Lipids and Membranes

Lipids

• Lipids are compounds that are soluble in non-polar organic solvents, but insoluble in water.

• Can be hydrophobic or amphipathic

Major Lipid Classes

•Acyl-lipids - contain fatty acid groups as main non-polar group

• Isoprenoids – made up of 5 carbon isoprene units

Lipid Subclasses

Function of major acyl-lipids

• Phospholipids – membrane components• Triacylglycerols – storage fats and oils• Waxes – moisture barrier• Eicosanoids – signaling molecules

(prostaglandin)• Sphingomyelins – membrane

component (impt. in mylein sheaths)• Glycospingolipids – cell recognition

(ABO blood group antigen)

Function of major isoprenoid lipids

• Steroids (sterols) – membrane component, hormones

• Lipid Vitamins – Vitamin A, E, K• Carotenoids - photosynthetic accessory

pigments• Chlorophyll – major light harvesting pigment• Plastoquinone/ubiquinone – lipid soluble

electron carriers• Essential oils – menthol

Fatty acids• Amphipathic molecule• Polar carboxyl group• Non-polar hydrocarbon tail• Diverse structures (>100

different types)• Differ in chain length• Differ in degree of

unsaturation• Differ in the position of double

bonds• Can contain oxygenated

groups

Fatty acid nomenclature

• Short hand nomenclature describes total number of carbons, number of double bonds and the position of the double bond(s) in the hydrocarbon tail.

C18:1 9 = oleic acid, 18 carbon fatty acid

with a double bond positioned at the ninth carbon counting from and including the carboxyl carbon (between carbons 9 and 10)

C1

C2C3

C4C5

C6C7

C8C9

C10C11

C12C13

C14C15

C16C17

C18O

HO

Fatty acid nomenclature

• Omega () notation – counts carbons from end of hydrocarbon chain.

• Omega 3 fatty acids advertised as health promoting

• Linoleate = 18:3 9,12,15 and 18:33,6,9

C1

C2C3

C4C5

C6C7

C8C9

C10C11

C12C13

C14CH15

C16C17

C18O

HO

Common saturated fatty acids

common name IUPAC name melting point (Co)

12:0 laurate dodeconoate 44

14:0 myristate tetradeconoate 52

16:0 palmitate hexadeconoate 63

18:0 stearate octadeconoate 70

20:0 arachidate eicosanoate 75

22:0 behenate docosanoate 81

24:0 lignocerate tetracosanate 84

Common unsaturated fatty acids

common name IUPAC namemelting point

(Co)

16:0 palmitate hexadeconoate 63

16:1 9 palmitoleate cis-9-hexadeconoate -0.5

18:0 stearate octadeconoate 70

18:1 9 oleate cis-9- octadeconoate 13

18:2 9,12 linoleate cis-9,12- octadeconoate -9

18:3 9,12,15 linolenate cis-9,12,15- octadeconoate -17

20:0 arachidate eicosanoate 75

20:4 5,8,11,14 arachindonate cis- 5,8,11,14-eicosatetraenoate -49

Physical Properties of Fatty acids

• Saturated chains pack tightly and form more rigid, organized aggregates

• Unsaturated chains bend and pack in a less ordered way, with greater potential for motion

18:0 18:1 18:3

70o 13o -17o

Melting points of fatty acids affect properties of acyl-

lipids• Membrane fluidity determined by

temperature and the degree of fatty acid unsaturation of phospholipids

• Certain bacteria can modulate fatty acid unsaturation in response to temperature

• Difference between fats and oils • Cocoa butter – perfect melt in your mouth

fat made of triacylglycerol with 18:0-18:1-18:0 fatty acids

• Margarine is hydrogenated vegetable oil. Increase saturation of fatty acids. Introduces trans double bonds (thought to be harmful)

Unusual fatty acids can function analogously to unsaturated fatty

acids

Major acyl-lipids•Phospholipids – membrane components

•Triacylglycerols – storage fats and oils

•Waxes – moisture barrier

•Eicosanoids – signaling molecules (prostaglandin)

•Sphingomyelins – membrane component (impt. in mylein sheaths)

•Glycospingolipids – cell recognition (ABO blood group antigen)

Phospholipids

• Phospholipids are built on glycerol back bone.

• Two fatty acid groups are attached through ester linkages to carbons one and two of glycerol.

• Unsaturated fatty acid often attached to carbon 2

• A phosphate group is attached to carbon three

• A polar head group is attached to the phosphate (designated as X in figure)

Common membrane phospholipids

P

O

OO

O

H

CH2

HCH2C

O O

C C OO

R1 R2

P

O

OO

O

CH2

CH2

HCH2C

O O

C C OO

R1 R2

CH2

NH3

P

O

OO

O

CH2

CH2

HCH2C

O O

C C OO

R1 R2

CH2 COO

NH3

P

O

OO

O

CH2

CH2

HCH2C

O O

C C OO

R1 R2

CH2

NH3C CH3

CH3

Phophtidate Phophatidylethanolamine Phophatidylserine Phophatidylcholine

Enzymes used to Dissect Phospholipid Structure

P

O

OO

O

X

CH2

HCH2C

O O

C C OO

R1 R2

phospholipase D

phospholipase C

phospholipase A2phospholipase A1

Plasmalogens•Plasmalogens have hydrocarbon at carbon 1 attached thru vinyl ether linkage

•Polar head group could be ethanolamine or choline

•Important component of membranes in central nrevous system

P

O

OO

O

CH2

CH2

HCH2C

O O

CH C O

HC

CH2

NH3

R1

H2C

R2

Sphingolipids• Sphingolipids named from Sphinx

due to mysterious role• Abundant in eukaryotic membranes,

but not found in bacteria• Structural backbone made of

sphingosine• Unbranched 18 carbon alcohol with

a trans double bond between C4 and C5

• Contains an amino group attached to C2 and hydroxyl groups on C1 and C3

C4

HC

C6

C7

C8

C9

C10

C11

C12

C13

C14

C13

C16

C17

C18

C1 C2 C3

OH

H

H

NH3

OH

H

Ceramides

• Sphingosine with fatty acid attached to carbon 2 by amide linkage

• Metabolic precursors to sphingolipids

C4

HC

C6

C7

C8

C9

C10

C11

C14

C13

C14

C15

C16

C17

C18

C1 C2 C3

OH

H

H

NH

OH

H

CO

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11

C12

C13

C14

C15

C16

C17

C18

Sphingomyelin

• has phosphocholine group attached to C1 of ceramide.

• Resembles phosphatidylcholine

• Major component of myelin sheaths that surround nerve cells

C4

HC

CH2

C8

C1 C2 C3

H

H

NH

OH

H

CO

R1

OP

O

O

O

CH2

CH2

NH3C CH3

CH3

12

Cerebrosides

• contains one monosaccharide residue attached to C-1 of ceramide

• Glucose and galtactose are common

• Can have up to 3 more monosaccharide residues attached to sugar on C1

• Abundant in nerve tissue• Up to 15% of myelin sheath

made up of cerebrosides

C4

HC

CH2

C8

C1 C2 C3

H

O

NH

OH

H

CO

R1

OOH

OH

OH

CH2OH H

12

-D-galactose

Gangliosides• Gangliosides have oliosaccharide containing N-

acetylneuraminic acid attached to C1 of ceramide

• Diverse class of sphingolipid due to variety of olgosaccharide species attached

• Oligosaccharide moiety present on extracellular surface of membranes

• ABO blood group antigens are gangliosides

• Impt in cell recognition, cell-cell communication

Defects in sphingolipid metabolism lead to disease

state• Tay-Saachs disease is a genetic defect in

gangliosides degradation. Gangliosides accumulate in spleen and brain. Leads to retardation in development, paralysis, blindness, and early death.

• Niemann-Pick disease is a genetic defect in sphingomyelin degradation. Causes Sphingomyelin accumulation in brain, spleen and liver. Causes mental retardation. Children die by age 3 or 4.

Triacylglycerols (TAG)

• Fats and oils• Impt source of metabolic fuels• Because more reduced than

carbos, oxidation of TAG yields more energy (16 kJ/g carbo vs. 37 kJ/g TAG)

• Americans obtain between 20 and 30% of their calories from fats and oils. 70% of these calories come from vegetable oils

• Insulation – subcutaneous fat is an important thermo insulator for marine mammals

C9

CH2

HCH2C

C16

O O

C OC1

O

C2

C3

C4

C5

C6

C7

C8

C10

C11

C12

C13

C14

C15

C17

C18

C9

C16

C2

C3

C4

C5

C6

C7

C8

C10

C11

C12

C13

C14

C15

C17

C18

O

C O

C9

C16

C2

C3

C4

C5

C6

C7

C8

C10

C11

C12

C13

C14

C15

C17

C18

Olestra

•Olestra is sucrose with fatty acids esterified to –OH groups

•digestive enzymes cannot cleave fatty acid groups from sucrose backbone

•Problem with Olestra is that it leaches fat soluble vitamins from the body

isoprenoids• Isoprenoids are derived from the

condensation of 5 carbon isoprene units• Can combine head to head or head to tail• Form molecules of 2 to >20 isoprene units• Form large array of different structures

Terprenes

Steroids

• Based on a core structure consisting of three 6-membered rings and one 5-membered ring, all fused together

• Triterpenes – 30 carbons• Cholesterol is the most common steroid

in animals and precursor for all other steroids in animals

• Steroid hormones serve many functions in animals - including salt balance, metabolic function and sexual function

cholesterol

• Cholesterol impt membrane component

• Only synthesized by animals

• Accumulates in lipid deposits on walls of blood vessels – plaques

• Plaque formation linked to cardiovascular disease

Steroids

Many steroids are derived from cholesterol

• Barrier to toxic molecules • Help accumulate nutrients • Carry out energy transduction • Facilitate cell motion • Modulate signal transduction • Mediate cell-cell interactions

Membranes

The Fluid Mosaic Model

• The phospholipid bilayer is a fluid matrix

• The bilayer is a two-dimensional solvent

• Lipids and proteins can undergo rotational and lateral movement

• Two classes of proteins: – peripheral proteins (extrinsic proteins) – integral proteins (intrinsic proteins)

The Fluid Mosaic Model

Motion in the bilayer• Lipid chains can bend, tilt and rotate • Lipids and proteins can migrate

("diffuse") in the bilayer • Frye and Edidin proved this (for

proteins), using fluorescent-labelled antibodies

• Lipid diffusion has been demonstrated by NMR and EPR (electron paramagnetic resonance) and also by fluorescence measurements

• Diffusion of lipids between lipid monolayers is difficult.

fusion

After 40 minutes

Flippases• Lipids can be moved from one

monolayer to the other by flippase proteins

• Some flippases operate passively and do not require an energy source

• Other flippases appear to operate actively and require the energy of hydrolysis of ATP

• Active flippases can generate membrane asymmetries

Membranes are Asymmetric

In most cell membranes, the composition of the outer monolayer is quite different from that of the inner monolayer

Membrane Phase Transitions

• Below a certain transition temperature, membrane lipids are rigid and tightly packed

• Above the transition temperature, lipids are more flexible and mobile

• The transition temperature is characteristic of the lipids in the membrane

Phase Transitions

• Only pure lipid systems give sharp, well-defined transition temperatures

• Red = pure phospholipid

• Blue = phopholipid + cholesterol

Structure of Membrane Proteins

• Integral (intrinsic) proteins • Peripheral (extrinsic) proteins • Lipid-anchored proteins

Peripheral Proteins

• Peripheral proteins are not strongly bound to the membrane

• They can be dissociated with mild detergent treatment or with high salt concentrations

Integral Membrane Proteins

• Integral proteins are strongly imbedded in the bilayer

• They can only be removed from the membrane by denaturing the membrane (organic solvents, or strong detergents)

• Often transmembrane but not necessarily

• Glycophorin, bacteriorhodopsin are examples

Seven membrane-spanning alpha helices, connected by loops, form a bundle that spans the bilayer in bacteriorhodopsin.

The light harvesting prosthetic group is shown in yellow.

Bacteriorhodopsin has loops at both the inner and outer surface of the membrane.

It displays a common membrane-protein motif in that it uses alpha helices to span the membrane.

Lipid-Anchored Proteins

• Four types have been found: – Amide-linked myristoyl

anchors – Thioester-linked fatty acyl

anchors – Thioether-linked prenyl

anchors – Glycosyl phosphatidylinositol

anchors

Amide-Linked Myristoyl Anchors

• Always myristic acid • Always N-terminal • Always a Gly residue that links

Thioester/ester-linked Acyl Anchors

• Broader specificity for lipids - myristate, palmitate, stearate, oleate all found

• Broader specificity for amino acid links - Cys, Ser, Thr all found

Thioether-linked Prenyl Anchors

• Prenylation refers to linking of "isoprene"-based groups

• Always Cys of CAAX (C=Cys, A=Aliphatic, X=any residue)

• Isoprene groups include farnesyl (15-carbon, three double bond) and geranylgeranyl (20-carbon, four double bond) groups

Glycosyl Phosphatidylinositol Anchors

• GPI anchors are more elaborate than others

• Always attached to a C-terminal residue

• Ethanolamine link to an oligosaccharide linked in turn to inositol of PI

Membrane transport

• Membranes are selectively permeable barriers

• Hydrophobic uncharged small molecules can freely diffuse across membranes.

• Membranes are impermeable to polar and charged molecules.

• Polar and charged molecules require transport proteins to cross membranes (translocators, permeases, carriers)

Transport of non-polar molecules

• Non-polar gases, lipids, drugs etc…• Enter and leave cells through diffusion.• Move from side with high concentration

to side of lower concentration.• Diffusion depends on concentration

gradient.• Diffusion down concentration gradient

is spontaneous process (-G).

Transport of polar or charged compounds

Involves three different types of integral membrane proteins

1. Channels and Pores2. Passive transporters3. Active transporters

Transporters differ in kinetic and energy requirements

Channels and Pores

• Have central passage that allows molecules cross the membrane.

• Can cross in either direction by diffusing down concentration gradient.

• Solutes of appropriate size and charge can use same pore.

• Rate of diffusion is not saturable.

• No energy input required

Porins

• Present in bacteria plasma membrane and outer membrane of mitochondria

• Weakly selective, act as sieves • Permanently open• 30-50 kD in size• exclusion limits 600-6000 • Most arrange in membrane as

trimers

Passive Transport (Facilitated Diffusion)

• Solutes only move in the thermodynamically favored direction

• But proteins may "facilitate" transport, increasing the rates of transport

• Two important distinguishing features: – solute flows only in the favored

direction – transport displays saturation kinetics

Three types of transporters

• Uniporter – carries single molecule across membrane

• Symport – cotransports two different molecules in same direction across membrane

• Antiport – cotransports two different molecules in opposite directions across membrane.

Saturation Kinetics of transport

•Rate of diffusion is saturable.

•Ktr = [S] when rate of transport is ½ maximun rate.

•Similar to M-M kinetics

•The lower the Ktr the higher the affinity for substrate.

• Transporters undergo conformational change upon substrate binding

• Allows substrate to transverse membrane

• Once substrate is released, transported returns to origninal conformation.

Active Transport Systems

• Some transport occur such that solutes flow against thermodynamic potential

• Energy input drives transport • Energy source and transport

machinery are "coupled" • Like passive transport systems

active transporters are saturable

Primary active transport• Powered by direct source of energy(ATP,

Light, concentration gradient)

Secondary active transport• Powered by ion concentration gradient.• Transport of solute “A” is couple with

the downhill transport of solute “B”.• Solute “B” is concnetrated by primary

active transport.

Na+-K+ ATPase

• Maintains intracellular Na low and K high

• Crucial for all organs, but especially for neural tissue and the brain

• ATP hydrolysis drives Na out and K in

Na+-K+ ATPase• Na+ & K+ concentration gradients are

maintained by Na+-K+ ATPase• ATP driven antiportsystem.• imports two K+ and exports three Na+ for

every ATP hydrolyzed• Each Na+-K+ ATPase can hydrolyze 100

ATPs per minute (~1/3 of total energy consumption of cell)

• Na+ & K+ concentration gradients used for 2o active transport of glucose in the intestines

1o active transport of Na+

2o active transport of glucose

Transduction of extracellular signals

• Cell Membranes have specific receptors that allow cell to respond to external chemical stimuli.

• Hormone – molecules that are active at a distance. Produced in one cell, active in another.

• Neurotransmitters – substances involved in the transmission of nerve impulse at synapses.

• Growth factors – proteins that regulate cell proliferation and differentiation.

• External stimuli(first messenger) – (hormone, etc…)• Membrane receptor – binds external stimuli• Transducer – membrane protein that passes signal to effector

enzyme• Effector enzyme – generates an intracellular second messenger• Second messenger – small diffusible molecule that carrier

signal to ultimate destination

G-Proteins• Signal transducers.• Three subunits, () and anchored to

membrane via fatty acid and prenyl group• Catalyze hydrolysis of GTP to GDP.• GDP bound form is inactive/GTP bound form

active• When hormone bound receptor complex

interacts with G-protein, GDP leaves and GTP binds.

• Once GTP -> GDP G-protein inactive • GTP hydrolysis occurs slowly (kcat= 3min-1)

good timing mechanism

Epinephrine signaling pathway• Epinephrine regulation of glycogen

degradation• Fight or Flight response• Ephinephrine primary messenger• G-protein mediated response.• G-protein activates Adenyl-cyclase to

produce cAMP• cAMP is the second messenger• Activates protein kinase• Activates glycogen phosphorylase

Effect of Caffeine

• Caffeine inhibits cAMP phosphodiesterase , prevents breakdown of cAMP.

• Prolongs and intensifies Epinephrine effect.

Phosphatidylinositol (PI) Signaling Pathway

• G-protein mediated• G-protein activates phospholipase C

(PLC)• PLC cleaves PI to form inositol-

triphosphate (IP3) and diacylglycerol (DAG) both act as 2nd messengers

• IP3 stimulates Ca2+ releases from ER

• DAG stimulates Protein kinase C