LIPID CHEMISTRY

LIPID OF BIOLOGICAL IMPORTANCEDefinition:

Lipids are organic compounds, which have the following common properties:

1- They are esters of fatty acids or substances associated with them in nature.

2- Most of them are insoluble in water but soluble in fat solvents (non polar solvents) e.g.: benzene, chloroform, acetone and ether.

Classification: Lipids are classified into three main groups:

Simple Lipids Compound lipidsDerived Lipids

Esters of fatty acids with various types of alcohols

Esters of fatty acids with various types of alcohols in addition to a

prosthetic group

They are produced by hydrolysis of the first two

groups

Simple lipids:

NUTERAL FAT WAXES

Esters of three fatty acids with glycerol.= TAG

Esters of fatty acids with higher

molecular weight monohydric

alcohols.

Complex lipids (Compound or Conjugated lipids)

Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue.

Glycolipids (glycosphingoLipids:)

containing a fatty acid, sphingosine, and

carbohydrate.Proteolipids: fatty acid,

alcohol and protein

Fatty Acids• Fatty acids that occur in natural fats

are usually monocarboxylic acids containing an even number of carbon atoms. The chain may be saturated (containing no double bonds) or unsaturated (containing one or more double bonds).

I- Saturated Fatty Acids (SFA):

• They contain no double bonds.• They are either short chain (from C2 to C10)

or long chain (from C12 to C24)

• All have the following general formula:• CH₃- (CH₂)n- COOH• Where n = Total number of carbons – 2

I- Saturated Fatty Acids (SFA):

• A- Short chain fatty acids include:• Acetic acid (C2) • Butyric acid (C4)• B- Long chain fatty acid, the most common

include mainly:• Palmitic acid (C16) • Stearic acid (C18) • Arachidic acid (C20)

• A- Short chain fatty acids include:• Acetic acid (C2) • Butyric acid (C4)• B- Long chain fatty acid, the most common

include mainly:• Palmitic acid (C16) • Stearic acid (C18) • Arachidic acid (C20)

II- Unsaturated Fatty Acids (USFA):

• They contain one or more double bonds.• Unsaturated fatty acids are classified

according to the number of double bonds in their chains into two main groups:

• 1- Monoethenoid: one double bond:• Palmitoleic acid• Oleic acid

II- Unsaturated Fatty Acids (USFA):

• Polyethenoid • They have more than one double bond in their

structure, termed polyunsaturated fatty acids (PUFA).

• Linoleic acid • Linolenic acid • Arachidonic acid

Nutritional classification of fatty acids:• a)- Essential Fatty Acids: • They are not synthesized in our body, so it is essential to

take them in diet. They include Linolenic and Linoleic acids.

• Deficiency of essential fatty acids produces: • Fatty liver and sterility in adults.• Impaired growth and dermatitis in infants.• Sources of PUFA: They are present mainly in fish and

vegetable oils e.g.: maize, cottonseed, linseed, olive, sun flower and soya been oils.

• b)- Non Essential Fatty Acids:• They include all other fatty acids because they are formed

in our body in good amounts mainly from carbohydrates. It is not essential to take them in diets.

•

• a)- Essential Fatty Acids: • They are not synthesized in our body, so it is essential to

take them in diet. They include Linolenic and Linoleic acids.

• Deficiency of essential fatty acids produces: • Fatty liver and sterility in adults.• Impaired growth and dermatitis in infants.• Sources of PUFA: They are present mainly in fish and

vegetable oils e.g.: maize, cottonseed, linseed, olive, sun flower and soya been oils.

• b)- Non Essential Fatty Acids:• They include all other fatty acids because they are formed

in our body in good amounts mainly from carbohydrates. It is not essential to take them in diets.

•

Physical Properties of Fatty Acids

1. Solubility in water: fatty acids are insoluble in water but soluble in

fat solvents.2. Physical state at room temperature: The saturated fatty acids are solid at room

temperatureUnsaturated long chain fatty acids are liquids

due to the presence of cis double bonds.

Chemical Reactions of Fatty Acids• 1. Ester Formation:• 2. Salt formation: • 3. Reduction:• 4. Rancidity:• It is the development of bad odor and taste

(bad flavor) of fats and oils.• Rancidity is due to exposure of fats or oils to

high temperature, oxygen, moisture or humidity, light and metals (act as catalysts).

I- Simple Lipids• They are esters of fatty acids with alcohols,

according to the types of alcohols there are two main sub-groups:

• 1. Neutral fats or Triacylglycerol (TAG): They are esters of three fatty acids with glycerol.

• 2. Waxes: They are esters of one fatty acid with long chain monohydroxyalcohol higher than glycerol.

•

I- Simple Lipids

Sphingosine containing lipids

• SphingosineFA

Ceramide

Phosphocholine

Sphingomyline

Importance of glycolipids:

• They are found mainly in the myelin sheath and cell membrane of RBCs. They act as cell membrane receptors for hormones and external stimuli also they provide recognition properties.

III- Derived Lipids• They are produced by hydrolysis of either

simple or conjugated lipids. • They include the following :• 1. Fatty acids.• 2. Alcohols.• 3. Steroids.• 4. Carotenoids.• 5. Fat soluble vitamins: as vitamins A, D, E &

K.•

Steroids

Classification of steroids

•Steroid hormones.•Sterols.•Bile acids.•

Cholesterol• It is the most important animal sterol. It is

present either free (non esterified) or esterified with fatty acid to form cholesteryl ester. Free cholesterol contains 27 carbon atoms.

• Distribution of cholesterol: It is widely distributed in all tissues but higher concentrations are present in brain, nervous tissue, liver, adrenals, gonads, skin and adipose tissue.

Cholesterol• Precursor of cholesterol: It is formed

from active acetate (acetyl-CoA).• Blood level of cholesterol: Normally it is

present in the plasma in concentrations ranging from 100 to 200 mg/dL (30% as free cholesterol and 70% as cholesteryl esters).

• .

Importance and derivatives of cholesterol:

• It is important constituent of cell membranes.• It is converted into bile acids and bile salts in

the liver.• It is the precursor of all steroid hormones.• It can be oxidized in the liver into 7-dehydro

cholesterol which can be converted under the skin into vitamin D3 by ultra violet

Bile Acids• 1. Primary bile acids: they are formed in the liver

from cholesterol and they include:• - Cholic acid• - Chenodeoxy cholic acid• 2- Secondary bile acids: they are formed of

primary bile acid in large intestine. • - Deoxycholic acid• - Lithocholic acid

Bile salts: • They are formed by conjugation of cholic

acid with glycine (80%) or taurine (20%) then they are excreted by liver in bile as sodium glycocholate or sodium taurocholate. Bile salts pass to the intestine where they are reabsorbed and return back to the liver to be excreted again in bile (entero-hepatic circulation).

•

Importance of bile salts:• Conversion of cholesterol to bile salt is an

important mechanism for removal of excess cholesterol from blood.

• They are good emulsifying factors important for digestion and absorption of fats.

• They prevent precipitation of cholesterol in the bile as cholesterol stones.

• They stimulate liver cells to secrete more bile (choleretic effect).

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