4. COMPOUND OR COMPLEX LIPIDS:
This term includes:
1. Glycolipids
2. Gangliosides
3. Phospholipids
5.
6. Types of complex lipids:
Glycolipids
Gangliosides
Phospholidids
Streroids and sterols
Terpenes (isoprene complexes)
7. 1.Glycolipids (Glycosphingolipids)
These are ceramide-sugar(s).
Ceramide (sphingosine-fatty acid complex) and
glycolipids can be represented as below:
Ceramide+one more hexoses=glycolipids
8. Sphingosine :
Sphingosine is an 18-carbon monosaturated alcohol
containing:
An amino(-NH2) group.
A long chain fatty acid(22-26)is attach to sphingosine
at its( –NH2) group forming what is called ceramide.
Thus ceramide is sphingosine-fatty acid complex.
9.
10. Formation of glycolipids:
Glycolipids are formed when one or more sugars are
attached to ceramide at its terminal –OH group.
Glycolipids can be represented as ceramide sugars.
In certain cases dihydrosphingosine replaces
sphingosine.
11. Occurence:
Glycolipids occur mostly in white matter of the brain
and in the myelin sheaths of nerve fibers.
Various glycolipids such as cerebron, nervon,
oxynervon and kerasin differ from each other in the
type of fatty acid and the type of sugar present in their
molecules.
A common fatty acid present in these compounds is a
24-carbon containing fatty acid namely cerebronic
acid.
12. Glycolipids containing one sugar unit are called
cerebrosides.
Examples of cerebrosides:
Ceramide-glucose
Ceramide- galactose
Occurance:
these occur in plasma membranes of non-neural and
neural tissues respectively.
13. A more complex glycolipid is ceramide- trihexoside;
Its composition is ceramide-glucose-galactose-
galactose.
It can be converted to ceramide
dihexoside(ceramide-glucose-galactose).
Glycolipids have been found to be the determinants of
blood group antigens, A and B.
14. Note :
The sugar molecules present in glycolipids may be
sulfated forming sulfatides also called sulfolipids.
15. 2.Gangliosides:
In this the basic core structure is mostly ceramide-
glucose-galactose-N- acetylgalactosamine and N-
acetyleneuraminic acid.
Neuraminic acid is one of the sialic acids.
16. Occurance:
Ganglioside occur in brain, spleen,
RBCs and nerve cells.
In the brain gangliosides contribute
about 6% of the membrane lipids in
gray matter.
17. Examples:
One important example of ganglioside is GM1.
It occurs in the intestinal cell membrane and act as the
site of attachment to cholera toxin.
Molecular Formula: C73H131N3O31
Its structure is ceramide-glucose- glactose-N-
acetylegalactosamine -galactose
18. GM2 has structure ceramide-glucose-N-
acetylneuraminic acid
Galactose –N-acetylegalactosamine.
Symbol M in these gangliosides means they contain
only one molecule of sialic acid; more complex
gangliosides can have several of these acids.
19. Globosides:
Closely related to glycolipids are compounds called
globosides, said to be mucolipids.
The core structure of globosides is ceramide-glucose-
galactose-galactose-N- acetygalactosamine.
22. Occurance:
Sphingomyelins are present in large amounts in
brain and nerve tissues and in smaller amount in
other tissues and blood.
Because sphingomyelins contain
phosphocholine or phosphoethanolamine,
therefore they possess polar groups resembling
glycerophospholipids.
Sphingomyelin are more stable molecules than
glycerophospholipids.
23. B.Glycerophospholipids:
These are also called phosphoglycerides or glycerol
phosphatides;
These are the phospholipids containing
Glycerol
Fatty acid
H3PO4
And in many cases nitrogenous bases as well
24. Occurance:
They occur in all cells, plant as well as animals.
They are very important as constituent of cellular
structure is shown by the fact that they do not undergo
rapid mobilization during starvation as happens to
neutral fats.
These compounds are not freely soluble in water but
are water- miscible; they do not make true solutions in
water but they disperse forming micelles.
They are good emulsifying agents.
25. Types of glycerophospholipids:
There are various types of glycerophospholipids:
Phosphatidic acids
Lecithins
Cephalins
Plasmalogens
phosphatidylglycerol
26. Phosphatidic acid:
In these acids, one of the fatty acids
of a triacylglycerol is replaced by
H3PO4.
Occurance:
These are parent compounds of all
glycerophospholipids.
They are present in cells in small
amounts only.
27. Lecithins:
These are derivatives of L- phosphatidic acid in which
choline (a nitrogenous base) is joined to
H3PO4(phosphatidic acid+ choline + lecithine);in other
words they are phosphatidyl- cholines.
Choline is trimethylethanolamine and is a quaternary
ammonium compound which is as NaOH.
Depending upon the type of fatty acids R’ and R’’ , there
are many types of lecithins.
Both of its fatty acids may be saturated , both may be
unsaturated or one may be saturated, while the other is
unsaturated.
28. Properties of lecithins:
Lecithins are soluble in all fat- solvents except acetone.
They are white waxy materials.
They are the most abundant of the phospholipids in
cell membranes, serum and bile.
They are good emulsifying agents for fats.
Lecithins are convertible to lysolecithins by the
enzymatic removal of one of a fatty acid group
attached to either carbon No.1 or 2: the latter
compounds cause hemolysis of erythrocytes.
29. The enzyme catalyzing the conversion of lecithin
lysolecithin is called phospholipase A and is found
in snake venoms .
Lecithins containing two molecules of palmatic acid,
i.e. dipalmitoyl lecithin occur in surfactant, a
substance has secreted by type- 2 cells of the lung
alveoli.
Surfactant has a surface tension lowering effect and is
of great help in facilitating lung expansion at birth
and later.
30. The potentiality of the lungs to expand after birth can
be predicted by a chemical analysis of the amniotic
fluid.
If the ratio of lecithin to sphingomyelin in this fluid is
high , it means normal condition.
If the ratio of lecithin to sphingomyelin in this fluid is
high , it means normal condition.
31. Cephalin:
These are structurally identical with lecithins except
the base choline is replaced by either of ethanolamine,
serine , inosite ( occuring as myoinositol) or threonine,
forming phosphatidylethanolamine ,
phosphatidylinositol or threonine respectively.
32. Occurance:
Some authors prefer to reserve the term cephalins for
phosphatidylethanolamine only.
Cephalins are so named because they occur in high
concentration in brain tissue.
Cephalins have properties generally similar to lecithins
and occur associations with them; however , they are
more acidic than lecithins.
They also take part in clotting of blood.
33. Plasmalogens:
The plasmalogens resemble lecithin and cephalin in
structure, but instead of containing a fatty acid the
contain a vinyl ethyl substituent at carbon No.1 of
glycerol and on hydrolysis liberate a high fatty
aldehyde instead of a fatty acid.
In most plasmalogens the base ethanolamine though
choline, serine or inositol may be present.
34. Occurance:
Plasmalogens are chiefly found in skeletal muscle,
heart , brain , liver , platelets.
Plasmalogens appear to be resistant to phospholipases.
Only of the related compounds is the platelet
activating factor which is released from basophils; it
stimulates the aggregation of platelets and release
serotonin from them.
35. Phosphatidylglycerol:
In this case a glycerol molecule is attached to the
phosphoric acid part of the phosphatidic acid.
This substance gives rise to diphosphatidylglycerol also
called cardiolipin.
Occurance:
They occur in mitochondria.
36. 4.Steroids And Sterols:
A large number of compounds found in nature in the
non- saponifiable fraction of lipid belong to the class
of compounds called steroid.
In most of the natural steroids a methyl group is
present at carbon No. 13 and one is usually present at
carbon No 10 .
A subgroup of steroid is sterols which contain one or
more hydroxyl groups and carbonyle or carboxyl
group; there names end in –ol.
37. Some of the natural compounds belonging to the
steroids are cholestrol , ergosterol, bile acids, male and
female sex hormones and the hormones of the adrenal
cortex.
Cardiac glycosides and some alkaloids also have
steroid components.
38. Cholesterol:
It is the most abundant animal sterol.
It was first isolated in 1784 from gall stones
(cholesterol=solid bile).
39. Occurance:
It occur in all animal tissues.
It is most abundant in the adrenal cortex(10%) where it
acts as the precursor of steroid hormones, followed by
nervous system(2%).
Normal plasma level ranges from 150 to 220 mg/dl but
level of 200 mg/dl is at present considered to be the
maximum desirable.
Some 140 grams of cholesterol may be present in an
adult human body.
40. Cholesterol occurs in all animals but only in few higher
plants.
It generally crystallizes in the form of white rhombic
plates.
It is without taste and odor .
• It is insoluble in water but soluble In chloroform ,
ether and acetone(fat solvents).
• It is also very soluble in liquid fat and in solution of
bile salts.
• It is precipitated by digitonin which is a glycoside
present in the leaves and seeds of digitalss.
41. It is solid at room temperature and melts at 150C .
Cholesterol forms esters due to the presence of –OH
group in its molecule.
Cholesterol occurs in tissues in free as well as bound
form, i.e. as esterified with fatty acid.
Cholesterol occur in plasma membrane of tissue cells
and in the plasma lipoproteins.
42. Dehydrocholesterol:
This sterol occur in many tissues but that which is
present in the skin has the special role.
It is converted to vitamin D3 , I.e. cholecalciferol when
the skin is exposed to ultraviolet rays.
Exposure to sunlight is also the results in the
formation of vitamin D3 in the skin because sunlight
contains some u.v. rays; for this reason, vitamin D3 is
also called sunlight vitamin.
43. Ergosterol:
It is the main sterol of fungi and yeast .
Its name originates from the fact that it was first
isolated from ergot , a fungus which grows on plants
specially rye.
when irradiated with u.v. rays it is converted to
ergocalciferol which is the same as vitaminD2.
44. 5.Terpenes(isoprene complexes):
These are hydrocarbons containing carbon and
hydrogen in the ratio of 10: 16; these compounds
contain multiple units of the five carbon containing
hydrocarbon, namely isoprene.
Isoprene has the following structure:
45. The number of isoprene units greatly varies in
different terpenes.
Terpenes may be either
Linear
Or cyclic molecules
Some substances have both linear and cyclic terpenes
in their molecules.
Terpene includes vitamins A, E and K; coenzyme Q
also belong to these compounds.
46. Dolichol is a 17 to 21 isoprene unit containing alcohol
that act as sugar carrier.
During the addition of polysaccharides units to certain
proteins and lipids (forming glycoproteins and
glycolipids respectively) the sugar unit to be anchored
on cell membranes are chemically activated by
attachment to dolichol.
47. Other compound related to terpenes are
Camphor
Menthol
Limolene
Which are present in oil of camphor , mint and lemon
respectively.
48. Functions of lipids:
1) They are good sources of energy, as they provide
9Kcal=approximately 38KJ(1 cal =4.184 J)/g fat utilize
in the body.
2) Lipids in food also act as carriers of fat soluble
vitamins and nutritionally essential fatty acids
(linoleic acid and alpha linolenic acid).these two
fatty acids are needed by the body, but they cannot
be synthesized in the body.
3) Lipids also make the food more palatable and serve
to decrease its mass.
49. 4) The dietary lipids decrease gastric motility and
secretions and have a high satiety value.
5) Body fat provides contour to the body and also
gives anatomical stability to organs like kidneys.
6) Fats are good energy reservoirs in the body.
Adipose tissue is best suited for this purpose due to
its very little water content and high energy content.
7) Lipids act as electrical insulators in the nervous
tissue allowing rapid propagation of action potential,
i.e. depolarization waves or nerve impulses along
myelinated nerve fibers.
50. 8) Lipids are an integral part of cell protoplasm and
cell membranes. Glycolipids occur particularly in the
outer face of plasma membrane and contribute to the
carbohydrate of cell surface.
9) Some lipids act as precursors of very important
physiological compounds. For example,
cholesterol is the precursor of steroid hormones; 7-
dehydrocholesterol and ergosterol are the precursors
of vitamins D3 and D2 respectively.
10) Presence of lipids like cholesterol and others in skin
makes it highly resistant to the absorption of
water-soluble substances and also to the action of
many chemical agents.
51. 11) The sphingosine-containing lipids have a role in the
transmission of the nerve impulses across
synapses as they form part of the post synaptic
membrane receptors.
12) Eicosanoides which include prostanoids, likotriens
and lipoxins possess very important physiological
compounds.
13) In certain cases derivatives of lipids act as
intracellular messengers after they are released.
14) The terpene called dolichol helps in adding sugars to
proteins and lipids in the cell membrane.
52. 15) The platelet-activating factor is a lipid(
plasmalogen); it aggregates platelets and has many
other functions.
16) Ceramide is an important 2nd messenger regulating
pathways such as the cell cycle, aging and
apoptosis(programmed cell death).
17) Cardiolipin occurs in the inner membrane of
mitochondria where it plays a vital role in
mitochondrial functions; it is also believed to be
concerned with apoptosis.
53. Relationship with diseases:
I. Lipids along with proteins form lipoproteins that act
as transporters of various lipids from one tissue to
another. Abnormalities of lipoproteins are
responsible for diseases such as atherosclerosis
causing ischemic diseases.
II. Storage of abnormally high levels of certain lipids
occur due to lack of lysosomal acid hydrolases
resulting in lipid storage diseases.
54. III. One of the lecithins, dipalmitoyl lecithin, is
produced by the alveolar type II cells of the lungs;
the substance called surfactant has detergent- like
surface tension lowering properties preventing
collapse of pulmonary alveoli during expiration; it
also facilitates inspiration by the same property. Its
absence in the new born may prove fatal by
producing the infant respiratory distress
syndrome(IRDS).
IV. Gangliosides M1 present in the intestinal mucosa
acts as the receptor for cholera toxin.
