Chapter 20Chapter 20

LipidsLipids

LipidsLipids• 3 major roles in biochemistry3 major roles in biochemistry

• Store E within fat cells (vs plants – cellulose) Store E within fat cells (vs plants – cellulose) • Membranes to separate compartments (e.g. lipid Membranes to separate compartments (e.g. lipid bilayer)bilayer)

• Chemical messengers (e.g. hormones)Chemical messengers (e.g. hormones)

• Classification: (4 groups/types) Classification: (4 groups/types) 1. simple lipids (fats, waxes)1. simple lipids (fats, waxes)

2. complex lipids (phospho/glyco-lipids)2. complex lipids (phospho/glyco-lipids)

3. steroids (cholesterol)3. steroids (cholesterol)

4. prostaglandins, thromboxanes, leukotrienes4. prostaglandins, thromboxanes, leukotrienes

(inflammation)(inflammation)

Structure of Structure of TriglycerideTriglyceride• Triglyceride:Triglyceride: an ester of glycerol with three (1-3)

fatty acids (F.A.s).

1.

2.

3.

Confirming your knowledge:Draw the structural formula of a triglyceride of: myristic, palmitic and oleic acid and glycerol

See table 20.1 p 496

+ 3 F.A.s

+ 3 H2O

Properties of TriglyceridesProperties of Triglycerides• 1. Physical properties depend on the fatty acid components:• Melting points, Oils and Fats

•Longer Carbon chain(s) higher M.P.•More d. bonds (unsaturated) lower M.P.

• RECALL•Oils = liquid at room temp. b/c lots of d.bonds (as cis form)

•Fats = solid at room temp. b/c few d. bonds (stack easily)

NOTE: Natural form of d. bonds in fatty acids/ oils = always cis or trans?

e.g. the Essential Fatty Acid (EFA) linoleic acid

CHALLENGE QUESTION

Linoleic acid has M.P. of -5 *C.What would happen to it’s M.P. if you converted the

cis d. bonds to trans and why?

2. Hydrogenation2. Hydrogenation• reduction of some or all d. bonds of an unsaturated

triglyceride w/ H2/transition metal catalyst.

• Why? Easier kitchen use (e.g. Crisco oil etc)• Better for baking, mixing, longer shelf life, cheaper

• Formation of mainly trans isomers (Trans fats) • e.g. partially hydrogenated vegetable oil

• Drawbacks: unnatural, • body does not recognize “trans” form• Inflammation, weight gain, heart disease (higher

cholesterol)

Properties of TriglyceridesProperties of Triglycerides

3. Saponification3. Saponification

O

O

_Na+

O

O

_Na+

• Hydrolysis of esters: using base• Produces glycerol + mixture of fatty acids aka (SOAPS)

Properties of TriglyceridesProperties of Triglycerides

O

O

_Na+

Note: Hard water disrupts soapsMg2+ or Ca2+

Binds to fatty acids

see chem conn. p.500

Polar HeadGreasy tail

Complex LipidsComplex Lipids• 1. Phospholipids

contain an alcohol, two fatty acids, and a phosphate ester.

a. in glycerophospholipids, the alcohol is glycerol.

(plasma membranes)

b. In sphingolipids, the alcohol is sphingosine.

(function in coatings neurons)2. GlycolipidsComplex lipids that contain a

carbohydrate.

polar

Non polar((Water))

((Water))

glycerophospholipidglycerophospholipid

Soap = “ Micelle”

(traps dirt/grease )

20.5 Membranes20.5 Membranes• Complex lipids form the membranes around cells and small

structures within cells.

• complex lipids spontaneously form into a lipid bilayer with a back-to-back arrangement of lipid monolayers.

1. Polar (hydrophilic) head groups are in contact with the aqueous environment.

2. Nonpolar (hydrophobic) tails are buried in bilayer shielded from environment.

- driving force of lipid bilayers = hydrophobic interaction.

• The arrangement of hydrocarbon tails in the interior can be rigid (saturated fatty acids) or fluid (unsaturated fatty acids).

((water))

((water))

2.

1.

Fluid Mosaic ModelFluid Mosaic Model

Lipid types most likely found in membranes: glycolipids, & cholesterol

transport

1a. 1a. GlycerophospholipidsGlycerophospholipids

• second most abundant group of naturally occurring lipids.• Occur in plant and animal membranes: ( 40% -50% phosphoacylglycerols) (50% - 60% proteins)

• most abundant glycerophospholipids derived from phosphatidic acid

The three most abundant fatty acids in phosphatidic acids are palmitic (16:0), stearic (18:0), and oleic (18:1).

1a. Glycerophospholipids1a. Glycerophospholipids• A phosphatidic acid

• 1. glycerol is esterified with: 2. two molecules of fatty acid 3. one phosphoric acid.

1.

H

H

2.

3.H

1a. Glycerophospholipids 1a. Glycerophospholipids cont.cont.

3 other types: -Ethanolamine

-Serine -Inositols

GlycerophospholipidsGlycerophospholipidsChallenge Question (Homework)Which Glycerophospholipids will have the greatest solubility in Water: ethanolamine?

choline?serine?

CH2

CH

CH2

O

O

O

O

O P OCH2CH2N(CH3)3

O

O-

+

palmitic acid

linolenic acid

glycerol

choline

HOHO

OHOH

OH

OH

NH3+

HOCH2CHCOO-

HOCH2CH2N(CH3)3

HOCH2CH2NH2

inositol phosphatidylinositol

Name and FormulaName of Glycerophospholipid

ethanolamine

+choline lecithin

cephalin

serine cephalin

glycerol

?

?

?

SphingolipidsSphingolipids• Found in the coatings of nerve axons (myelin).• Contain the long-chain aminoalcohol, sphingosine, from which this class of compounds in named.

OH

(CH2)12CH3

HO

NH2

SphingosineOH

(CH2)12CH3

HO

NHCR

A ceramide(an N-acylsphingosine)

OPOCH2CH2N(CH3)3

(CH2)12CH3

HO

NHCR

A sphingomyelinO

O-+

O O

Sphingolipids make up Sphingolipids make up myelinmyelin

See chem. Connections 20E p.506, Swine flu, watch out!

GlycolipidsGlycolipids• GlycolipidGlycolipid:: a complex lipid that contains a carbohydrate.• The carbohydrate is either glucose or galactose.

• The cerebrosides are ceramide mono- or oligosaccharides.

O

(CH2)12CH3

HO

NHCR

O

H

HO

H

HO

H

HOH

H

OHO

a-glycosidic bond

a unit of -D-glucopyranose

a ceramide

SteroidsSteroids• SteroidsSteroids:: a group of plant and animal lipids that have this tetracyclic ring structure.

A B

C D

Know this backbone

A. CholesterolA. Cholesterol• Cholesterol is the most abundant steroid in the human body, and most important. Why?

1. It is a component in plasma membranes in all animal cells.

2. It is the precursor of all steroid hormones and bile acids.

HO

-CONFIRMING your Knowledge-How many stereocenters exist in cholesterol?

Gallstone of cholesterol

B. LipoproteinsB. Lipoproteins• Cholesterol, along with fats, are transported by lipoproteins

Lipoprotein

Composition (% dry weight)

ProteinsCholesteroland esters

Phospho-lipids

Tri-glycerides

High-densitylipoprotein (HDL)

Low-densitylipoprotein (LDL)

Very-low densitylipoprotein (VLDL)

Chylomicrons

33 30 29 8

25 50 21 4

10 22 18 50

1-2 8 7 84

proteins

Cholest

Mostly fat

LipoproteinsLipoproteins• Figure 20.5 Schematic of a low-density lipoprotein (LDL).

Cholesterol TransportCholesterol Transport• Cholesterol made in Liver• Transport of cholesterol from the liver starts out as VLDL.

• VLDL is carried in serum (blood) to fat or muscle tissues and . . . deposits proteins and triglycerides. .

• As fat is removed, (diameter shrinks) its density increases

(i.e. VLDL LDL) • LDL stays in the plasma for about 2.5 days.

1. LDL carries cholesterol to cells, where specific LDL receptors bind it.

2. After binding, LDL is taken into cells where enzymes liberate free cholesterol and cholesteryl esters.

3. LDL receptor then recycled to “Pit”

Cholesterol Transport Cholesterol Transport cont.cont.

1.

2.

2.

3.

3.

• What happens to patients who have limited LDL receptors on there cell membranes?

• (i.e. familial hypercholesterolemia)

Challenge Question?Challenge Question?

Cholesterol TransportCholesterol Transport1. High-density lipoproteins (HDL) transport cholesterol: f/ tissues liver and also transfer cholesterol to LDL.• While in the serum, free cholesterol in HDL is converted to cholesteryl esters.2. In the liver, HDL binds to the liver cell surface and transfers its cholesteryl esters to the cell.• These esters are used for the synthesis of steroid hormones (estrogen, testosterone) and bile acids.• After HDL has delivered its cholesteryl esters to liver cells, it reenters circulation.

1.

2.

HO

Levels of LDL and HDLLevels of LDL and HDL• Most of the cholesterol is carried by LDL.• If sufficient LDL receptors on the surface of cells, LDL is removed from circulation and its concentration in blood plasma drops.

• number of LDL receptors is controlled by a feedback mechanism.

• When [cholesterol] inside cells is high, the synthesis of LDL receptors is suppressed. (inside liver)

•Normal plasma levels: 175 mg/100 mL.

HO

HO HO

HO

“hypercholesterolemia” plasma levels: 680 mg/100 mL!

Not enough LDL receptors

Levels of LDL and HDLLevels of LDL and HDL• These high levels of cholesterol can cause premature atherosclerosis (myocardial infarction: MI) and heart attacks.

• High LDL means high cholesterol content in the plasma because LDL cannot get into cells.

• Therefore, high LDL together with low HDL is a symptom of faulty cholesterol transport and a warning of possible atherosclerosis.

• Get tested?!? . . Simple blood test, Planned parenthood, Longs etc.

• Reg. Exercise, Diets low in cholesterol and sat. fat reduce serum cholesterol

• The commonly used statin drugs (Lipitor) inhibit the synthesis of cholesterol by blocking HMG-CoA reductase in the liver

Treating High Treating High CholesterolCholesterol

Steroid HormonesSteroid Hormones• Androgens:Androgens: male sex hormones

• synthesized in the testes• responsible for the development of male secondary sex characteristics

AndrosteroneTestosteroneO

OH

H3C

H3C H3C

H3C

O

HO