INTRODUCTION
Lipid are substances that are
1- water insoluble and
2- extractable by non-polar solvents.
Complex lipids of humans fall into two broad categories:
(1) neutral, non-polar lipids, ( triacylglycerols and cholesceryl esters)
(2) polar lipids (phospholipids and glycolipids)
The polar lipids are amphipathic, containing a hydrophobic and hydrophilic region in the same
molecule.
The hydrophobic and hydrophilic regions in glycerophospholipids are bridged by a glycerol moiety
and in sphingomyelin and glycosphingo lipids by sphingosine.
Triacylglycerol is found primarily in storage sites in adipose tissue, whereas polar lipids occur
primarily in cellular membranes.
Complex lipids have many roles:
1. structure of membranes,
2. some glycerophospholipids are required for the activity of membrane enzymes
3. inositol containing phospholipids serve as precursors of signalling molecules
4. Glycosphingolipids have a role in cell-cell recognition, phagocytosis, contact inhibition , and
rejection of transplanted tissues and organs.
5. Antigenic determinants of blood groups are primarily glycolipid in nature.
6. Cholesterol is important in atherosclerosis
7. various sphingolipids accumulate in genetic disorders called sphingolipidoses.
PHOSPHOLIPIDS
Two major classes of acylglycerolipids are: triacylglycerols and glycerophospholipids which
have as their core the C3 polyol, glycerol.
Two primary alcohol groups of glycerol are not stereochemically identical, and in the case of
phospholipids, it is usually the same hydroxyl group esteri ed to the phosphate residue.
The stereospeci c numbering system designates di erent hydroxyl groups.
When the stereospeci c numbering (sn) system is employed, the pre x sn-is used before the
name of the compound.
Glycerophospholipids usually contain a sn-glycerol 3-phosphate moiety.
Although each contains the glycerol moiety as a fundamental structural element, neutral
triacylglycerols and charged ionic phospholipids have very di erent physical properties and
functions.
Phospholipids Contain Phosphatidic Acid Linked to a Base
-Phospholipids are polar, ionic lipids composed of 1, 2-diacylglycerol and
a phosphodiester bridge chat links the glycerol backbone to some base, usually a nitrogenous
one, such as choline , serine, or ethanolamine.
-The most abundant phospholipids in human tissues are
1.phosphatidylcholine (also called lecithin),
2.phosphatidylethanolamine , and
3.phosphatidylserine.
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, At physiologic pH , phosphatidylcholine and phosphatidylethanolamine have no net charge and
exist as dipolar zwitterions, whereas phosphatidylserine has a net charge of - 1, causing it to be
an acidic phospholipid.
Phosphatidylethanolamine (PE) is related to phosphatidylcholine in that trimethylation of PE
produces lecithin.
Most phospholipids contain more than one kind of fatty acid per molecule:
1.Phosphatidylcholine (PC) contains mostly palmitic acid or stearic acid in the sn-1 position and
primarily unsaturated 18 carbon fatty acids oleic, linoleic, or a-linolenic in the sn-2 position.
2.Phosphatidylethanolamine has the same saturated fatty acids as PC at the sn-1 position but
contains more of the long-chain polyunsaturated fatty acids, namely, linoleic acid, arachidonic
acid, and docosahexaenoic acid at the sn-2 position.
3.Phosphatidylinositol, an acidic phospholipid that occurs in mammalian membranes, often
contains almost exclusively stearic acid (18:0) in the sn-1 position and arachidonic acid in the
sn-2 position.
4.Phosphatidylglycerol composed of a polyol polar head group, occurs in relatively large
amounts in mitochondrial membranes and pulmonary surfactant and is a precursor of cardiolipin.
Phosphatidylglycerol and phosphatidylinositol both carry a formal charge of - 1 at neutral pH and
are therefore acidic lipids.
Cardiolipin, a very acidic (charge - 2) phospholipid, is composed of two molecules of
phosphatidic acid linked together covalently through a molecule of glycerol.
It occurs primarily
- in the inner membrane of mitochondria of metabolically active tissues (e.g., heart muscle)
- in bacterial membranes.
Cardiolipin is present in the membrane of Treponema palladium and is the antigen detected in the
Wasserman test for syphilis.
Barth syndrome is a rare X-linked mitochondrial disorder caused by a defect in the T AZ gene,
which encodes the protein ta uzin which is required for cardiolipin synthesis.
Patients with this inherited disorder exhibit cardiomyopathy, skeletal myopathy, and abnormal
mitochondria.
Phospholipids mentioned so far contain only O-acyl residues attached to glycerol.
0-(1-alkenyl) substituents occur at C1 of the sn-glycerol in phosphoglycerides in combination with
an 0-acyl residue esteri ed to the C2 position; compounds in this class are known as
plasmalogens or plasmenyl lipids.
Relatively large amounts of ethanolamine plasmalogen (also called plasmenylethanolamine) occur
in myelin with lesser amounts in heart muscle where choline plasmalogen is abundant.
platelet activating factor (PAF) is a major mediator of:
-hypersensitivity,
-acute in ammatory reactions,
-allergic responses, and
-anaphylactic shock.
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Lipid are substances that are
1- water insoluble and
2- extractable by non-polar solvents.
Complex lipids of humans fall into two broad categories:
(1) neutral, non-polar lipids, ( triacylglycerols and cholesceryl esters)
(2) polar lipids (phospholipids and glycolipids)
The polar lipids are amphipathic, containing a hydrophobic and hydrophilic region in the same
molecule.
The hydrophobic and hydrophilic regions in glycerophospholipids are bridged by a glycerol moiety
and in sphingomyelin and glycosphingo lipids by sphingosine.
Triacylglycerol is found primarily in storage sites in adipose tissue, whereas polar lipids occur
primarily in cellular membranes.
Complex lipids have many roles:
1. structure of membranes,
2. some glycerophospholipids are required for the activity of membrane enzymes
3. inositol containing phospholipids serve as precursors of signalling molecules
4. Glycosphingolipids have a role in cell-cell recognition, phagocytosis, contact inhibition , and
rejection of transplanted tissues and organs.
5. Antigenic determinants of blood groups are primarily glycolipid in nature.
6. Cholesterol is important in atherosclerosis
7. various sphingolipids accumulate in genetic disorders called sphingolipidoses.
PHOSPHOLIPIDS
Two major classes of acylglycerolipids are: triacylglycerols and glycerophospholipids which
have as their core the C3 polyol, glycerol.
Two primary alcohol groups of glycerol are not stereochemically identical, and in the case of
phospholipids, it is usually the same hydroxyl group esteri ed to the phosphate residue.
The stereospeci c numbering system designates di erent hydroxyl groups.
When the stereospeci c numbering (sn) system is employed, the pre x sn-is used before the
name of the compound.
Glycerophospholipids usually contain a sn-glycerol 3-phosphate moiety.
Although each contains the glycerol moiety as a fundamental structural element, neutral
triacylglycerols and charged ionic phospholipids have very di erent physical properties and
functions.
Phospholipids Contain Phosphatidic Acid Linked to a Base
-Phospholipids are polar, ionic lipids composed of 1, 2-diacylglycerol and
a phosphodiester bridge chat links the glycerol backbone to some base, usually a nitrogenous
one, such as choline , serine, or ethanolamine.
-The most abundant phospholipids in human tissues are
1.phosphatidylcholine (also called lecithin),
2.phosphatidylethanolamine , and
3.phosphatidylserine.
fi fi ff fi ff fi
, At physiologic pH , phosphatidylcholine and phosphatidylethanolamine have no net charge and
exist as dipolar zwitterions, whereas phosphatidylserine has a net charge of - 1, causing it to be
an acidic phospholipid.
Phosphatidylethanolamine (PE) is related to phosphatidylcholine in that trimethylation of PE
produces lecithin.
Most phospholipids contain more than one kind of fatty acid per molecule:
1.Phosphatidylcholine (PC) contains mostly palmitic acid or stearic acid in the sn-1 position and
primarily unsaturated 18 carbon fatty acids oleic, linoleic, or a-linolenic in the sn-2 position.
2.Phosphatidylethanolamine has the same saturated fatty acids as PC at the sn-1 position but
contains more of the long-chain polyunsaturated fatty acids, namely, linoleic acid, arachidonic
acid, and docosahexaenoic acid at the sn-2 position.
3.Phosphatidylinositol, an acidic phospholipid that occurs in mammalian membranes, often
contains almost exclusively stearic acid (18:0) in the sn-1 position and arachidonic acid in the
sn-2 position.
4.Phosphatidylglycerol composed of a polyol polar head group, occurs in relatively large
amounts in mitochondrial membranes and pulmonary surfactant and is a precursor of cardiolipin.
Phosphatidylglycerol and phosphatidylinositol both carry a formal charge of - 1 at neutral pH and
are therefore acidic lipids.
Cardiolipin, a very acidic (charge - 2) phospholipid, is composed of two molecules of
phosphatidic acid linked together covalently through a molecule of glycerol.
It occurs primarily
- in the inner membrane of mitochondria of metabolically active tissues (e.g., heart muscle)
- in bacterial membranes.
Cardiolipin is present in the membrane of Treponema palladium and is the antigen detected in the
Wasserman test for syphilis.
Barth syndrome is a rare X-linked mitochondrial disorder caused by a defect in the T AZ gene,
which encodes the protein ta uzin which is required for cardiolipin synthesis.
Patients with this inherited disorder exhibit cardiomyopathy, skeletal myopathy, and abnormal
mitochondria.
Phospholipids mentioned so far contain only O-acyl residues attached to glycerol.
0-(1-alkenyl) substituents occur at C1 of the sn-glycerol in phosphoglycerides in combination with
an 0-acyl residue esteri ed to the C2 position; compounds in this class are known as
plasmalogens or plasmenyl lipids.
Relatively large amounts of ethanolamine plasmalogen (also called plasmenylethanolamine) occur
in myelin with lesser amounts in heart muscle where choline plasmalogen is abundant.
platelet activating factor (PAF) is a major mediator of:
-hypersensitivity,
-acute in ammatory reactions,
-allergic responses, and
-anaphylactic shock.
fl fi ff
