

lipid kinases

H

H

*


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Phosphatidylinositol (4,5) bis-phosphate

Important signaling molecule = PI(4,5)P2

Converted to PI(3,4,5)P3by PI 3-kinase

PI(3,4,5)P3serves as docking site for

certain signaling proteins (PH domain)

Pleckstrin Homology (PH) Domain

*


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glycolipidsPC SM

PS PE

In healthycells, PS is found exclusively on the inner leaflet

Lipids are asymmetrically distributed

across biological membranes

*


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plasma membrane is

made in the endoplasmicreticulum

new phospholipids are

inserted into thecytoplasmic side of the

ER membrane

enzymes (flippases)transfer phospholipids

across the plasma

membrane

*


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Newmembrane is distributed throughout the

endomembrane system through vesicular transport

lumen

Cytoplasmic Surface

extracellular

*


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Membrane proteins perform these essential functions


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4 functional classes of membrane proteins

glucose transporterNa-K ATPase

integrinsankyrin

!-adrenergic recInsulin rec

adenylyl cyclasephospholipase C


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Proteins constitute ~ 50% of the mass of biomembranes

(myelin = 25 % ; inner mito memb = 75%)

~ 1/3 of all proteins are membrane associated


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Integral vs Peripheral Membrane Associations

Integral membrane proteins

peripheralmembrane

proteins

*


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Membrane Associated Protein Strategies

transmembrane"helical proteins

transmembrane

!barrel proteins

tetheredproteins

*


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*


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A Bacterial Porin


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Ionic detergent Non-ionic detergent

Useful detergents for isolation of membrane proteins

*


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Figure 10-31 Molecular Biology of the Cell( Garland Science 2008)

*


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Halobacterium salinarum *


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Bacteriorhodopsin is a light-powered proton pump

*


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Spectrin-rich cortical network supports erythrocyte membrane

**


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Demonstration of lateral movement of proteins in cell membrane


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*Fluorescence RecoveryAfter Photobleaching


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Membrane proteins are usually restricted to specific regions

guinea pig sperm


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Four ways to restrict the distribution of membrane proteins

Stabilized by

cell-cell junctions

Tether to cortex Tether to ECM

Restricted by

tight junctions

*


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Epithelial cells use tight junctionsto prevent movementbetween apicaland basolateralmembrane domains *


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Figure 10-28a Molecular Biology of the Cell( Garland Science 2008)

*The Glycocalyx


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The Glycocalyx


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Lots of glycosylated residuesSer, Thr, Asn

Structure stabilized by

disulfide bridges

Most cysteine side chainsexist in the reduced state

-SH Not -S-S-

Glycosylation is absent

Its a Jungle out there !! *


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*Leukocyte Extravasation


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rea lec 11 biomembranes fp (revised)

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