Lipids and Membranes - كلية الطب · 2020. 1. 22. · Lipids • Lipids: a heterogeneous class of naturally occurring organic compounds classified together on the basis of

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Lipids and

Membranes

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Learning Objectives 1. What Is the Definition of a Lipid?

2. What are the functions of lipids ?

3. What Are the Chemical Natures of the Lipid

Types?

4. What are phospholipids & what are their

functions ?

5. Which Are the Lipid-Soluble Vitamins, and What

Are Their Functions?

6. What are waxes, sphingolipids & glycolipids ?

7. What Are Steroids & their derivatives ?

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Lipids • Lipids: a heterogeneous class of naturally occurring

organic compounds classified together on the basis of common solubility properties • insoluble in water, but soluble in nonpolar organic solvents

including diethyl ether, chloroform, methylene chloride, and acetone

• Amphipathic in nature

• Lipids include

Open Chain forms

• fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, glycolipids,

• lipid-soluble vitamins

• prostaglandins, leukotrienes, and thromboxanes

Cyclic forms

• cholesterol, steroid hormones, and bile acids

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What Are Functions of Lipids ?

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Functions of Lipids • Source of energy

• Cellular membranes

• Excellent insulators

• Thermal insulators

• Electrical insulators (in membranes)

• Special roles:

• Signals

• Hormones, Mediators, or growth factors.

• Coenzymes

• Vitamins

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Fatty Acids • Fatty acid: an unbranched-chain carboxylic acid,

most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or phospholipids of biological membranes

• Usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized)

• FA that contain C=C, are unsaturated: If they contain only C-C bonds, they are saturated

• Length of fatty acid plays a role in its chemical character

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8-8 Fig. 8-1a, p. 194

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8-13 Fig. 8-1f, p. 194

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• In most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare

• Unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point

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Fatty Acids

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Fatty Acids

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Fatty acid composition of three food fats

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In the shorthand notation for fatty acids • saturated FA: the number of carbons then (:) then (0)

are shown

• unsaturated FA: the number of carbons then (:) then the number and location of double bonds in the chain raised above delta symbol (Δ) are shown, separated by a colon

• Note: Counting starts from C of COOH

• Examples: palmitic (16:0), stearic (18:0), and oleic acids (18:1Δ9) = double bond between C9 and C10

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Essential Fatty Acids

• They have to be supplied in the diet without

exception.

• They are all polyunsaturated fatty acids: the C20

fatty acid arachidonic acid (20:4 Δ 5,8,11,14) and the

two C18 acids linoleic acid (18:2 Δ 9,12) and

linolenic acid (18:3 Δ 9,12 ,15 ).

• The animal organism requires arachidonic acid to

synthesize eicosanoids.

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• Nutritional fats contain palmitic,

stearic, oleic acid, and linoleic

acid particularly often.

Unsaturated fatty acids are usually

found at the central C atom of

glycerol.

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Triacylglycerols • Triacylglycerol (triglyceride): an ester of glycerol

with three fatty acids

• natural soaps are prepared by boiling triglycerides

(animal fats or vegetable oils) with NaOH, in a

hydrolysis reaction called saponification (Latin, sapo,

soap)

A triacylglycerol(a triglyceride)

CH2 O- CR

O

O

O

CH2 O- CR''

NaOH, H2 O

CH2 OH

CH2 OH

1,2,3-Propanetri ol(Gl ycerol, gl yceri n)

+

RCOO-Na+

Sodi umSoaps

HOCH

R' CO-CH

R' COO- Na+

R' ' COO-Na+

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Soaps • Soaps form water-insoluble salts when used in

water containing Ca(II), Mg(II), and Fe(III) ions

(hard water)

+

+

A sodium soap(soluble in water as mi cell es)

Cal ci um salt of a fatty aci d(i nsol uble i n water)

2 CH3 ( CH2 ) 1 4 COO-Na

+Ca

2 +

[ CH3 ( CH2 ) 1 4 COO-] 2 Ca

2 +2 Na

+

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Hydrolysis of Triglycerides • Chemical hydrolysis,

by saponification as

mentioned earlier

• Enzymatic digestion

by lipase enzyme

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Phosphoacylglycerols • Phosphoacylglycerols (= phospholipids) are the

second most abundant group of naturally

occurring lipids

• found almost exclusively in plant and animal cell

membranes, which typically consist of 40% -50%

phosphoacylglycerols and 50% - 60% proteins

• the most abundant phosphoacylglycerols are derived

from phosphatidic acid, a molecule in which glycerol is

esterified with two molecules of fatty acid and one of

phosphoric acid

• the three most abundant fatty acids in

phosphatidic acids are palmitic (16:0), stearic

(18:0), and linoleic (18:1)

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Phosphatidic acid

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Phosphatidic acid

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Phosphatidic acid

CH2

CH

CH2 -O- P-O-

O

O

O

O gl ycerol

palmi tic aci d

steari c acid

O

O-

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Phosphatidyl esters • further esterification with a low-molecular-

weight alcohol with phosphate group gives a phosphoacylglycerol or phosphatidyl ester

• the most common of these low-molecular-weight alcohols are:

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Phosphatidyl esters

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Phosphatidyl esters

Name and Formula Name of Phospholipid

ethanolamine

+choline phosphatidylchol ine

(l ecithin)

phosphatidylethanolamine(cephal in)

seri ne phosphatidylseri ne

NH3+

HOCH2 CHCOO-

HOCH2 CH2 N( CH3 ) 3

HOCH2 CH2 NH2

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Phosphatidyl esters inositol phosphatidylinositol

HO

HOOH

OHOH

OH

HOCH2 CHCH2OH

gl ycerol

phosphatidylglycerol

OHphosphatidylgl ycerol

HOCH2 CHCH2OPOCH2 CHOCR3

OH

O-

O

CH2 OCR4

O

O

di phosphatidyl glycerol(cardiolipin)

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Phosphatidyl esters • A lecithin

CH2

CH

CH2

O

O

O

O

O P OCH2 CH2 N( CH3) 3

O

O-

+

palmi tic acid

steari c acid

gl ycerol

choline

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Pulmonary surfactant

Molecular Dynamics simulation of DPPC lipid bilayer

formation in two phase systems

reduces surface tension

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Waxes • A complex mixture of esters of long-chain

carboxylic acids and alcohols

• Found as protective coatings for plants and

animals

Biological wax Beeswax, is an ester of palmitic acid

with the alcohol triacontanol.

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Sphingolipids • contain sphingosine,

a long-chain (18 C)

aminoalcohol from

which this class is

named

• Found in plants and

animals

• Abundant in nervous

system

choline

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Glycolipids • Glycolipid: a compound in which a carbohydrate

is bound to an -OH of the lipid

• many glycolipids are derived from ceramides

• Ceramide + simple sugar= cereboside (found in nerve

and brain cells)

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8-39 Fig. 8-8, p.190

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Multiple sclerosis

It is the most common autoimmune disorder affecting

the central nervous system.

The insulating covers of nerve cells in the brain and spinal

cord are damaged.

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Steroids • Steroids: a group of plant and animal lipids that

have this tetracyclic ring (four fused rings) structure

• Composed from three six-membered rings (A, B and C) and one five-membered ring (D)

• The features common to the ring system of most

naturally occurring steroids are illustrated here

A B

C D

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Cholesterol

H3 C

HO

H3 C

H H

H

• The steroid of most interest is cholesterol

• Cholesterol is the major component of biological

membranes (especially animal membranes)

• The precursor of other steroids (sex hormones and

vitamin D)

cholesterol

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Androgens • Androgens: male sex hormones

• synthesized in the testes

• responsible for the development of male secondary

sex characteristics

AndrosteroneTestosterone

O

H

OH

H

H3 C H

H

H3 C H3 C

H

HH3 C

H

O

HO

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Estrogens • Estrogens: female sex hormones

• synthesized in the ovaries

• responsible for the development of female secondary

sex characteristics and control of the menstrual cycle

Progesterone Estradiol

H3 C

H

HH3 C

H

C= O

H

O

CH3

OH

HO

H

H

H

H3 C

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