Chapter 12

Lipids and Cell Membranes

The surface of a soap bubble is a bilayer formed by detergent molecules.

The polar heads(red) pack together, leaving the hydrophobic groups(green).

The boundaries of the cells = Biological membranes (the barriers that define the inside and the outside of a cell).

Prevent molecules leaking out and unwanted molecules from diffusing in.

Allow the cell to take up specific molecules and remove unwanted ones.

Internal membranes : mitochondria, chloroplasts, peroxisomes, lysosomes…

Many common features underlie the diversity of biological membranes

1. Membranes are sheetlike structures, only two molecules thick.(60Å~100Å)

2. Membranes consist of lipids and proteins.

3. Membrane lipids are small molecules that have both hydrophilic and hydrophobic moieties.

4. Specific proteins mediate distinctive functions of membranes (pumps, channels, receptors, energy transducers and enzymes).

5. Membranes are noncovalent assemblies.

6. Membranes are asymmetric.

7. Membranes are fluid structures.

8. Most cell membranes are electrically polarized.

12.1 Fatty acids are key constituents of lipidsFatty acid names are based on their parent hydrocarbons

-Fatty acid : long hydrocarbon chains of various lengths and degrees of unsaturation terminated carboxylic acid groups.-Fatty acid name is derived from the name of its parent hydrocarbon. -Ex> C18 saturated fatty acid = octadecanoic acid (18:0) parent hydrocarbon = octadecane C18 with one double bond = octadecenoic acid(18:1) C18 with two double bond = octadecadienoic acid(18:2) C18 with three double bond = octadecatrienoic acid(18:3)

16:0

18:1

- Fatty acid carbon atoms are

numbered starting at the carboxyl

terminals.

-Carbon atom 2 = α

-Carbon atom 3 = β

-Methyl carbon atom at the distal

end of the chain = ω-carbon atom-The position of double bond = Δ-Ex> cis-Δ9 = cis double bond between carbon 9 and 10.

-Double bond counting from the distal end = ω-carbon as number1.-Ex> ω-3

Fatty acids vary in chain length and degree of unsaturation

-16~18 carbon fatty acid are most common.-The properties of fatty acid and of lipids derived from them are dependent on chain length and degree of saturation.※ Unsaturated fatty acid = lower melting points saturated fatty acid = higher melting points (same length)

12.2 There are three common types of membrane lipids

※ Three major kinds of membrane lipids

1.Phospholipids

2.Glycolipids

3.Cholesterol

※ Lipids have a variety of biological roles :

1.Fuel molecules.

2.Signal molecules and messengers in signal

transduction.

3.Components of membranes.

Phospholipids are the major class of membrane lipids

-Phospholipids are abundant in all

biological membranes.

- Composed with 4 components.

-Platform are built may be glycerol,

a three carbon alcohol, or

sphingosine.

-One or more fatty acids

-Platform to which the fatty acids are attached

-Phosphate

-alcohol

-Phosphoglyceride : phospholipid derived from glycerol.

-C1 and C2 are esterified to the carboxyl groups of the two

fatty acid chains.

-C3 hydroxyl group is esterified to phosphoric acid.

-When no more addition → the simplest fatty acid,

phosphatidate

They can be attached to phosphate group as alcohol moieties.

- Sphingomyelin : backbone is sphingsine(amino alcohol

that contains a long, unsaturated hydrocarbon chain).

-The amino group of the sphingosine bachbone is linked

to a fatty acid by amide bond.

-The primary hydroxyl group is esterified to

phosphorylcholine.

Membrane lipids can include carbohydrate moieties

-Glycolipid : sugar containing lipid.-Derived from sphingosine.-Amide group is acylated by a fatty acid.

-Primary hydroxyl group is linked to one or more sugars.

-Sugar residues always on the extracellular side of the membrane.

Cholesterol is a lipid based on a steroid nucleus

-Cholesterol : a lipid with different structure. steroid, built from four linked hydrocarbon rings.

Hydrocarbon

Hydroxyl group

-Only found in virtually all animal membranes.

-Almost 25% of the membrane lipids in nerve cell.

-Absent from some intracellular membranes.

Archaeal membranes are built from ether lipids with branched chains

- Archaea membranes differ from eukaryotes or bacteria membranes.

1.Nonpolar chains are joined to a glycerol backbone by ether than ester linkages.(ether is more resistant to hydrolysis)

2.Alkyl chains are branched.(branched chain is more resistant to oxidation)

3.Stereochemistry of the central glycerol is inverted.

A membrane lipid is an amphipathic molecule containing a hydrophilic and a hydrophobic moiety

-Membrane lipids are amphipathic.

-Contains both a hydrophilic and

hydrophobic moiety.

-Overall shape : rectangular.

-Polar head : hydrophilic, red circle.

-Hydrocarbon chain : hydrophobic,

straight or wavy lines.

12.3 Phospholipids and glycolipids readily form bimolecular sheets in aqueous media

-Polar head groups favor contact with

water, hydrocarbon chains interact

with one another.

-Micelle : globular structure.

-Lipid bilayer : two lipid sheets.

Idealized view

Realistic view

-A micelle is a limited structure (less

than 200Å in diameter)

-Bimolecular sheet can extend to

millimeter or more.

-Lipid bilayers are held together by

many noncovalent interactions

(hydrophobic), they are cooperative

structures.

-Hydrophobic interactions have three

significant biological consequences.

1.Extensive

2.Close on themselves (no edges)

3.Self-sealing (no holes)

Lipid vesicles can be formed from phospholipids

-Lipid vesicles, or liposomes are aqueous

compartments enclosed by a lipid bilayer.

-Can be used to study membrane permeability or to

deliver chemicals to cells.

-Liposomes are formed by suspending a suitable lipid, such as phosphatidylcholine, in an aqueous medium.

-Sonicating

-Vesicles formed.(diameter of about 500Å)-Ions or molecules can be trapped in the vesicles.

-Molecule containing vesicles can be separated by dialysis or gel filtration chromatography.

-A bilayer membrane is formed across a 1-mm hole in

a septum that separates two aqueous compartments.

-This arrangement permits measurements of the

permeability and electrical conductance of lipid

bilayers.

Lipid bilayers are highly impermeable to ions and most polar molecules

-Lipid bilayer membranes have a very low permeability

for ions and most polar molecules. Why?

-Na+ and K+ traverse the membranes 109 times as

slowly as does H2O.

1/109

12.4 Proteins carry out most membrane processes

Erythrocyte

Photoreceptor in

retinal ro

d cell

SR in m

uscle ce

ll

-Specific proteins mediate membrane functions.

-Membranes differ in their protein contents.-Ex> Myelin(nerve fiber) = 18% Plasma membrane = 50% Energy transduction membrane(mitochondria and chloroplast) = 75%

- Membranes contains many proteins, but has a distinct protein composition.

Proteins associate with the lipid bilayer in a variety of ways

- Integral membrane protein : interact with the

hydrocarbon chains of membrane lipids(a, b, and c).

Can be released by a detergent or an organic

solvent.

- Peripheral membrane protein : bound to

membranes primarily by electrostatic and hydrogen

bond interactions with the head groups of lipid(d and

e). Can be released by adding salts or changing the

pH.

Proteins can span the membrane with alpha helices

-Bacteriorhodopsin : uses light energy to transport

protons from inside the cell to outside → proton

gradient.

-Almost α helices; 7 closely packed α helices.

-Span its 45Å width.

-Examination of the primary structure of

bacteriorhodopsin : most of the amino acid in α helices

are nonpolar(yellow) and only a very few are

charged(red).

-Membrane-spanning α helices are the most common

structural motif in membrane proteins.

A channel protein can be formed from beta strands

-Porin : built from β strands and contain

essentially no α helices.

-Each strand is hydrogen bonded to its neighbor in

an antiparallel arrangement.

-The outside surface of porin is nonpolar. Interact

with the hydrocarbon chains.

-The inside of the channel is hydrophilic and is

filled with water.

-Alternation of hydrophobic and hydrophilic

amino acids along each β strand. Why?

-Hydrophobic – yellow

Embedding part of a protein in a membrane can link the protein to the membrane surface

- Prostaglandin H2 synthase-1 : catalyzes the conversion of arachidonic acid into prostaglandin H2 in two steps.

1. Cyclooxygenase reaction

2. Peroxidase reaction

- Prostaglandin H2 promotes inflammation and modulates gastric acid secretion

- Lies along the outer surface of the membrane firmly

bound by a set of α helices with hydrophobic

surfaces that extend from the bottom of the protein

into the membrane.

- Partially embedded.

-The location of prostaglandin H2 synthase-1 in the

membrane is crucial to its function.

-The substrate(arachidonic acid) : hydrophobic, generated

by the hydrolysis of membrane lipids.

-The substrate reaches the active site through a

hydrophobic channel(yellow).

-Drug(aspirin) block the channel.

(by transfer acetyl group to Ser530)

Membrane proteins: all alpha helix or beta sheet?

Some proteins associate with membranes through covalently attached hydrophobic groups

Palmitoyl group attached to a cystein by a thioester bond.

Farnesyl group attached to a cystein at the carboxyl terminus.

Glycosylphosphatidylinositol(GPI) anchor attached to the carboxyl terminus.

By enzymesGlcNAc

mannose

Transmembrane helices can be accurately predicted from amino acid sequences

Phe : hydrophobic amino acid. unfavorable(+15.5kJ/mol)

Arg : positively charged amino acid. favorable(-51.7kJ/mol)

30Å ≈ 20 residues

-Window : the span of 20 residues chosen for this

calculation.

-The free energy change for each window is plotted

against the first amino acid at the window to create a

hydropathy plot.

- ≥80kJ/mol in a hydropathy plot : membrane spanning α

helix.

- Some membrane proteins contain membrane-

spanning features that escape detection by these

plot.

12.5 Lipids and many membrane proteins diffuse rapidly in the plane of the

membrane-Lateral diffusion : lipids and many membrane proteins are in lateral motion.- FRAP(fluorescence recovery after photobleaching) 1. Cell-surface component is labeled with a fluorescent chromophore. 2. A small region is viewed through a fluorescence microscope. 3. Fluorescent molecules in this region are destroyed by laser. 4. This region is monitored. 5. Bleached molecules leave and unbleached molecules enter. 6. Recover the fluorescent intensity in this region.

-The rate of recovery depends on the lateral

mobility of the component, which can be expressed in

terms of a diffusion coefficient, D.

-The average distance S = (4Dt)1/2

-Rhodopsin ‘s D = 0.4μm2/s-Fibronectin’s D = 10-4μm2/s (because it is anchored to actin filaments on the inside of the plasma membrane through integrin.)

The fluid mosaic model allows lateral movement but not rotation through the membrane

-Fluid mosaic model : membranes are two-dimensional solutions

of oriented lipids and globular proteins.

- lipid bilayer’s roles

: a solvent for integral membrane proteins.

: a permeability barrier.

-The lateral diffusion can be

rapid.

-The transition of a molecule

from one membrane surface to

the other is very slow =

transverse diffusion or flip-flop.

Membrane fluidity is controlled by fatty acid composition and cholesterol content

-Membrane transport or signal transduction depend to

the fluidity of the membrane lipids.

- The transition from rigid to

the fluid state takes place

rather abruptly as the

temperature is raised above

Tm, the melting temperature.

-Tm depends on the length of the fatty acid chains and on their degree of unsaturation.-A cis double bond produces a bend in the hydrocarbon chain. This bend interferes with a highly ordered packing of fatty acid chains, and so Tm is lowered.

-Bacteria regulate the fluidity of their membranes by varying the number of double bonds and length of fatty acid chains

-In animals, Cholesterol is the key regulator of membrane fluidity, (hydroxyl group : phospholipid head group, hydrocarbon tail : nonpolar core of the bilayer)-Cholesterol disrupts the interactions between fatty acid chain increasing the membrane fluidity. -Cholesterol forms specific complexes with some phospholipids to make lipid rafts

All biological membranes are asymmetric

-Membranes are structurally and functionally

asymmetric.

-Different component and enzymatic activity between

outer and inner membrane.

※ Na+ - K+ PUMP

-Na+ out of the cell.

-K+ into the cell.

12.6 Eukaryotic cells contain compartments bounded by internal membranes

Plasma membranePermeability barrier -Gram positive bacteria and

archaea.-When staining, violet color-Single membrane surrounded by cell wall (thick peptidoglycan layer)

-Gram negative bacteria (E.coli)-When staining, pink color-Double membrane-Thin cell wall (peptidoglycan) between them.-LPS (lipopolysaccharide)-Small molecules permeable due to porin

Protection

-Eukaryotic cells are

distinguished from

prokaryotic cells by the

presence of membranes

inside the cell that form

internal compartments.

(peroxisome, mitochondria,

nucleus)

- Membranes must be able to separate or join together so that cells and compartments may take up, transport, and release molecules.

- Many cells take up molecules through the process of receptor-mediated endocytosis.

- Hormones, antibodies, transport proteins

- The vesicle containing the LDL fuses with a lysosome→ degradation.

- Invaginated membrane Breaks off and fuses to form a vesicle.

※ LDL receptor : recycled.※Cholesterol and amino acid : store or use ※ virus and toxin also use receptor-mediated endocytosis

Iron is essential but Free iron ions are toxic because of free radical formation.

Special transport is necessary

- The reverse process (the fusion of a vesicle to a

membrane) is a key step in the release of

neurotransmitters from a neuron into the synaptic cleft.