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Lipid Chemistry
Lipids are heterogeneous group of organic compounds related, either
actually or potentially to fatty acids
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Common Properties:


Relatively insoluble in water





Soluble in no polar solvents such as ether, chloroform, benzene, and acetone
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Simple Lipids
i
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The fatty acids linked to the glycerol are
usually different
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It is colorless,
syrupy taste, sweet and very hygroscopic (tend to absorb moisture from air)
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They are obtained mostly
from hydrolysis of natural fats and oils
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All of them possess a long hydrocarbon chain
and a terminal carboxyl group
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According to chain length of fatty acids they can be:
 Low fatty acid chains: containing 10 carbon atoms or less (2-10)
e
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g Palmitic (16 C) and Stearic (18 C)
b
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Monounsaturated fatty acids e
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2
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g Linoleic 18:2:9,12 , alpha Linolenic 18:3:9,12,15 ,and Arachidonic
20:4:5,11,14 which is a precursor of prostaglandins
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Biological classification of fatty acids from nutritional point of view:
 Essential fatty acids: are polyunsaturated fatty acids which are
Linoleic and Lionolenic
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 Non essential fatty acids: are saturated fatty acids and
mammals can synthesize them from other precursors
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It becomes
essential if it’s precursor linoleic acid is missing in the diet
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They are an
integral part of cell membranes throughout the body and affect the function of cell receptors in
these membranes
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Importance of arachidonic acid:

2

It is an omega 6 fatty acid that has important role in call membrane fluidity and transport
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Triacylglycerols

Simple triacylglycerols
which contains a single kind
of fatty acids in all three
ester positions e
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Tristearin, Tripalmitin,
Triolein

Mixed triacylglycerols which
contain 2or more different
fatty acids in the molecule
e
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1,3- Distearopalmitin

Biological Importance of True Fats:
1) They form reserve food in animal and plants
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This depot fat is mobilized during starvation to produce energy
and so its amount is variable, thus true fats are known as variable element of fat
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3 Kcal)
3) True fats are also found as supporting material in some parts of body as around kidney
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Complex Lipids
I
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There are two classes of
phospholipids, those that have glycerola as backbone Glycerophosphlipids and those that have
spingosine as a backbone Sphingophospholipids
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a)

Glycerophospholipids:

The precursor compound is the phosphoric ester of glycerol (glycerol-3-phosphate)
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It is
found that the L -isomer of glycerol phosphate is found in the natural phosphoglycerols
(phosphoglycerides)

Phospholipids because they have phosphorylation bases as polar, hydrophilic heads in addition
their hydrocarbon non polahydrphobic tail are considered
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It is present mainly as
intermediate compound during the synthesis of phosphoglycerols and
triacylglycerols
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It has no base
 Lecithin (phosphatidyl choline): is most abundant phosphoglyceride in
plants and animals
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It is present in the animal’s cell
membrane
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This phospholipid is the major lipid
components of lung surfactants
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It’s
absence from the lungs of premature infants causes respiratory
distress syndrome
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The
base is ethanolamine
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 Phosphatidyl Serine: is cephalin-like and present in many tissues
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 Lipositol(phosphatidyl inositol): The polar head is cyclic heroes sugar
alcohol inositol (myoinositol)
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An inositol phosphatidyl
containing the 3 moles of phosphoric acid (1,4,5) is present in the
brain and muscles
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They are important membrane components both in plants and
animal cells
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The amino group of sphinogosine is connected by an amine linkage to a long chain saturated or
monounsaturated fatty acids, resulting in a compound called ceramide
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They contain a phosphoryl
choline as their polar head
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Glycolipids (glycosphinolipids)

Are widely distributed in every tissue of the body, particularly in nervous tissue as brain
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Acidic

Neutral (cerebrosides)
According to the
monosaccharide bound to
the ceramide they are

Sulfatides

Gangliosides

(Galactocerbroside

(Glucerebroside to
which one or more
sialic acid are
added)

Sulfate)

Galactocerbroside

Glucocerebroside

C
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Steroids:
 Steroids have a similar cyclic nucleus known as steroid nucleus:
resembling phenanthrene (rings A,B, and C) to which a cyclopentane
ring (D) is attached giving a sterane ring
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 There is methyl group at C10 (makes it C19)
 There is methyl group at C13 (makes it C18)
 So this ring is composed of 17 carbon atoms beside the methyl groups
Steroids include:






Sterols
Steroid hormones
Saponins and Cardiac glycosides
Bile acids and salts
Vitamin D

a) Sterols
This group of steroids has hydroxyl group OH at C3
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There is 2 types of sterols animal sterols (cholesterol and its derivative 7-dihydrocholestrol)
and plant sterols (ergosterols andd 22-dihydroergosterols)
Cholesterol Biomedical importance:




It is the main sterol in human body
Major constituent of plasma membrane
Precursor of sex hormones, cortical hormones, vitamin D and bile acids

Sources:



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Formed in body from acetyl CoA
Present in diet; egg yolk, meat,liver and brain

Chemistry of Cholesterol:
It is a solid alcohol of 27C and contains Stefan ring and has double bond between C5 and C6
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Cholesterol derivatives:
7-dehydrocholesterol is stored under the skin, and by the effect of ultraviolet rays is
transformed to cholecalciferol (vit
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It is present in lower plants as yeasts and
mounds e
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ergosterol ( provitamins D2) and has a double bond between C22 and C23
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g
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Both
vitamins are of equal potency
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Are obtained in liver by oxidation of cholesterol at C24 with removal of last 3 carbon
atoms
Are end products of cholesterol catabolism in body because body can’t breakdown
steroid nucleus

Types of bile acids:
o Primary bile acids:
 Choline acids: 3, 7, 12 Trihydroxycholanic acid (glycocholic and taurocholic are
primary bile acids)
 Chenodeoxy choline acid: 3,7 dihydroxy cholanic acid (glycochenodeoxycholic
and taurochenodeoxycholic are primary bile acids)
Primary biles can be conjugated with glycine or taurine to form glycocholic or taurocholic acids

8

Conjugation of primary bile acids with glycine or taurine mainly in their sodium or potassium
salts produce bile salts which are: Sodium or Potassium Glycocholate and Sodium or Potassium
taurocholate
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