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Lipids. Chapter 26. Lipids. Lipids: a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties. So, lipids are non-polar molecules having different structures. they are insoluble in water but are

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lipids

Lipids

Chapter 26

lipids1
Lipids
  • Lipids:a heterogeneousclass of naturally occurring organic compounds classified together on the basis of common solubility properties.
  • So, lipids are non-polar molecules having different structures.
    • they are insoluble in water but are
    • soluble in aprotic organic solvents, including diethyl ether, methylene chloride, and acetone
  • Any molecule extractable from plant or animal tissue using a non-polar solvent is a lipid.
lipid classifications
Lipid Classifications
  • Triglycerides,
  • Glycerol based phospholipids,
  • Sphingosine derivatives: sphingomyelins and gangliosides
  • Eicosinoids: prostaglandins, thromboxanes, leucotrienes and prostacyclins,
  • Steroids/sterols: cholesterol (C27), bile acids (C24), adrenocortical hormones (C21), and sex hormones (C19 & C 18),
  • fat-soluble vitamins.
26 1 triglycerides
26.1 Triglycerides
  • Triglyceride:an ester of glycerol with three fatty acids.

(or triacylglycerol)

a fatty acids
A. Fatty Acids
  • Fatty acid: a carboxylic acid derived from hydrolysis of animal fats, vegetable oils, or membrane phospholipids.
    • nearly all have an even number of carbon atoms, most between 12 and 20, in an unbranched chain.
    • the three most abundant are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1).
    • 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; a greater degree of unsaturation gives a lower melting point.
slide6

B. Table 26.1

# before colon

= # carbons

# after colon

= # double bonds

triglycerides
Triglycerides
  • Physical properties depend on the fatty acid components.
    • melting point increases as the number of carbons in its hydrocarbon chains increases and as the number of double bonds decreases.
    • triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils.
    • triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats.
triglycerides1
Triglycerides
  • example: a triglyceride derived from one molecule each of palmitic acid, oleic acid, and stearic acid, the three most abundant fatty acids in the biological world.

A very non-polar molecule

26 2 a soaps and detergents

C

H

O

C

R

2

3

N

a

O

H

R

C

O

C

H

C

H

O

C

R

C

H

O

H

2

2

-

+

A triglyceride

3

R

C

O

N

a

C

H

O

H

(a triester of glycerol)

C

H

O

H

2

1,2,3-Propanetriol

(Glycerol; Glycerin)

26.2 A.Soaps and Detergents
  • Natural soaps are prepared by boiling lard or other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap).

O

O

saponification

+

O

O

+

Sodium soaps

soaps and detergents fig 26 4
Soaps and Detergents, Fig. 26.4
  • a soap micelle: nonpolar (hydrophobic) hydrocarbon chains cluster in the inside and polar (hydrophilic) carboxylate groups lie on the surface.
soaps and detergents
Soaps and Detergents
  • micelle: a spherical arrangement of organic molecules in water clustered so that their hydrophobic parts are buried inside the sphere and their hydrophilic parts are on the surface of the sphere and in contact with water.
  • when soap is mixed with water-insoluble grease, oil, and fats, the nonpolar parts of the soap micelles “dissolve” these nonpolar dirt molecules and they are carried away in the polar wash water.
soaps and detergents1
Soaps and Detergents
  • Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water).
c synthetic detergents
C. Synthetic Detergents
  • The design criteria for a good detergent are:
    • a long hydrocarbon tail of 12 to 20 carbons.
    • a polar head group that does not form insoluble salts with Ca(II), Mg(II), or Fe(III) ions.
  • The most widely used synthetic detergents are the linear alkylbenzenesulfonates (LAS).
  • Detergents are salts of strong acids.
synthetic detergents

1

.

H

S

O

2

4

C

H

(

C

H

)

C

H

3

2

1

0

2

2

.

N

a

O

H

-

+

S

O

N

a

C

H

(

C

H

)

C

H

3

3

2

1

0

2

Sodium 4-dodecylbenzenesulfonate

(an anionic detergent)

Synthetic Detergents
  • Also added to detergent preparations are:
    • foam stabilizers.
    • bleaches.
    • optical brighteners.

Dodecylbenzene

26 5 a phospholipids
26.5 A.Phospholipids
  • Phospholipids are the second most abundant group of naturally occurring lipids.
    • they are found almost exclusively in plant and animal membranes, which typically consist of 40% -50% phospholipids and 50% - 60% proteins.
    • the most abundant phospholipids are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid.
    • a second class of phospholipids are the sphingomyelins derived from N-acylsphingosine (ceramide) and a phosphate.
glycerol based phospholipids

-

C

H

-

O

-

P

-

O

2

-

O

C

H

C

H

2

Glycerol based phospholipids
  • A phosphatidic acid:
    • the three most abundant fatty acids in phosphatidic acids are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1).
    • further esterification with a low-molecular weight alcohol gives a phospholipid.
    • some common low MW alcohols are shown next.

O

stearic acid

O

O

O

O

palmitic acid

glycerol

phospholipids table 26 4
Phospholipids, Table 26.4
  • among the most common of these low-molecular-weight alcohols are:
glycerol based phospholipids1
Glycerol based phospholipids
  • A lecithin.

in aqueous solution, phospholipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers.

phosphatidic acid derivatives
Phosphatidic acid derivatives
  • Phosphatidyl Choline (lecithin)
  • Phosphatidyl Ethanolamine (cephalin)
phosphatidic acid derivatives1
Phosphatidic acid derivatives
  • Phosphatidyl Serine
  • Phosphatidyl Inositol
sphingolipids
Sphingolipids
  • Sphingosine
  • N-acylsphingosine (ceramide)
sphingosine based phospholipids

-

C

H

-

O

-

P

-

O

2

-

O

C

H

C

H

Sphingosine based phospholipids
  • N-acylsphingosine-P (ceramide phosphate)
    • the N-acyl fatty acid is usually unsaturated.
    • the alcohol normally attached to the phosphate is choline.

O

stearic acid

O

N

from serine

OH

from palmitic acid

sphingomyelins and cerebrosides
Sphingomyelins and Cerebrosides
  • Sphingomyelins
  • Cerebrosides
gangliosides
Gangliosides
  • Gangliosides
eicosanoids
Eicosanoids
  • Eicosanoids are a large family of compounds, all of which contain 20 carbons and are derived from polyunsaturated fatty acids. These include prostaglandins, prostacyclins, thromboxanes and leucotrienes.
eicosanoids1

H

O

C

O

O

H

C

O

O

H

N

C

O

O

H

H

H

N

N

H

Leukotriene C

(LTC

)

2

4

4

(a smooth muscle constrictor)

Eicosanoids
  • Leukotrienes are found primarily in white blood cells.
    • one function is constriction of smooth muscles, especially those of the lungs.

1

9

7

11

6

5

20

S

O

glycine

14

L-cysteine

L-glutamic acid

O

26 3 prostaglandins
26.3 Prostaglandins
  • Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid.
prostaglandins
Prostaglandins
  • Prostaglandins are not stored in tissues as such, but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers.
    • one such polyunsaturated fatty acid is arachidonic acid (notice the all cis configurations).
prostaglandins fig 26 5
Prostaglandins, Fig. 26.5
  • among the prostaglandins synthesized biochemically from arachidonic acid are:
26 4 steroids

(cyclopentanoperhydrophenanthrene)

26.4 Steroids
  • Steroids: a group of plant and animal lipids that have this tetracyclic ring structure.
  • The features common to the ring system of most naturally occurring steroids are illustrated on the next screen.
steroids
Steroids
  • the fusion of rings is trans and each atom or group at a ring junction is axial.
  • the pattern of atoms or groups along the ring junctions is nearly always trans-anti-trans-anti-trans.
  • the steroid system is nearly flat and quite rigid.
  • most have axial methyl groups at C-10 and C-13.
b biosynthesis of steroids
B. Biosynthesis of Steroids
  • The building block from which all carbon atoms of steroids are derived is the two carbon acetyl group of acetyl-CoA.

Stage 1: synthesis of isopentenyl pyrophosphate from three molecules of acetyl-CoA.

Stage 2: synthesis of cholesterol.

Stage 3: conversion of cholesterol to other steroids.

b biosynthesis of steroids1
B. Biosynthesis of Steroids

Cholesterol

glucocorticoid hormones

C-21(e.g., cortisone)

bile acids C-24

(e.g., cholic acid)

mineralocorticoid hormones

C-21(e.g., aldosterone)

progesterone C-21

sex hormones

(e.g., testosterone C-19

and estrone C-18)

bile acids
Bile Acids
  • Synthesized in the liver, stored in the gallbladder, and secreted into the intestine where their function is emulsify dietary fats and aid in their absorption and digestion.
glucorticoid hormones
Glucorticoid Hormones
    • synthesized in the adrenal cortex.
    • regulate metabolism of carbohydrates.
  • decrease inflammation.
    • involved in the reaction to stress.
mineralocorticoid hormones
Mineralocorticoid Hormones
  • synthesized in the adrenal cortex.
  • regulate blood pressure and volume by stimulating the kidneys to absorb Na+, Cl-, and HCO3-.
androgens
Androgens
  • Androgens - male sex hormones.
    • synthesized in the testes.
    • responsible for the development of male secondary sex characteristics.
estrogens
Estrogens
  • Estrogens - female sex hormones.
    • synthesized in the ovaries.
    • responsible for the development of female secondary sex characteristics and control of the menstrual cycle.
synthetic estrogens
Synthetic Estrogens
  • Progesterone-like analogs are used in oral contraceptives.
fig 26 10 cholesterol synthesis

H

O

C

H

3

-

C

H

-

C

-

S

-

C

o

A

O

O

H

3

3

3

-

-

O

O

P

P

O

O

2

2

6

6

C

terpenes

(

C

)

10

1

0

C

and C

terpenes

(

C

)

15

20

1

5

C

terpenes

(

C

)

30

3

0

Fig. 26.10 Cholesterol Synthesis

O

O

Acetyl Coenzyme A

(R)-Mevalonate

Isopentenyl pyrophosphate

Dimethylallyl pyrophosphate

Geranyl pyrophosphate

Farnesyl pyrophosphate

Squalene

Cholesterol

b biological membranes
B. Biological Membranes
  • Fluid mosaic model: a biological membrane consists of a phospholipid bilayer with proteins, carbohydrates, and other lipids embedded on the surface and in the bilayer.
    • fluid signifies that protein and lipid components of membranes “float” in the bilayer and can move freely along the plane of the membrane.
    • mosaic signifies that the various components of the membrane exist side by side, as discrete units rather than combining to form new molecules and ions, i.e the membrane is a non-covalent structure.
biological membranes
Biological Membranes

Fluid-mosaic model:

A biological membrane

showing the lipid bilayer

and membrane proteins

on the inner and outer

surfaces of the

membrane and

penetrating the

thickness of the

membrane.

26 5 fat soluble vitamins
26.5 Fat-Soluble Vitamins
  • Vitamins are divided into two broad classes on the basis of their solubility .
    • those that are fat soluble, and hence classified as lipids.
    • those that are water soluble.
  • The fat-soluble vitamins include A, D, E, and K.
a vitamin a
A. Vitamin A
  • occurs only in the animal world .
  • found in the plant world in the form of a provitamin in a group of pigments called carotenes.
vitamin a
Vitamin A
  • The best understood role of Vitamin A is its participation in the visual cycle in rod cells.
    • the active molecule is retinal (vitamin A aldehyde), which forms an imine with an -NH2 group of the protein opsin to form the visual pigment called rhodopsin.
    • the primary chemical event of vision in rod cells is absorption of light by rhodopsin followed by isomerization of the 11-cis double bond to the 11-trans configuration.
b vitamin d

H

O

V

itamin D

3

B. Vitamin D
  • A group of structurally related compounds that play a role in the regulation of calcium and phosphorus metabolism.
    • the most abundant form in the circulatory system is vitamin D3.
c vitamin e

F

our isoprene units, joined

O

H

head-to-tail, beginning here

and ending at the aromatic ring

V

itamin E (

a

-Tocopherol)

C. Vitamin E
  • Vitamin E is a group of compounds of similar structure, the most active of which is -tocopherol.
    • In the body, vitamin E functions as an antioxidant; it traps peroxy radicals of the type HOO• and ROO• formed as a result of oxidation by O2 of unsaturated hydrocarbon chains in membrane phospholipids.

O

d vitamin k

Vitamin K

Menadione

1

(a synthetic

vitamin K analog)

D. Vitamin K
  • The name of this vitamin comes from the German word Koagulation, signifying its important role in the blood-clotting process.

O

O

isoprene units

O

O

2

slide51
Lipids

End Chapter 26