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SIMPLE LIPIDS

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• Lipids are a heterogamous group of chemical compounds including:

• Fats• Oils• Waxes• Steroids• Other related compounds [they are related more by their physical

properties than their chemical properties.]

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Common properties of lipids:

• Relatively insoluble in water.• Soluble in nonpolar solvents such as:

– Ether .– Chloroform.– Benzene.– Acetone.

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Lipids are important dietary constituents:

• High energy value.• Essential fatty acids.• Fat soluble vitamins [ A D E K].

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• Lipids are stored in adipose tissue• It serves as thermal insulator• Nonpolar lipids serve as electrical insulator in

nerves.• Lipids and proteins (Lipoproteins) are important

constituents of cell and mitochondrial membranes.

• Lipoproteins serve as means of transport of lipids in blood.

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Knowledge of lipids biochemistry is necessary in understanding many important biochemical areas e.g.:

Obesity.Diabetes mellitus.Atherosclerosis.Nutrition: role of polyunsaturated fatty acids and fat soluble vitamins.etc……

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Classification of lipids:

• Simple lipids.

• Complex lipids.

• Precursor and derived lipids.

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Classification of lipids:

1. Simple lipids:They are esters of fatty acids with alcohols:1. Fats and oils: esters of fatty acids with glycerol

(triacylglycerols).

2. Waxes: esters of fatty acids with high molecular weight monohydric alcohol.

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2. Complex lipids: Esters of fatty acids with alcohols and molecules with

other groups.Phospholipids: Lipids containing:

Fatty acids Alcohol Phosphoric acid residue.

Glycolipids (glycosphingolipids):Lipids containing:

Fatty acidSphingosineCarbohydrate.

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3. Precursor and derived lipids:• This group includes:

– Fatty Acids.– Glycerol.– Cholesterol.– Steroid hormones.– Fatty aldehydes.– Fat soluble vitamins [ A D E K].– Some other alcohols.

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Esters ofAlcohols and Carboxylic Acids

RC OH

O

Carboxylic Acid Alcohol Ester

R OH R C O R

O

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Triacylglycerols in Simple Lipids

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Fatty Acids

• Fatty acids (FAs) exist as:• Esterified fatty acids: Fatty acids occur mainly as esters in natural fats

and oils (Triacylglycerols).

• Free fatty acids (FFA).

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• more than 90% of FAs found in plasma are in the form of FA esters.– Triacylglycerols (triglycerides).– Cholesterol esters – PhospholipidsALL are contained in lipoproteins.

• Unesterified FAs are circulated in association with albumin.

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Physiological Functions of Fatty Acids:• Energy Source: During fasting, blood level of FFA

increases.• FAs can be oxidized by many tissues including liver

and muscle to produce energy.• FAs are structural components of membrane lipids.• FAs are attached to some intracellular proteins.• FAs are precursors of hormone-like prostaglandins.• FAs esterified as TAGs stored in adipose tissue serve

as major energy reserve in the body.

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Structure of Fatty Acids:• FAs consist of:

– Hydrophobic hydrocarbon chain.– Carboxyl group.

The anionic group has an affinity for water giving the FA its amphipathic nature, having both a hydrophilic and a hydrophobic regions.

Long chain fatty acids (LCFA) are highly water insoluble, hence they need transport in blood through plasma albumin.

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Structure of a fatty acid.

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• the carbons in FAs are named as follows:

– Carbon atoms are numbered from the carboxyl carbon (carbon no. 1).

– C-2, the carbon at which the carboxyl group is attached also called: α- carbon.

• C- 3 : β- carbon

• C- 4: γ- carbon

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126 5 4 3

– Carbon of the terminal methyl group is known as ω-carbon.

– The carbons of FAs can be counted beginning at the ω-carbon or methyl end of the chain.

– ω9 indicates a double bond between the 9th and the 10th carbons counting from ω- carbon.

– Arachidonic acid is called ω-6 FA.

– Δ is used for indicating the number and position of the double bonds,

• Δ9 indicates double bond between C9 and C10 of a fatty acid. 20

• In animals, additional double bonds are introduced only between the existing double bonds e.g.:– ω9– ω6– ω3– Leading to a series of fatty acids known as: ω9, ω6 and ω3 fatty acids.

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Chain length of FAs:

• FAs are denoted as follows:• Ex: Arachidonic acid 20:4(5,8,11,14)

– The number before the colon indicates the number of carbons in the chain.

– The numbers after indicates the number and positions of double bonds.

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Arachidonic acid

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• Fatty acids that occur in natural fats are always straight chain derivatives containing an even number of carbon atoms.

• Fatty acids are named after corresponding hydrocarbons with the same number and arrangements of carbon atoms.

• oic been substituted for –e.• Saturated acids end in –anoic.• Unsaturated acids end in –enoic.

25Some fatty acids of physiologic importance

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Saturation of Fatty AcidsSaturated Fatty Acids:

• Saturated FAs: contain no double bonds.• Short chain saturated fatty acids have low

melting point.• Short chain saturated fatty are liquid at room

temperature.• Medium and long chain fatty acids have high

melting point.• Medium and long chain fatty acids are solid at

room temperature.

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S aturated fatty acids (all have C –C single bonds):

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Unsaturated Fatty Acids:

• Unsaturated FAs: contain one, or more double bond, being:1. Monounsaturated FAs.2. Polyunsaturated FAs.

• Double bonds are nearly always in cis- rather than trans- position.

• Double bonds are always spaced by 3 carbons.

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Unsaturated Fatty acids Have nonlinear chains that do not allow molecules to pack closely.

Have low melting points, are liquids at room temperature.

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( CH 2)7 C O H

O

omega-6 fatty acid(linoleic acid)

( C H 2 ) 7 C O H

O

6ω1

ω1

omega-3 fatty acid(linolenic acid)

Polyunsaturated Fatty acids:Polyunsaturated fatty acids such as:Linoleic acidLinolenic AcidAre nutritionally essential fatty acids

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Sat. Fatty Acids Formula Melting Point (oC) Butyric C4H8O2 Liquid Palmitic C16H22O2 63 Stearic C18H36O2 70 Unsat. Fatty Acids Formula Melting Point (oC) Oleic C18H34O2 Liquid Linoleic C18H32O2 Liquid Linolenic C18H30O2 Liquid

Melting point of some saturated and unsaturated fatty acids

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- A fat is a triacylglycerol that is solid at room temperature

- An oil is a triacylglycerol that is liquid at room temperature

- Usually fats come from animal sources and contain more saturated fatty acids

- Oils come from plant sources and have a high content of unsaturated fatty acids

Fats and Oils

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