Chapter 11: Biological Membranes and Transport

Dr. Clower

Chem 4202

Lipid Aggregates

Lipids are not “free”

Virtually insoluble in water

Associate to form separate phase

– Reduces contact of nonpolar chain with H2O

– Solvate polar head groups

Micelles

Bilayers– Structural basis for biological membranes

Micelles Spherical 10 – 1000s of lipids Free fatty acids Detergents

Bilayer

Two monolayers (leaflets) 3 nm (30 Å) thick Lipids are structurally similar

– Glycerophospholipids

– Sphingolipids

Liposome

Bilayer folded back on itself Hollow sphere Maximum stability in aqueous environment

– Loss of hydrophobic edge of bilayer

Biological Membranes

Surround cells Partition two aqueous environments of different

concentrations Formed from lipid bilayers

– Inner and outer leaflet

Flexible– Change shape without compromising integrity

Lipid mobility– Transfer of lipid through bilayer

Transverse diffusion Lateral diffusion

Transverse Diffusion

“Flip-flop” From one bilayer

leaflet to the other Rare Very slow without

catalyst– Polar head pass

through anhydrous core

– Catalyst = flippase

Lateral Diffusion Exchange of neighboring lipids in same bilayer

leaflet

Measure with: – Fluorescence recovery after photobleaching (FRAP)– Single particle tracking

FRAP

Single Particle Tracking

Membrane Fluidity Changes in conformation of chains

keep interior in constant motion– Low viscosity in interior– Increases close to head (limited

mobility) Liquid-disordered state (fluid)

vs. liquid-ordered statevs. paracrystalline state (gel)

Temperature dependent Favored by unsaturated FAs,

shorter FAs Sterols

– Reduce fluidity– Reduce freedom of movement/rotation

Membrane Fluidity

Lipids synthesized by cells to keep fluidity constant

Membrane Structure and Assembly Contain lipids and proteins

– Percent composition varies with function

Lipids – Can be the same or different– Most commonly:

Glycerophospholipids, sphingolipids, and sterols

Membrane Proteins Composition varies

– More widely than lipids Catalyze chemical

reactions Relay information Transport across

membranes 3 classes

A. Integral/intrinsicB. Lipid-linkedC. Peripheral/

extrinsic

A. Integral Proteins

Strongly associate to membranes– Hydrophobic interactions

Difficult to separate from membrane– Need detergent, denaturant

Amphiphilic– Nonpolar section in membrane– Polar section(s) on one or both sides of

membrane Example: cyclooxygenase

COX-1 with NSAID

Intergral Membrane Proteins Types I - VI Transmembrane

proteins– Span membrane– 3 domains– Preference for one

face or the other– Sugar residues

outside

Transmembrane Domain

Hydrophobic region Domain structure

– a-helix– b-barrel

Protein tertiary structure difficult to determine– 10-20% are integral– 1% structure determined

Predict presence when > 20 nonpolar AA residues

Use hydropathy index

Hydropathy Index

Free energy change accompanying movement of AA side chain from hydrophobic solvent into water– Charged or polar =

exergonic– Aromatic, aliphatic =

endergonic

Glycophorin ASingle a-helix

Bacteriorhodopsin7 helices connected by hydrophilic loops

Threonine and Tyrosine Interact with both polar and nonpolar regions Located on surface

Tyr = orange Thr = red Charged = blue

Rhodopseudomonas viridis Photosynthetic

reaction center 1200 residues 1st protein determined

by crystallography 4 non-identical

subunits Transmembrane

section = 11 a-helices Red = prosthetic

groups

b-barrel b-sheets not found in membrane interior b-barrels are 16-20 stranded anti-parallel sheet Typically 7-9 residues to span Alternate residues (at least) are hydrophobic

– Interact with lipid Ex: porins

– Found in membranes of gram-negative bacteria– Trimers of identical subunits – Barrel forms channel

Allows entry of charged/polar molecules R groups in channel can be polar

Membrane Proteins with b-Barrel Structure

B. Lipid-linked proteins

Covalently attached to lipids (anchor) Not as strongly associated as integral;

more strongly associated than peripheral 3 varieties

1. Prenylated proteins

2. Fatty acylated proteins

3. GPI-linked proteins

1. Prenylated proteins Lipid synthesized

from isoprene Linkage to Cys

residue at C-terminus

2. Fatty Acylated Proteins Myristic acid (14:0)

– Links to amine N of Gly at N-terminus

Palmitic acid (16:0)– Thioester linkage to

internal Cys

3. GPI-linked Proteins Glycosyl-

phosphatidylinositol Exterior surface only Glycerophospholipid

linked to tetrasaccharide – (3 Man; 1 Glc) – linked to C-terminus

through ethanolamine phosphate

C. Peripheral/Extrinsic Proteins

Easy to separate from membranes

Associate with membranes by binding at surface to lipids or integral proteins– H-bond or electrostatic

Do not bind lipids Regulate membrane-

bound enzymes or limit mobility of integral proteins (tether to intracellular structures)

Assembly of Membranes Fluid-mosaic model Proteins move in membranes due to lipid mobility Leaflets not equivalent in composition or function

Transport across Membranes

Nonmediated– Diffusion of nonpolar

molecule through membrane– From high concentration to

low concentration

Mediated– Through action of specific

proteins– Carrier proteins– Integral protein channels

Carrier Proteins Shuttle amino acids, ions, sugars etc. into cells Hydrophobic on outside Specific for ligands/substrates

Integral Protein Channels Means by which hydrophilic molecules/ions move

through hydrophobic membrane Typically selective for one molecule/ion Channel = protein complex

– Transverse cell membrane– Hollow, hydrophilic core– Hydrophobic outside interact with lipids

Transport Systems Integral proteins with binding sites on

either side of membrane Reversible process More than one type of molecule can be

transported Ex: lactose transporter of E. coli

– Lactose and H+

Summary of Transport Types

Chapter 11 Problems

3-4, 6, 11-15, 18