Lipids chemistry

“You are what you eat !”

Thursday, June 9, 2016

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Rajesh Chaudhary

Department of Biochemistry, NGMC, ChisapaniThursday, June 9,

2016

Rajesh Chaudhary 2

Biomedical importance and function

Thursday, June 9, 2016Rajesh Chaudhary

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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 acids12

Mono unsaturated fatty acid (Oleic acid, C18:1)

A saturated fatty acid (Palmitic acid, C16)

A polyunsaturated fatty acid (Linoleic acid, C18:2)

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.

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 𝑂𝑚𝑒𝑔𝑎−6 𝑓𝑎𝑡𝑡𝑦 𝑎𝑐𝑖𝑑

𝑂𝑚𝑒𝑔𝑎−3 𝑓𝑎𝑡𝑡𝑦 𝑎𝑐𝑖𝑑 Cardiovascular diseases

Optimal ratio is 1:1 to 4:1, but North American diet contains

10:1 or 30:1.

Key convention in PUFA


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The length and degree of

unsaturation affects the

melting point of

unsaturated fatty acids.

The most stable

conformation is the fully

extended form.

Geometric isomerism of D9, 18:1 fatty

acids (Oleic and Elaidic acids)

Thursday, June 9, 2016

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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 alkyl long chain + ether-linked to glycerol @ both ends)

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 Sphingolipids lacks “Phosphate” group

but can be simple sugar or complex oligosaccharides

at their polar end – known as “Glycolipids”.

Some common types of storage and

membrane lipids


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Glycerophospholipids26

1. The fatty acids in the “glycerophospholipids” can be any of wide variety.

2. Distribution of molecular species is specific for different organisms, different

tissues of the same organism, and different glycerophospholipids in the same

cell or tissue.

3. In general, glycerophospholipids contain C16 or C18 @ C-1 and a C18 or

C20 @ C-2.

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 tails.

Doesn’t contain “glycerol”.

3 subclasses of Sphingolipids

1. sphingomyelins

2. neutral glycolipids

3. and, gangliosides

Ceramide

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.


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