Lipids

1
Lipids

• Chapter 26

2
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
• 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.

3
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.

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26.1 Triglycerides

• Triglyceride an ester of glycerol with three
  fatty acids.

(or triacylglycerol)
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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 (160),
  stearic acid (180), and oleic acid (181).
• 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.

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B. Table 26.1
before colon carbons
after colon double bonds
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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.

8
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
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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
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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.

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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.

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Soaps and Detergents

• Soaps form water-insoluble salts when used in
  water containing Ca(II), Mg(II), and Fe(III) ions
  (hard water).

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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.

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Synthetic Detergents

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

Dodecylbenzene
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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.

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Glycerol based phospholipids

• A phosphatidic acid
• the three most abundant fatty acids in
  phosphatidic acids are palmitic acid (160),
  stearic acid (180), and oleic acid (181).
• 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
glycerol
palmitic acid
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Phospholipids, Table 26.4

• among the most common of these low-molecular-weigh
  t alcohols are

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Glycerol based phospholipids

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

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Phosphatidic acid derivatives

• Phosphatidyl Choline (lecithin)
• Phosphatidyl Ethanolamine (cephalin)

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Phosphatidic acid derivatives

• Phosphatidyl Serine
• Phosphatidyl Inositol

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Sphingolipids

• Sphingosine
• N-acylsphingosine (ceramide)

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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
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Sphingomyelins and Cerebrosides

• Sphingomyelins
• Cerebrosides

24
Gangliosides

• Gangliosides

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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.

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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
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26.3 Prostaglandins

• Prostaglandins a family of compounds that have
  the 20-carbon skeleton of prostanoic acid.

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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).

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Prostaglandins, Fig. 26.5

• among the prostaglandins synthesized
  biochemically from arachidonic acid are

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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.

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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-t
  rans.
• the steroid system is nearly flat and quite
  rigid.
• most have axial methyl groups at C-10 and C-13.

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A. Cholesterol
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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.

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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)
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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.

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Glucorticoid Hormones

• synthesized in the adrenal cortex.
• regulate metabolism of carbohydrates.
• decrease inflammation.
• involved in the reaction to stress.

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Mineralocorticoid Hormones

• synthesized in the adrenal cortex.
• regulate blood pressure and volume by stimulating
  the kidneys to absorb Na, Cl-, and HCO3-.

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Androgens

• Androgens - male sex hormones.
• synthesized in the testes.
• responsible for the development of male secondary
  sex characteristics.

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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.

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Synthetic Estrogens

• Progesterone-like analogs are used in oral
  contraceptives.

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Fig. 26.10 Cholesterol Synthesis
O
O
Acetyl Coenzyme A
(R)-Mevalonate
Isopentenyl pyrophosphate
Dimethylallyl pyrophosphate
Geranyl pyrophosphate
Farnesyl pyrophosphate
Squalene
Cholesterol
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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.

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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.

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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.

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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.

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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.

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Vitamin A
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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.

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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
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D. Vitamin K

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

O
isoprene units
O
O
O
2
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• Lipids

End Chapter 26