Lipids

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“You are what you eat !”
Thursday, November 06, 2014
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Rajesh Chaudhary
LIPIDS
Department of Biochemistry, KMC, Duwakot
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Biomedical importance and function
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Phospholipids and Sterols are major structural elements of cell membrane.
Role as enzyme cofactor.
Electron carrier.
Light-absorbing pigment.
Hydrophobic-anchor for proteins.
Emulsifying agent in digestive tract.
Categories of biological lipids
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Eight major categories of Biological lipids
Lipids Vs. Fatty acids Vs. Triacylglycerol
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Simplest lipid constructed from Fatty acids are Triacylglycerol, also referred to as TG, Fat or Neutral fat.
How are ester link formed?
Triacylglycerol (TG)
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Examples of simple TG: Tripalmitin, Tristearin, Triolein.
Types of Triacylglycerols
Simple
Complex

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1. Triglycerides as storage form of energy in Eukaryotes and higher vertebrates 2. Role of Lipases
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What is the advantage of having TG as a stored fuel rather than polysaccharide such as Glycogen and Starch?
1. The carbon atom of fatty acid are more reduced than glycogen and starch, thus, gives more energy.
(A gram for gram)
2. TG are hydrophobic thus unhydrated. Therefore, organisms don’t have to carry extra weight of water.
Classification of Lipids
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1. Simple lipids
Examples: Fats and Waxes.
2. Complex lipids
Examples: Phospholipids (glycerophospholipids, sphingophospholipids)
Glycolipids: glycosphingolipids
3. Precursor and derived lipids
Examples: fatty acid, glycerol, steroids, fatty aldehydes, ketone bodies.
Simple lipids
Complex lipids
Classification of Lipids
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Fatty acids are aliphatic carboxylic acid.
Are either: 1. Saturated and 2. Unsaturated
1. Saturated: doesn’t contain double bond
2. Unsaturated: contains double bond
2. Unsaturated
2.1. Monounsaturated (E.g.: Oleic acid)
2.2. Polyunsaturated (E.g.: polyethenoid, polyenoic acids)
2.3. Eicosanoids (E.g.: Prostanoids, Lipoxin and Leukotrienes)
Nomenclature
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Saturated acids ends in “-anoic” acid.
Example: Octanoic acid
Unsaturated acids with double bonds end in “-enoic” acid.
Example: Octadecenoic acid
Examples of saturated, monounsaturated and polyunsaturated fatty acids
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Basic rules of nomenclature in Lipids
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1. Carbon atoms are named from the carboxyl carbon. (Carbon No. 1)
2. The rest of the carbon following are named as: 2,3,4… and so on – also known as “a”, “b”, “g” and so on….
3. For polyunsaturated fatty acids: The no. of carbon is named from the opposite side of the carboxyl carbon and given the no. (Carbon No. 1) – and called “omega (w)”.
Delta (D) is used for indicating the position and number of double bonds.
Nomenclature
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Fatty acids are hydrocarbon derivatives (Length: C4 to C36).
Monounsaturated fatty acids
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… acids containing one double bond.
Figure. Oleic acid. n-9 is equivalent to w9.
Examples
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Palmitic acid: abbreviated form  16:0
Oleic acid: 18:1
Most commonly occurring fatty acids have even no. of carbon chain  12 – 24.
Common pattern of location of double bonds in Fatty acids
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In most monounsaturated fatty acids, double bond is between C9 and C10. (D9)
Other double bonds of polyunsaturated fatty acids (PUFA) are at D12 and D15.
Exception: Arachidonic acid
In most naturally occurring PUFA have cis-configuration.
Trans-FA are produced by fermentation in rumen of ruminants.
Key convention on Polyunsaturated Fatty Acids (PUFA)
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Double bond between the 3rd. And 4th. Carbon atom from methyl end is of special importance in human nutrition.
Alternative nomenclature is used for PUFA rather than conventional one.
1. Carbon atom to the most distant site from the carboxylic terminal gets the name “w” – the carbon no. 1.
Example: Omega-3 fatty acid.

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Key convention in PUFAS
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Example: Omega-3 PUFA (a-linolenic acid)
(ALA; 18:3(D9,12,15))
(EPA; 20:5(D5,8,11,14,17)
(DHA; 22:6(D4,7,10,13,16,19)
An imbalance in 푂푚푒푔푎−6 푓푎푡푡푦 푎푐푖푑 푂푚푒푔푎−3 푓푎푡푡푦 푎푐푖푑  Cardiovascular diseases
Optimal ratio is 1:1 or 4:1, but North American diet contains 10:1 or 30:1.
Geometric isomerism of D9, 18:1 fatty acids (Oleic and Elaidic acids)
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Effect of temperature on structure of lipids (Higher Vs. Lower)?
Presence of acyl chains on same side vs opposite side around carbon containing double bond
Arachidonic acid
Physical and Physiologic properties of Fatty acids
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Melting point of even numbered carbon fatty acids increase with chain length and decrease according to unsaturation.
Triacylglycerol containing three saturated fatty acids of 12 carbons or more Vs. if, fatty acid residues are 18:2
Membrane lipids are liquid at room temperature.
Structural Lipids in Membranes
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What are the structural lipids in membrane?
1. Glycerophospholipids (2 fatty acids + glycerol) – hydrophobic region
2. Galactolipids & Sulfolipids (2 fatty acids + 1 glycerol) but lacks Phosphate
3. Archael tetraether lipids (2 acyl long chain + ether-linked to glycerol)
4. Sphingolipids (1 fatty acids + fatty amine (Sphingosine))
5. Sterol
Phospholipids Vs. Glycolipids
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In Glycerophospolipids and some Sphingolipids, a polar head group is joined to hydrophobic moiety with phosphodiester linkage – these are called “Phospholipids”.
But, some lacks “Phosphate” group an can be simple or complex – known as “Glycolipids”.
Some common types of storage and membrane lipids
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Glycerophospholipids
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… called “Phosphoglycerides”.
2 fatty acids + 1st. And 2nd. Carbon of glycerol.
Glycerol is Prochiral.
No asymmetric carbon atom.
Some glycerophospholipids have ether-linked fatty acids.
Glycerophospholipids
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1
2
… released from leukocyte called “basophils” and stimulate platelet aggregation and release of serotonine (a vasoconstrictor)
Sphingolipids
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… 4th. Large class of membrane lipids.
… contains one polar head and two non-polar tail.
Doesn’t contain “glycerol”.
3 subclasses of Sphingolipids
1. sphingomyelins
2. neutral glycolipids
3. and, gangliosides
Sphingolipids
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Sphingolipids at cell surface are sites of biological recognition.
Many of these are specially prominent in the plasma membranes of neurons.
 Carbohydrate moieties of certain sphingolipids define the human blood groups.
NOTE: Phospholipids and Sphingolipids are degraded in lysosomes.
Sterols
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… structural lipids present in the membrane of most eukaryotic cells.
Cholesterol is one major sterol.
Phospholipids are the main lipid constituents of membranes
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… is derivative of Phosphatidic acid.
1. Phosphatidylcholines (Lecithins)  in membranes.
2. Phosphatidylinositol  Precursor of second messengers
3. Cardiolipin  major lipid in mitochondrial membrane.
4. Lysophospholipids  intermediate in metabolism of phosphoglycerols.
5. Plasmalogens occur in brain & muscle.
6. Sphingomyelins  in Nervous system

Lipids

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