1

Lecture 28 (11/27/17) Lipids & MembranesA. Lipids

1. Roles2. Classes

a. Fatty Acidsb. Fatsc. Waxesd. Membrane lipidse. Terpenes

B. Membranes1. Introduction2. The 4 S’s3. Models for Membrane structure

a. Old Modelb. Datac. Fluid Mosaic Modeld. Testing the model

4. The Red-Blood Cell Membrane5. Membrane Asymmetry

a. Lipidsi. transverseii. lateral

b. Proteini. anchoringii. glycoproteins

6. Membrane Fluidity

•Reading: Ch10; 361-368, 370, 372, 376Ch11; 387-393, 395-401

•Problems: Ch10 (text); 1,3,4,8,10,12,14,16Ch10 (study-guide: applying); 1,3,4Ch10 (study-guide: facts);1-5,6-8Ch11 (text); 3,4,5,6,7,9,10,13,14Ch11 (study-guide: applying); 1,2,3Ch11 (study-guide: facts);1-5,8,9-13,19

NEXT•Reading: Ch7; 241-246

•Problems: Ch7 (text); 1,2,4,9,10Ch7 (study-guide: facts); 1,3,7,8,9,11,12

Lipids & Membranes

2

Lipids

anything “greasy”

• Membrane structure– main structure of cell membranes

• Storage of energy– reduced compounds: lots of available energy– hydrophobic nature: good packing

• Signaling molecules– paracrine signals (act locally)– steroid hormones (act body-wide)– growth factors– vitamins A and D (hormone precursors)

• Vitamins, Cofactors, and secondary products– Vitamins E & K: antioxidant & blood clot formation, resp.– coenzyme Q: ATP synthesis in mitochondria– Pigments, e.g., tomatoes, carrots, pumpkins, some birds– Water repellant in feathers and hides– Insulation & bouyancy control in marine mammals (blubber)

Lipids: Roles• Membrane structure

3

FunctionsofMembranes• Define the boundaries of the cell• Allow import and export

– Selective import of nutrients (e.g. lactose)– Selective export of waste and toxins (e.g. antibiotics)

• Retain metabolites and ions within the cell• Sense external signals and transmit information into the cell• Provide compartmentalization within the cell

– separate energy-producing reactions from energy-consuming ones– keep proteolytic enzymes away from important cellular proteins

• Produce and transmit nerve signals• Store energy as a proton gradient • Support synthesis of ATP

Lipids: Roles

Electron Micrograph of Biological MembranesLipids: Roles

4

Lipids: ClassesBiological molecules that are characterized by low

solubility in water, that is, are relatively hydrophobic.Classes of Lipids

1. Fatty acids

2. Fats (triglycerides)

3. Waxes

4. Membrane Lipids

5. IsoprenesTABLE 10-2 Eight Major Categories of Biological Lipids

Category Category code Examples

Fatty acids FA Oleate, stearoyl-CoA, palmitoylcarnitine

Glycerolipids GL Di- and triacylglycerols

Glycerophospholipids GP Phosphatidylcholine, phosphatidylserine, phosphatidyethanoloamine

Sphingolipids SP Sphingomyelin, ganglioside GM2

Sterol lipids ST Cholesterol, progesterone, bile acids

Prenol lipids PR Farnesol, geraniol, retinol, ubiquinone

Saccharolipids SL Lipopolysaccharide

Polyketides PK Tetracycline, erythromycin, aflatoxin B1

2 & 3 are sometimes classified together as ”simple lipids”

They have a high hydrocarbon content

Lipids: Fatty Acids• Carboxylic acids with hydrocarbon chains containing between

4 to 36 carbons– Almost all natural fatty acids have an even number of carbons. – Most natural fatty acids are unbranched.

• Biologically, most are found in ester linkages as the pKa is ~3.0, and would otherwise be very acidic.

• TWO CLASSES– Saturated: no double bonds

between carbons in the chain– Unsaturated: ≥1 double bonds

between carbons in the chain• Monounsaturated: one double

bond between carbons in the alkyl chain

• Polyunsaturated: more than one double bond in the alkyl chain – never conjugated

m.p. > 37 °C

m.p. < 20 °C

⇌–

+ H+

pKa≈3

5

ConformationofFattyAcids• The saturated chain tends to adopt extended conformations. • The double bonds in natural unsaturated fatty acids are in a cis

configuration, which kinks the chain.

Lipids: Fatty Acids

C18:0 = stearate

C18:1 = oleate

Lipids: Fatty Acids

Turkey 1 18-20 12-14 18-20 25-30

Saturated FA Unsaturated FA

6

Lipids: Fatty Acids

Need to Know: Common names, structure, symbol

Nomenclature• Fatty acids can be described by:

– systematic name: cis-9-octadecanoic acid– common name: oleic acid– delta numbering of carbon skeleton: 18:1Δ9

• describes location of the first carbon of the alkene in relationship to the carbonyl carbon

– omega numbering of carbon skeleton: 18:1ω9

• describes location of the first carbon of the alkene in relationship to the terminal methyl

• Omega-3 fatty acids are essential nutrients.– Humans need them but cannot synthesize them.– They include a-linoleic acid (ALA)(18:3w 3,6,9), Eicosapentaenoic acid (EPA),

and Docosahexaenoic Acid (DHA).• although DHA (22:6), and EPA (20:5) can be synthesized from ALA

Lipids: Fatty Acids

20:5w3,6,9,12,1522:6w3,6,9,12,15,1822:6(D3,6,9,12,15,18) Docosahexaenoic Acid (DHA)

18:1w9

7

Solubility and Melting Point of Fatty Acids• Solubility

– decreases as the chain length increases

• Melting Point – decreases as the

chain length decreases

– decreases as the number of double bonds increases

Lipids: Fatty Acids

Melting Point and Double Bonds• Saturated fatty acids pack in a fairly orderly way.

– extensive favorable interactions• Unsaturated cis fatty acids pack less orderly due to the kink.

– less-extensive favorable interactions• It takes less thermal energy to disrupt disordered packing of

unsaturated fatty acids.– Explains the lower melting point of unsaturated cis fatty acids.

Lipids: Fatty Acids

What kind of interaction?

…. van der Waals

8

Trans Fatty Acids• Trans fatty acids form by partial hydrogenation (reduction) of

unsaturated fatty acids.– done to increase shelf life or stability at high temperature of oils used in

cooking (especially deep frying)– Or to convert plant oils to margarine, a solid fat (partially hydrogenated

polyunsaturated oils).• A trans double bond allows a given fatty acid to adopt an extended

conformation.• Trans fatty acids can pack more regularly and show higher melting

points than cis forms.• Consuming trans fats increases risk of cardiovascular disease.

– Avoid deep frying partially hydrogenated vegetable oils.– Current trend: reduce trans fats in foods (Wendy’s, KFC).

Lipids: Fatty Acids

(Eliadic acid)

Lipids: ClassesBiological molecules that are characterized by low

solubility in water, that is, are relatively hydrophobic.Classes of Lipids

1. Fatty acids

2. Fats (triglycerides)

3. Waxes

4. Membrane Lipids

5. Isoprenes

2 & 3 are sometimes classified together as ”simple lipids”

They have a high hydrocarbon content

9

Triacylglycerols (Nonpolar)• The majority of fatty acids in biological systems are

found in the form of triacylglycerols.– Solid ones are called fats.– Liquid ones are called oils.

• The primary storage form of lipids (body fat) • Less soluble in water than fatty acids due to the

esterification of the carboxylate group• Less dense than water: fats and oils float.

Lipids: Fat

TriacylglycerolsLipids: Fat

Tristearoyl glycerol

1-Myristoyl-2-stearoyl-3-palmitoleoyl glycerol

Name?

10

Fats Provide Efficient Fuel Storage• The advantage of fats over polysaccharides:

– Fats and oils carry more energy per carbon because they are more reduced.

– Fats and oils carry less water per gram because they are nonpolar.• Glucose and glycogen are for short-term energy needs and quick

delivery.• Fats are for long-term (months) energy needs, good storage, and slow

delivery.• Fats can be treated with alkaline (NaOH), which will hydrolyze the ester

bonds leading to glycerol and salts of the fatty acids…...soap! Process is called saponification.

Lipids: Fat

Fats Provide Efficient Fuel StorageLipids: Fat

11

Lipids: ClassesBiological molecules that are characterized by low

solubility in water, that is, are relatively hydrophobic.Classes of Lipids

1. Fatty acids

2. Fats (triglycerides)

3. Waxes

4. Membrane Lipids

5. Isoprenes

2 & 3 are sometimes classified together as ”simple lipids”

They have a high hydrocarbon content

• Waxes are esters of long-chain saturated fatty acids withand saturated or unsaturated long-chain alcohols.

• Insoluble and have high melting points• Variety of functions:

– waterproofing of feathers in birds– protection from evaporation in tropical plants and ivy– protection and pliability for hair and skin in vertebrates– storage of metabolic fuel in plankton– used by people in lotions, ointments, and polishes

Lipids: Waxes

12

Wax: The Material of the Honeycomb

Beeswax is a mixture of a large number of lipids, including esters of triacontanol (C30:0) and cerylanol (C26:0).

Lipids: Waxes

Ceryl alcohol(C26:0)

Myristic acid(C14:0)

Lipids: ClassesBiological molecules that are characterized by low

solubility in water, that is, are relatively hydrophobic.Classes of Lipids

1. Fatty acids

2. Fats (triglycerides)

3. Waxes

4. Membrane Lipids

5. Isoprenes

2 & 3 are sometimes classified together as ”simple lipids”

They have a high hydrocarbon content

13

Classification of Membrane LipidsTwo major categories based on the structure and function:

1. Lipids that contain phosphate2. Lipids that do not contain phosphate

–each can be further separated into:• Glycerol-based and sphingosine-based

Lipids: Membrane Lipids

Classification of Membrane LipidsTwo major categories based on the structure and function:

1. Lipids that contain phosphate2. Lipids that do not contain phosphate

–each can be further separated into:• Glycerol-based and sphingosine-based

Lipids: Membrane Lipids

These structures are similar to those of FAT…

SphingoglycolipidsSphingophospholipidsSphingolipids

14

Lipids: Membrane LipidsMembrane

Lipids

Glycerolphospholipids

Phosphatidic Acid

(more precisely, 1,2-distearoyl-phosphatidic acid)

General Structure of Glycerophospholipids

• Primary constituents of cell membranes• The phosphate group is negatively charged at physiological pH.• Two fatty acids form ester linkages with the first and second hydroxyl groups of L-

glycerol-3-phosphate.• Unsaturated fatty acids are commonly found connected to C2 of glycerol-3-

phosphate.• The highly polar phosphate group may be further esterified by an alcohol; such

substituent groups are called the head groups.

Lipids: Membrane Lipids

1-Palmitoyl-2-linoleoyl-phosphatidyl-X(name of alcohol)

What are these “head groups?”

15

Lipids: Membrane Lipids

1-Stearoyl-2-linoleoyl-phosphatidyl-choline

Examples of Glycerophospholipids

Polar Head group

Non-polar tails

P

O

O–

Lipids: Membrane LipidsExamples of Glycerophospholipids

H

Phosphatidyl glycerol

16

Examples of GlycerophospholipidsLipids: Membrane Lipids

SphingophospholipidsLipids: Membrane Lipids

Sphingophospholipids

17

Examples of SphingophospholipidsLipids: Membrane Lipids

Sphingosine (C18)CeramideSphingomyelinCholine Sphingomyelin

•Choline Sphingomyelin = Ceramide (sphingosine + amide-linked fatty acid) + phosphocholine attached to the alcohol•Sphingomyelin is abundant in myelin sheath that surrounds some nerve cells in animals.

§The backbone of sphingolipids is NOT glycerol.§The backbone of sphingolipids is a long-chain amino alcohol sphingosine.§A fatty acid is joined to sphingosine via an amide linkage, rather than an ester linkage as usually seen in other lipids (hence the name) = ceramide.§A polar phosphate group is connected to ceramide by a phospho-ester linkage = sphingomyelin.§A polar alcohol is connected by another phospho-ester linkage = Choline sphingomyelin or Ethanolamine sphingomyelin

Sphingophospholipids

Lipids: Membrane Lipids

Sphingomyelin

Sphingomyelin

PhosphoethanolamineEthanolamine

Choline

H3–

– –

18

Sphingomyelin is Structurally Similar to Phosphatidylcholine

Lipids: Membrane Lipids

SphingoglycolipidsLipids: Membrane Lipids

Sphingoglycolipids

19

Sphingoglycolipids

outer face of plasma membranes.

Lipids: Membrane Lipids

CeramideCerebrosideGlobosideGanglioside

SphingoglycolipidsLipids: Membrane Lipids

20

Lipids: ClassesBiological molecules that are characterized by low

solubility in water, that is, are relatively hydrophobic.Classes of Lipids

1. Fatty acids

2. Fats (triglycerides)

3. Waxes

4. Membrane Lipids

5. Isoprenes

2 & 3 are sometimes classified together as ”simple lipids”

They have a high hydrocarbon content

Cholesterol & Terpenes (Isoprenes)Lipids: Membrane Lipids

isoprene2 x isoprene = terpene

21

Lipids: Membrane LipidsCholesterol & Terpenes (Isoprenes)

Lipids Can Provide Pigment

• Cholesterol– Tri-terpene– steroid nucleus: four fused rings (lanosterol)– hydroxyl group (polar head) in the A-ring– various nonpolar side chains

• The tetracycle structure of Cholesterol is almost planar.

Lipids: Membrane Lipids

Need to Know: Structure, numbering

Cholesterol & Terpenes (Isoprenes)

22

Lipids: Membrane Lipids• Cholesterol and related sterols are present in the membranes of most eukaryotic cells.– modulate fluidity and permeability– thicken the plasma membrane– no sterols in most bacteria

• Mammals obtain cholesterol from food or synthesize it de novo in the liver.• Cholesterol, bound to proteins, is transported to tissues via blood vessels.– When in excess, cholesterol in low-density lipoproteins (LDLs) tends to deposit and clog arteries.• Bile acids and many hormones are derivatives of cholesterol.

A Bile Acid(made by liver)

Sex Hormones(made by the gonads)

A Metabolic Hormone(made by adrenal gland)

Cholesterol & Terpenes (Isoprenes)

17

-19

12

711

Lipids: Membrane Lipids

V. radiata Plasma Membrane* 0 32 – 39 4 2 2 – –

*This is the mung bean and the PM contains a large fraction of phosphatidic acid (21%). From Yoshida et al. (1986) Plant Physiol 82:807

23

IntroductionThe 4 S’s

SizeSolubilityShapeStability

Models for Membrane structureOld ModelDataFluid Mosaic ModelTesting the model

The Red-Blood Cell MembraneMembrane Asymmetry

Lipidstransverselateral

Proteinanchoringglycoproteins

Membrane Fluidity

Lipids: Membranes

Lipids: Membranes• All cells have a cell membrane, which separates the cell

from its surrounding.• Eukaryotic cells have various internal membranes that

divide the internal space into compartments (i.e., organelles).

• Membranes are complex lipid-based structures that form stable, dynamic, pliable “sheets”/barriers

• Membranes are composed of a variety of lipids and proteins

24

Lipids: Membrane ProteinsTABLE 11-1 Major Components of Plasma Membranes in Various Organisms

Components (% by weight)

Protein Phosphlipid Sterol Sterol type Other lipids

Human myelin sheath 30 30 19 Cholesterol Galactolipids, plasmalogens

Mouse liver 45 27 25 Cholesterol —

Maize leaf 47 26 7 Sitosterol Galactolipids

Yeast 52 7 4 Ergosterol Triacylglycerols, steryl esters

Paramecium (ciliated protist) 56 40 4 Stigmasterol —

E. coli 75 25 0 — —

Note: Values do not add up to 100% in every case because there are components other than protein, phospholipids, and sterol; plants, for example, have high glycolipid content.

Membrane Composition Is Highly Variable in Different Organisms…

and different organelles

The 4 S’sSize

SolubilityShape

Stability

Lipids: Membranes

25

Lipids: MembranesSize Polar Head group

Non-polar tails

8 Å

40-60 Å

It has a “trilaminar” structure as seen in the EM

ShapePolar Head group

Non-polar tails

Phospholipids:

Membranes:

What are the consequences of this shape?

Shape

• Two major structures are observed: –Bilayers/vesicles –micelles

• Structures formed depend on:– type of lipid– Concentration

• Both form spontaneously in aqueous solution and are stabilized by noncovalent forces, especially hydrophobic effect due to amphipathic molecules: large polar head & tail

–Examples that form micelles: fatty acids, sodium dodecyl sulfate–Examples that form bilayers: phospholipids, glycolipids

• Micelles are composed of a few dozen to a few thousand lipid molecules.

Lipids: Membranes

What is this concentration dependence?

26

SolubilityLipids: Membranes

• The first molecules, at low concentration, go to the air/liquid interface and form a monolayer.•Once that is crowded, they

“dissolve” in the water•Once the concentration is sufficient

to form aggregates, micelles or vesicles form.• Depending on the lipid, this

concentration is called the “Critical Micellular Concentration” (CMC).

Detergent

Phospholipid

• Consists of two leaflets (e.g., layers) of lipid monolayers

Vesicle(Liposome)OriginallycalledBangosomes

afterSirAlexBangham

Solubility:MembraneBilayerLipids: Membranes

– Forms when lipids with polar head groups and more than one lipid tail are in aqueous solution

• phospholipids• sphingolipids

– Hydrophilic head groups interact with water on both sides of the bilayer.

– Hydrophobic fatty acid tails are packed inside.

27

• Synthetic vesicle membranes can be made in vitro and can contain artificially inserted proteins.

• The central aqueous cavity can enclose dissolved molecules.

• They are useful artificial carriers of molecules (e.g., drugs).

• Vesicles fuse readily with cell membranes or with each other.

• Permeable to hydrophobic molecules (lipids, e.g., steroids) and water, but not permeable to large polar solutes and ions

• Dynamic and flexible structures

StabilityLipids: Membranes

• Most cells continually degrade and replace their membrane lipids.

• Phospholipids are degraded by phospholipases A−D.

Stability:BiochemicalLipids: Membranes

N. naja

C. adamanteus

28

IntroductionThe 4 S’s

SizeSolubilityShapeStability

Models for Membrane structureOld ModelDataFluid Mosaic ModelTesting the model

The Red-Blood Cell MembraneMembrane Asymmetry

Lipidstransverselateral

Proteinanchoringglycoproteins

Membrane Fluidity

Lipids: Membranes

ModelsforMembraneStructureLipids: Membranes

OLD MODEL (ca. 1940-1970)Sandwich model proposed by Danielli-Davson.

Based on the structures in the EM Bilayer

Membrane proteins(mostly have b-structure)

Scientifically, this is a good MODEL because it is clearly TESTABLE!This model makes several testable predictions:

1) Protein-lipid interactions should be mostly electrostatic; proteins should have lots of charged groups.

2) Should be able to “wash” nearly all membrane proteins off the membranes with high salt.

3) Isolated membrane proteins should show lots of b-structure4) Importantly, NO PROTEINS ON THE INSIDE

29

ModelsforMembraneStructureLipids: Membranes

TESTING OLD MODEL: DATA1) & 2) Wash isolated membranes with high-salt solutions or changes in pH.

Ø Removes some but not all proteinsØ This leads to an operational definition of peripheral (those that wash off

with 0.5 M salt), an integral (those that remain after washing) membrane proteins

d & c

a, b, & e

Integral Peripheral Amphitrophic and GPI-linked proteins

ModelsforMembraneStructureLipids: Membranes

TESTING OLD MODEL: DATA3) Isolated membrane proteins should show lots of b-structure.

Ø Peripheral membrane proteins looked like cytosolic proteins

Ø CD showed there was actually more a-helix than b-structure

Ø Integral membrane proteins had patches of hydrophobic residues in their sequence

30

ModelsforMembraneStructureLipids: Membranes

OMG!!! NOT smooth inside!

Oops, maybe proteins DO span the membrane.

TESTING OLD MODEL: DATA4) Importantly, NO PROTEINS ON THE INSIDE.

Ø So, lets look: performed Freeze-fracture EM on cell membranes

Ø This immediately became an explanation for Integral membrane proteins.