Lipids

• Fatty Acids• Triacylglycerols• Glycerophospholipids

Lipids consist of hydrophobic molecules that have great structural variability and are not polymeric.

In general, the lipids perform three biological functions:

1. Essential components of biological membranes.

2. Contain hydrocarbon chains that serve as energy stores.

3. Involved in many intracellular and intercellular signaling.

• Sphingolipids• Steroids, Eicosanoids• Lipid Bilayers

Fatty Acids

Fatty acids are carboxylic acids with long-chain hydrocarbon side groups. In plants and animals, the predominant fatty acid residues have C16 or C18 side chains.

Type of Fatty Acid

Saturated Fatty Acids

Unsaturated Fatty Acids

Polyunsaturated Fatty Acids

Content of Side Chain

no double bonds

contain double bonds

contain 2 or more double bonds

Common C18 Fatty Acids

HOOCCH2

CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH2CH2

CH3

HOOC

1

18

HOOC

1189

HOOC

1

18

9

HOOC

118

9

12

12 15

Stearic Acid18:0

Oleic Acid18:1

Linoleic Acid18:2

-Linolenic Acid18:3

Fatty acid double bonds are almost always have a cis configuration.

Table 9-1. Common Biological Fatty Acids

Common Name Systematic Name Structure

Saturated fatty acids12:0 Lauric acid Dodecanoic acid CH3(CH2)10COOH14:0 Myristic acid Tetradecanoic acid CH3(CH2)12COOH16:0 Palmitic acid Hexadecanoic acid CH3(CH2)14COOH18:0 Stearic acid Octadecanoic acid CH3(CH2)16COOH20:0 Arachidic acid Eicosanoic acid CH3(CH2)15COOH22:0 Behenic acid Docosanoic acid CH3(CH2)20COOH24:0 Lignoceric acid Tetracosanoic acid CH3(CH2)22COOH

Unsaturated fatty acids (all double bonds are cis)16:1 Palmitoleic acid 9-Hexadecenoic acid CH3(CH2)5CH=CH(CH2)7COOH18:1 Oleic acid 9-Octadecenoic acid CH3(CH2)7CH=CH(CH2)7COOH18:2 Linoleic acid 9,12-Octadecadienoic acid CH3(CH2)4(CH=CHCH2)2(CH2)6COOH18:3 -Linolenic acid 9,12,15-Octadecatrienoic acid CH3CH2(CH=CHCH2)3(CH2)6COOH18:3 -Linolenic acid 6,9,12-Octadecatrienoic acid CH3(CH2)4(CH=CHCH2)3(CH2)3COOH20:4 Arachidonic acid 5,8,11,14-Eicosatetraenoic acid CH3(CH2)4(CH=CHCH2)4(CH2)2COOH20:5 EPA 5,8,11,14,17-Eicosapentaenoic

acidCH3CH2(CH=CHCH2)5(CH2)2COOH

24:1 Nervonic acid 15-Tetracosenoic acid CH3(CH2)7CH=CH(CH2)13COOH

Triacylglycerols

H2C

HC

H2C

OH

OH

OH

H2C

HC

H2C

O

O

O

C R1

O

C R2

O

C R3

O

Glycerol Triacylglycerol

CO

H2C

HC

H2C

O

O

O

CO

CO 18

18

16

1-palmitoleoyl-2-linoleoyl-3-stearoyl-glycerol

1

2

3

Fats and Oils - Energy Reserves

Fats and oils exist primarily as triacylglycerols (or triglycerides).

Fats are solids at room temperature, oils are liquids.

Fats and oils are complex mixtures of triacylglycerols (composition)varies with organism). Plant oils are richer in unsaturated fatty acids (lower melting point and thus are liquids at room temperature).

Fats are a highly efficient form of energy. Fats are less oxidized than proteins or carbohydrates and thus produce more energy per unit mass upon complete oxidation. Also, fats are stored anhydrous.

Fats provide about six times the metabolic energy of an equal weight of hydrated glycogen.

Adipocytes (fat cells) synthesize and store triacylglycerols and adipose tissue is the most abundant tissue source.

Glycerophospholipids (or phosphoglycerides)

• major component of biological membranes• esters of glycerol-3-phosphate

Amphiphilic molecules: nonpolar aliphatic (hydrocarbon) “tails”polar phosphoryl-X “heads”

H2C

C

H2C

OH

H

O

HO

P OH

OH

O

H2C

C

H2C

O

H

O

O

P O

O-

O

C R1

O

CR2

O

X

L-glycerol-3-phosphate general structure of a glycerophospholipid

Common Classes of Phospholipids

X-OH

water

ethanolamine

choline

serine

glycerol

Formula of -X

-H

-CH2CH2NH3+

-CH2CH2N(CH3)3+

-CH2CH(NH3+)COO-

-CH2CH(OH)CH2OH

Name

phosphatidic acid

phosphatidylethanolamine

phosphatidylcholine (lecithin)

phosphatidylserine

phosphatidylglycerol

Table 9-2. H2C

C

H2C

O

H

O

O

P O

O-

O

C R1

O

CR2

O

X

Structure of a Phospholipid

1-stearoyl-2-oleoyl-3-phosphatidylcholine

Fig. 9-4

H2C

C

H2C

O

H

O

O

P O

O-

O

C

O

(CH2)16CH3

C

O

(CH2)7

CH2CH2N(CH3)3+

HC

HC(CH2)7H3C

Phospholipases hydrolyze phospholipids.

phospholipase A2

phospholipase A2

phospholipase A1

phospholipase C phospholipase D

H2C

C

H2C

O

H

O

O

P O

O-

O

C R1

O

CR2

O

X

H2C

C

H2C

O

H

O

HO

P O

O-

O

C R1

O

X

H2O

R2COOH

H2C

C

H2C

O

H

O

O

P O

O-

O

C R1

O

CR2

O

X

Phospholipases A2 - phospholipid complex

Fig. 9-6

Sphingolipids

H2C C C H

CH

OH

sphingosine

OH H

+H3N

HC

(CH2)12

CH3

H2C C C H

CH

OH

ceramide(R = fatty acid residue)

OH H

HN

HC

(CH2)12

CH3

C

R

O

• a major membrane component

• derivatives of the C18 amino alcohol sphingosine

• N-acyl fatty acid derivatives are known as ceramides

Steroids

A B

C D

cyclopentanoperhydrophenanthrene

Cholesterol

HO

cholesterol

Central steroid in the animal kingdom

Two Functions:

• Serves as an integral component in biological membranes

• Serves as the biosynthetic precursor to other steroid molecules

Adrenal Steroidogenesis

HO

cholesterol HO

pregnenolone

O

side chain cleavage

HO

O

OH

O

OH HO

O

OH

O

O

cortisol aldosterone

Sex Hormone Steroidogenesis

HO

cholesterol HO

pregnenolone

O

side chain cleavage

O

progesterone

OOH

HO

OH

O

aromatase

testosteroneestradiol

Eicosanoids

• Eicosanoids (C20) are derived from arachidonic acid (20:4)

• Four classes of eicosanoids:– prostaglandins– prostacyclins– thromboxanes– leukotrienes

COOH

158

11 1420

arachidonic acid(5,8,11,14-eicosatetraenoic acid)

Eicosanoid Biosynthesis

COOHarachidonic acid

phospholipids

phospholipase A2

cyclooxygenase 5-lipooxygenase

O

O

COOH

OH

PGH2

COOH

OH OH

LTB4

(leukotriene)

COOH

OH

HO

HO

COOH

OH

O

HO

COOH

OHOHO

HO

PGF2

(prostaglandin) PGI2

(prostacyclin)

TxB2

(thromboxane)