13 Lipids and Biological Membranes

8/2/2019 13 Lipids and Biological Membranes

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Lipids and

Membranes

A Angeles

Chem 40


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Lipids are the fourth major group of

biomolecules found in cells.

Lipids exhibit greater structural variety than

proteins, nucleic acids and carbohydrates. Lipids are largely hydrophobic.

Sparingly soluble in water

Soluble in organic solvents (e.g., CHCl3 and CH3OH)

Three major biological functions:

1. They serve as energy stores.

2. These molecules form lipid bilayers essential in cellularmembranes.

3. They participate in many intra- and intercellular

signalling pathways.


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Many lipids are ampiphatic. Although lipids do not form polymers, they

associate with each other non-covalently resulting

in supramolecular structures that correspond totheir function.


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Lipids as energy stores. Fats and oils used as stored forms of energy are

fatty acid derivatives. (At physiological pH, what's thepredominant form?)


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Lipids as energy stores.

Fatty acids contain carbon atoms in their most reduced

form (as opposed to carbohydrates, why?).

The hydrophobic quality of FAs result in efficient

compartmentalization in aqueous systems.


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Fatty Acid Naming System


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Packing of Saturated and Unsaturated FAs


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Packing of Saturated and Unsaturated FAs

Saturated FAs contain single bonds that are free to

rotate. Most stable conformation: extended

Unsaturated FAs contain double bonds that are cis.

Introduces a rigid 30o bend in the hydrocarbon chain

PUFAs contain more than one isolated double bonds


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The benefits of polyunsaturated fatty acids.

-linolenic acid (ALA; 18:3(9,12,15) is an essential

FA it must be sourced from the diet

ALA is the precursor of EPA and DHA

Imbalance in -6 and -3 in the diet is implicated

to an increased risk of cardiovasular diseases.


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The heart of oils and fats.


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Esterification of glycerol and FAs result in

triacylglycerols (or triglycerides, fats, neutral fats)


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The ic suffix of FAs becomes oyl in FA ester


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Triacylglycerols proved stored energy and

insulation.


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insulation.

Fat tissues in mammals are located under the skin,

the abdominal cavity and mammary glands. Moderately obese people with 15-20 kg stored TAG

could live off of fat stores for 2- 3 months.

In contrast, the human body can store less than adays supply of glycogen.

Ratio of energy derivation per weight basis

(TAG:glycogen) 6:1


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insulation.


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Saturated and Unsaturated Fats in Food Lipids


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Saturated and Unsaturated Fats in Food Lipids

Foods rich in lipids turn

rancid when exposed forlong periods to air (whichcontains O2) due to

ox a ve c eavage adouble bonds. Commercial vegetable oils are

subjected to partialhydrogenation.

Some cis double bonds aretransformed into trans doublebonds.


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Waxes serve as energy stores and water

repellents.

Biological waxes are esters of long chain (C14-

C36) FAs with long chain (C16-C30) alcohols. Planktons use waxes as primary metabolic fuel.

skin, hair, feathers; plants use them to prevent

excessive water loss and defense against

parasites.


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Waxes serve as energy stores and water

repellents.


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Lipids as membrane structural molecules.

Biological membranes are made from lipid bilayers.

Control of entry of ions and polar molecules Membrane lipids are ampiphatic.


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Lipids as membrane structural molecules.

There are five major types of

membrane lipids:1. Glycerophospholipids

2. Galactoli ids and Sulfoli ids

3. Tetraether lipids4. Sphingolipids

5. Sterols


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Glyerol 3-phosphate is the backbone of

glycerophospholipids.

Two fatty acids are esterified to C1 and C2 of

glycerol; a polar group is attached through a

phosphodiester linkage at C3.


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Glycerophospholipids are derivatives of

phosphatidic acid.


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Some glycerophospholipids have ether-linked

FAs

Found in large

quantities in hearttissue

Important in

inflammation andallergic reactions


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Chloroplasts contain galactolipids and

sulfolipids.


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Chloroplasts contain galactolipids and

sulfolipids.


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Tetraether lipids are found in archaeabacteria.

Long-chain branched hydrocarbons (C32) are linked

to two glycerol molecules

Ether bonds are more stable to at low pH and high

temperatures

Can span the entire membrane


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Sphingolipids are derivatives of sphingosine.

Sphingosine is structurally similar to glycerol.

Sphingolipids have a fatty acid linked by an amide

linkage to the amino group of sphingosine.


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Sphingomyelins and phosphatidylcholines are

structurally similar.


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The two other subclasses of sphingolipids

With the exception of

sphingomyelins, sphingolipidsDO NOT contain phosphate

groups.

Glycosphingolipids cerebrosidesand globosides

Gangliosides contains sialic acidresidues


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Glycosphingolipids as blood group

determinants.


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Membrane lipids are constantly synthesized

and degraded.

Phospholipid and sphingolipid degradation is

facilitated by enzymes in the lysosome.


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Sterols have four fused carbon rings.


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Sterols have four fused carbon rings.

Cholesterol is the most abundant steroid in animals. It is a major component of the plasma membrane (30-40

mol %) Modulates membrane fluidity

synthesize other sterols (e.g. stigmasterol,ergosterol respectively).

Prokaryotes cannot synthesize sterols but can

integrate exogenous sterols into their membranes.


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Lipids as Signals, Cofactors and Pigments

Storage and structural lipids play passive roles

within the cell. Some lipids have active roles in metabolic

processes.

1. Lipid hormones: intra- and extracellular messengers2. Enzyme cofactors

3. Emulsifying agents

Another function of lipids highly conjugated double bonds act as pigments

make up essential oils in plants that give distinctive

aromas


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Eicosanoids as paracrine hormones

Eicosanoids serve as potent signals in relation toreproductive function; inflammation, fever, pain;

formation of blood clots; regulation of blood pressure;gastric juice secretion.

Eicosanoids are derived from arachidonic acid (20:4-

6). Prostaglandins body temperature elevation, inflammationand pain, blood flow to specific organs, the circadianrhythm

Thromboxanes produced by platelets; aids in bloodclotting by reducing blood flow to the indicated site

Leukotrienes first found in leukocytes; implicated inasthmatic attacks and allergic reactions


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Eicosanoids as paracrine hormones

Steroid hormones carry messages between


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tissues.


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Steroids derived from cholesterol in animalsinclude the following families:

Androgens Mediate development of sexualcharacteristics and function (e.g., testosterone,estradiol)


tissues.

Progestins Regulate menstrual cycle (e.g.,progesterone) Glucocorticoids Regulate carbohydrate, protein

and lipid metabolism (e.g., cortisol)

Mineralocorticoids Regulate salt balance intissues (e.g., aldosterone)

Steroid hormones and transcription


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p

M di l li i f id


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Medical applications of steroid-receptor

interactions

Bil id D d b ll


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Bile acids Detergents secreted by gall

bladder that help solubilize lipids in diet

T li id


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Terpene lipids

This type of lipid commonly occurs in oils that give plants

their fragrance.

Monomer of terpenes: isoprene (C5H8)

Terpenes usually occur in multiples of 5 carbon atoms Monoterpene: C10

Sesquiterpene: C15

Diterpene: C20 Triterpene: C30 Precursor of cholesterol and other steroids

Terpene lipids


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Terpene lipids

Many monoterpenes are readily recognized by their characteristic

flavors or odors (limonene in lemons; citronellal in roses and

perfumes; menthol used in cough drops.


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Terpene lipids

Diterpenes include retinal

(the visual pigment in

rhodo sin and h tol

(found in chlorophyll.Gibberellic acid is a plant

hormone.


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Terpene lipids

The triterpene

lanosterol is a

constituent of woolfat and is also a

precursor to

c o es ero an e

other steroids.Lycopene is a

carotenoid found in

ripe fruit,especially

tomatoes.


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Terpene lipids as pigments

T li id th t l t i


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Terpene lipids that serve as electron carriers

Vitamins D A E and K are lipids that play


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Vitamins D, A, E and K are lipids that play

essential roles in animal physiology.

Vitamin D3 (cholecalciferol) regulation of Ca2+

uptake

Vitamins D A E and K are lipids that play


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Vitamin D3 deficiency causes rickets (stuntedgrowth and deformed bones due to insufficient

bone mineralization)


i i i i


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Vitamin A (retinol) visual pigment of vertebrate

eye -carotene is the precursor of retinol




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Vitamin E (tocopherol) an antioxidant; free-

radical scavengers, thus protects unsaturated fattyacids



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Vitamin K (Koagulation) cofactor for

posttranslational modification of blood clottingproteins

Th Bi l i l M b


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The Biological Membrane



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Th Bi l i l M b


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defines the external boundaries of cells andregulates the passage of molecules in and out of the

cell for eukaryotes, it compartmentalizes the different

components essential in energy conservation and cell to cell

signalling

Membranes are flexible, self-sealing and selectivelypermeable to solutes.

Th Bi l i l M b


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Cell membrane flexibility permits:

shape changes cell growth

Because membranes are self-sealing, membranescan undergo fusion and fission without leaking

cellular components.

Th Bi l i l M b


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http://www.csc.mrc.ac.uk/microscopy/



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Lipid composition of organelle membranes vary

probably due to specialized function



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All biological membranes share fundamental

properties Selective permeability

5-8 nm thick


Trilaminar phospholipid bilayer Asymmetric functionality protein and lipid composition

FLUIDITY!



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Plasma membrane lipids are distributed

asymmetrically.



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Proteins have various associations with the

membrane Integral/intrinsic membrane proteins highly

associated with the membrane


Peripheral membrane proteins associated with themembrane through electrostatic interactions with

hydrophilic moieties

Amphitropic proteins can be found in the cytosol and

in association with membranes



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The fluidity of the membrane is temperature-

dependent.

Transition temperature



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Transition temperature of the bilayer increases with

chain length and degree of saturation of FAs

Bacteria and cold-blooded animals (e.g., fish) can

modif the FA com osition of their membrane li ids

g

to suit ambient temperature Cholesterol can decrease membrane fluidity; at the

same time broadens transition temperature

A membrane plasticizer



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Lipids and proteins diffuse laterally in the bilayer.

Membrane Transport


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Nonmediated transport: diffusion of substance acrossan electrochemical gradient (e.g. O2, steroidal

hormones) Mediated transport: transport proteins enable ions

+ + + - . ., , , , . .,

amino acids, sugars, nucleotides) to traverse the cellmembrane Passive-mediated transport or facilitated diffusion

Transport through an electrochemical gradient

Active transport Transport against a gradient

Must be coupled to a sufficiently exergonic process (i.e., a reactionwith G


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Membrane Transport


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Membrane Transport


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Membrane Transport


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Membrane Transport


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Membrane Transport


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13 Lipids and Biological Membranes

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