Lecture 2 : Lipids

Lipids are compounds, present in plants and animals, that are soluble in non polar solvent, such as chloroform or eter, but not soluble in water.

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Five (5) types of lipids:

• Energy-storage lipids: triacylglycerols

• Membrane lipids: phospholipids

• Emulsification lipids: bile

• Messenger lipids: steroid hormones

• Protective lipids: biological waxes

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FATTY ACIDS:SaturatedUnsaturated

GLYCERIDES:NeutralPhosphoglycerides

COMPLEX LIPID:LipoproteinsGlycolipids

NONGLYCERIDES:SphingolipidsSteroidWaxes

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Fatty acids: a component of many lipids• Different types of fatty acids:

– Saturated fatty acid

• Biosynthesized fatty acids are further classified as:– Long-chain: contain 12-26 carbons– Medium-chain: contain 6-10 carbons– Short-chain: contain 4-6 carbons

– Unsaturated fatty acid• Monounsaturated fatty acid: exp (20:1).• Polyunsaturated fatty acids (PUFA): exp (20:3).

• Monocarboxylic acids.

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Physical properties

of fatty acids• the length and degree of unsaturation.

Solubility in H2O

Chain Length

Melting Point

• chain > > solubility < <

• unsaturation > > melting point < < as – due to the lack of compact packing as a result of ‘kinks’ introduced by the double bonds

melting point > >

7D.B.0 1 2 3 4

meltingtemperature All unsaturated

fats are oils at room temperature

16:0  6016:1 1

18:0 6318:1 1618:2 -518:3 -1120:0 75

F. A. M. P. (0C)

20:4 -50  1 D.B.

Cis D.B.

2 D.B.

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Some fatty acids and their common names:14:0 myristic acid; CH3(CH2)12-COOH

16:0 palmitic acid; CH3(CH2)14-COOH

18:0 stearic acid; CH3(CH2)16-COOH

18:1 cis9  oleic acid CH3(CH2)7CH=CH(CH2)7-COOH

Double bonds in fatty acids usually have the cis configuration. Most naturally occurring fatty acids have an even number of carbon atoms.

CO

O 1

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fatty acid with a cis-9 double bond

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18:2 cis9,12  linoleic acidCH3(CH2)4CH=CHCH2CH=CH(CH2)7-COOHis used to synthesize arachidonic acid, which regulates blood pressure, clotting et

18:3 cis9,12,15  a-linonenic acidCH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7-COOHis used to synthesize eicosapentaenoic acid (EPA) and docosahexanoic acid (DHA) which are required for normal brain function and development (an omega-3)

20:4 cis5,8,11,14  arachidonic acid

20:5 cis5,8,11,14,17 eicosapentaenoic acid(an omega-3)

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Waxes

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ester of fatty acids and O

C30H61 O C C15H31

O

C16H33 O C C15H31

Important in fruits:1. Natural protective layer in fruits, vegetables, etc.

2. Added in some cases for appearance and protection.Beeswax (myricyl palmitate),Spermaceti (cetyl palmitate)

O || R - C - O - R1

Waxes :long chain

alcohol

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C-OHCHC-OH

HH

HH

HO-

glycerol

Monoacylglycerol/monoglyceride

H-O-C-CH2C-R1

O

Diacylglycerol/diglyceride

R2-CH2-C-OHO

H-O-C-CH2C-R3

O

Triacylglycerol = Triglyceride

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The chemical reactions of triacylglycerols are similar to those of alkenes and esters.

Hydrogenation of double bonds in unsaturated fatty acids by H2 (compare to alkenes).

Chemical Properties of Triacylglycerols

Hydrolysis of ester bonds by water in the presence of an acid, a base, or an enzyme (compare to esters).

TAG : * energy reservoirs * insulator* digested in the small intestine by the enzyme pancreatic lipase

* saponification : formation of soapsMonoacylglycerols are absorbed through the intestinal cells, re-converted to TG and assembled into lipoproteins

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Triglycerides• Stored as droplets of

adipose tissue in body fat.

Fat stored in this cell

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Hydrogenated Fats• Hydrogenation of

vegetable oils to margarine or shortening, not only convert unsaturated bond to saturated, but also may converts some of cis unsaturated bonds to trans configuration

• Diet high in trans fatty acids risks heart disease, cancer.

cis trans

*

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PYROLYSIS

Hydrogenation: addition of two hydrogen atoms over a double bond. Some food products are partially hydrogenated

O O // // CH2 - O - C - R1 3 CH2 - O - C - R

O // CH - O - C - R2 O

O // // CH2 = CH - C - H CH2 - O - C - R3

fatty acids

acrolein-o2

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- COOH ( fatty acid)

- C = C - + O2 - C - C - - CHO - C=O O - O

(Peroxide)

H2O

Oxidation

Oxidation: addition of oxygen (derived from air), lead to oxidize the double bonds present in a fatty acid to produce carboxylic acid, and other carbonyl compound (keton, aldehyd) through peroxide intermediate.

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Rancidification

O //Fat R - CH2 - CH2 - C - O - H

O O // //R - C - CH2 - C - O - H R - C - C + CO2

// O Keton

Lipase

-Oksidase

+

the decomposition of fats and other lipids by hydrolysis and or oxidation

Keton or acids produced from lipid oxidation /rancidification process have low-molecular-mass and volatile. This volatile compounds are reponsible for rancid odor and flavor in some food products. Antioxidant is usually added to prevent lipid oxidation which lead to rancidity.

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• Glycerol esterified with 2 fatty acids.

• Third position is phosphate

• Other end of phosphate can combine with other molecules to form various derivatives

• Amphipathic –emulsifying agents

• component of cell membrane

PHOSPHATE

OH

H

OH

H

H

OHH

OH

H

H OHPhosphatidylinositol

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O P O

O

O

H2C

CH

H2C

OCR1

O O C

O

R2

X

glycerophospholipid

a polar region: glycerol, carbonyl O of fatty acids, Pi, & the polar head group (X)

non-polar hydrocarbon tails of fatty acids (R1, R2).

Each glycerophospholipid / phosphoacylglycerol possess:

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O P O

O

O

H2C

CH

H2C

OCR1

O O C

O

R2

H

H

Phosphatidic acidPhosphatidyl

-OH

H

OH

H

H

OHH

OH

H

H OHPhosphatidylinositol

Phosphatidylinositol has roles in cell signaling, as well as component of cell membrane

CH2 CH2 N CH3

CH3

CH3

+

Phosphatidylcholine/lechitin It is a common membrane lipid

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phosphatidyl choline = lechitinLecithin, a common food additive, is a phospholipid,

Which embedded in cell membranes.

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Cell Membrane

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Structure of Phosphatidyl choline with 2 fatty acids, oleate and palmitate

OCH3

N+

CH3

CH3

O

(CH2)14 CH3O

O

O

(CH2)7 CH

CH

(CH2)7 CH3

CH2

CH

CH2

OP

O

O

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Sphingolipids

• Sphingosine instead of glycerol

• Derivatization at ester C1

• Fatty acids added at amine

• Different types are found in plasma membrane and myelin sheaths

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2

H2CHC

O H

C H

N C H

C

CH2

CH 3

H

( )12

sphingosine

Sphingolipids/ceramide

H

O H

C

R

O

OH

The amino group of sphingosine can form an amide bond with carboxylate group of a fatty acid, to yield a ceramide Ceramides usually include a polar

head group, esterified to the terminal OH of the sphingosine

PO O

OH2C

H2CN+

CH3

H3C

CH3

OH

Sphingomyelin, a ceramide with a phosphocholine or phosphethanolamine head group, is a common constituent of plasma membranes

Sphingomyelin

Sphingomyelins are found abundantly in the myelin sheath that surrounds the nerve fibers

PO O

OH2C

H2CN+

CH3

H3C

CH3

OH O

H H

H

OHH

OH

CH2OH

H

A cerebroside is a sphingolipid (ceramide) with a monosaccharide such as glucose or galactose as polar head group.

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Steroid Nucleus

Consists of:• 3 cyclohexane rings.• 1 cyclopentane ring.• no fatty acids.

steroid nucleussteroid nucleus

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CH3

CH3CH3

CH3

HO

CH3

Cholesterol• is the most abundant steroid in the

body. • has methyl CH3- groups, alkyl chain,

and -OH attached to the steroid nucleus.

OH

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Cholesterol:-important constituent of cell

membranes-has a rigid ring system and

a short branched hydrocarbon tail.

OH

hydrophobicamphipathic.

Keeps the lipids in membrane from aggregrating, eliminate of excess cholesterol in intestinal tract and artery.

- Precursor to Bile Acids:- Crusial role in lipid

digestion- break up large lipid to

smaller one, make it can be broken easily by enzymes Precursor to steroid hormones

that regulate gene expression

Precursor to Vitamin D

A normal, open artery.

An artery clogged by cholesterol plaque

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Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids.

OH

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. The OH group of cholesterol forms hydrogen bonds with polar phospholipid head groups.

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Lipoproteins: LDL, HDL, Etc…

• combine lipids with proteins & phospholipids.

• are soluble in water because its surface consists of polar lipids.

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Types of Lipoproteins

• differ in density, composition, and function.• include low-density lipoprotein (LDLs) and high-

density lipoprotein (HDLs).

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Transport of Lipoproteins in the Body

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LDL• » circulate in the blood• Build up in arteries;

lead to heart attack

HDL

• level ≥ 130 mg/dL, reflects an increased risk of heart disease

• level < 40 mg/dL, is thought to increase the risk for heart disease.

• cholesterol lipoprotein, that returns to the liver ; removes cholesterol out of the bloodstream;

• believed to prevent heart attacks