LIPID CHEMISTRY
Definition:-
Lipids are heterogeneous group of compounds related to fatty acids and include fats, oils, waxes and related compounds.
Function of lipids:-
1- High source of energy when stored in adipose tissues
2- Acts as medium for fat soluble vitamins
3- It serves as insulating material in the subcutaneous tissues
4- The fat contents of the nerve tissues acts as pad for the nerve
Classification of lipids:
A)-Simple lipids: these are esters of fatty acids with alcohols. According to the alcohol, they are subclassified into:-
1- Neutral fats: these are esters of fatty acids with glycerol. They are also called triglycerides or triacylglycerols.
- Neutral fats are either fats or oils.
- Fats are solid triacylglycerols due to high content of saturated fatty acids.
- Oils are liquid triacylglycerols due to high contents of unsaturated fatty acids.
- Glycerol is trihydric alcohol can be esterified with one, two or three fatty acids forming monoglycerides, diglycerides or triglycerides respectively.
2- Waxes:- These are esters of fatty acids with alcohol higher than glycerol.
B)- Compound lipids:-
These are esters of fatty acids with alcohols containing other groups and according to these groups they are subclassified into:-
1- Phospholipids:- these contain in addition to fatty acids and alcohol, a phosphate radical. They also have nitrogen containing bases. The alcohol of phospholipids is usually glycerol.
2- Glycolipids:- contain in addition to fatty acids and alcohol, a carbohydrate radical.
3– Proteolipids:- contain protein radical in addition to fatty acids and alcohols.
C)- Derived lipids:-
These include substances obtained by the hydrolysis of the above groups. They also include substances associated with them in nature and related to them in properties and metabolism.
Fatty Acids
- Fatty acids (F.A.) are momocarboxylic acids obtained from the hydrolysis of fats.
- They are either saturated (containing no double bonds) or unsaturated (containing one or more double bonds).
- F.A. usually containing an even number of carbon atoms (because they are synthesized from two carbon units).
- In F.A. the carbon atoms are numbered from the carboxylic end, which is known as carbon No. 1
- The carbon atom adjacent to the carboxyl group is known as carbon No. 2 or α-carbon, and the next carbon as carbon 3 or β- carbon and the next carbon 4 or γ-carbon, …etc.
- The methyl carbon at the other end of F.A. is known as omega carbon (ω-carbon).
Saturated fatty acids:-
Saturated fatty acids do not contain any double bonds or other functional groups along the chain. The term "saturated" refers to hydrogen, in that all carbons (apart from the carboxylic acid [-COOH] group) contain as many hydrogens as possible. In other words, the omega (ω) end contains 3 hydrogens (CH3-) and each carbon within the chain contains 2 hydrogens (-CH2-). The first member of saturated F.A. is acetic acid and the most common being palmitic acid and stearic acid.
Some saturated fatty acids are:
• Butyric: CH3(CH2)2COOH
• Lauric: (dodecanoic acid): CH3(CH2)10COOH
• Myristic: (tetradecanoic acid): CH3(CH2)12COOH
• Palmitic: (hexadecanoic acid): CH3(CH2)14COOH
• Stearic: (octadecanoic acid): CH3(CH2)16COOH
• Arachidic: (eicosanoic acid): CH3(CH2)18COOH
Unsaturated F.A.:-
These contain double bonds; they can be sub-divided according to the number of unsaturation into:-
A- Monoethenoid (Monounsaturated F. A.):-
These contain one double bond:
e.g.:- Palmitoleic acid (C16):
CH3 - (CH2)5 – HC9 = 10CH - (CH2)7 - COOH
- Oleic acid (C18):
CH3 - (CH2)7 - HC9 = 10CH - (CH2)7 – COOH
B- Polyethenoid (Polyunsaturated) F. A.:- These contain two or more double bonds:-
1- Tow double bonds: e.g. Linoleic acid 18:2;9,12
18 carbon atoms; 2 double bonds (number of unsaturation), the first between carbon 9–10 and the other between 12–13.
2- Three double bonds: Linolenic acid 18:3;9,12,15.
3- Four double bond: e.g.: Arachidonic acid 20:4;5,8,11,14.
Arachidonic acid found in small quantity with linoleic acid and linolenic acid mainly in peanut oil.
:sm255:
Essential fatty acids:
- The above mentioned three polyunsaturated fatty acids (Linoleic, Linolenic and Arachidonic acids) can't be synthesized by the body at sufficient rates; therefore they must be supplied in our diet.
- They protect against atherosclerosis as they form esters with cholesterol.
- Vegetable oils as corn and olive oils are rich in essential fatty acids and are beneficial in treatment of hypertension and heart diseases.
- Arachidonic acid is an important precursor of a group of biologically active compounds (Eicosanoids) known as prostaglandins and related compounds.
Alcohols:
Alcohols found in lipids are glycerol, cholesterol and higher alcohols (e.g.; cetyl alcohol found in waxes). Glycerol is colourless, odourless, sweet liquid which is miscible with water.
Compound LipidsI-
Phospholipids:
Phospholipids or phosphatides are compound lipids containing phosphate radicals, they include:-
1- Phosphatidic acid (Diacylglycerol phosphate):-
Consists of glycerol which is esterified with saturated F.A. at position (1) and unsaturated F.A at position (2) and phosphoric acid at position (3).
:sm255:
2- Cardiolipin:
Consists of 2 molecules of phosphatidic acid connected together by phosphate groups through one molecule of glycerol.
:sm255:
3- Lecithin (Phosphatidyl choline):-
It consists of phosphatidic acid, the phosphate of which is esterified with nitrogenous base (choline). By hydrolysis lecithin produce glycerol + saturated F.A. + unsaturated F.A. + choline base.
4- Cephalin (Phosphatidyl ethanolamine):-
5- Lipositol (Phosphatidyl inositol): inositol attached to phosphatidic acid
6- Phosphatidyl serine:- It is a cephaline like phospholipids but amino acid serine rather than ethanolamine found
7- Lysophospholipids (Lysophosphatides):
If the fatty acid in position 2 of lecithin or cephalin was removed, we get lysolecithin and lysocephalin respectively.
:extra170:
8- Plasmalogens:-
The compounds constitute as much as 10% of the phospholipids of brain and muscles. They are similar to lecithin and cephalin but amino acid in position (1) is replaced by enol form of fatty aldehyde in ether linkage instead of ester linkage of F.A.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] 9- Sphingomylein:
Differ from all other types of phospholipids in that they contain amino alcohol sphingosine instead of glycerol. A fatty acid usually saturated or monounsaturated is attached in amide linkage to the amino group of sphingosine forming ceramide. A phosphate is esterified to the one position of sphingosine and choline is esterified to the phosphate. Sphingomyleins are found in large quantities in brain and nerve tissues.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Distribution of phospholipids:-
Lecithins, cephalins, plasmalogens and sphingomyleins are found in almost all tissues, being essential components of cell membranes and in the blood plasma. High concentrations are found in nerve tissue, egg yolk and liver. In addition, sphingomyleins is found in the lungs, plasmalogens in muscles and cardiolipin in the heart. Phosphatidic acid is formed as intermediate in the biosynthesis of phospholipids and triglycerides, but not found in tissues to any sufficient extent. Lysolecithin is found in very small quantities in the blood plasma.
II- Glycolipids:
These are compound lipids containing a carbohydrate radical. They also contain sphingosine and therefore they can be classified with sphingomyleine as sphingolipids.
Glycolipids include:-
1- Cerebrosides:- these consist of sphingosine and galactose or rarely glucose. The F.A connected to the amino group of sphingosine in amide linkage and the sugar is connected to the primary alcohol group of sphingosine in glucosidic linkage.
2- Sulfolipids (sulfatides):- These consist of cerebrosides in which the galactose is esterified at C3 with sulfuric acid. They are found in the brain.
3- Gangliosides:- Therse are similar to cerebrosides but contain a more complex carbohydrate radical and F.a is usually stearic or oleic acids. They are also found in the brain.
III- Proteolipids:-
This is a group of lipid surrounds the protein, making the molecule insoluble in water and soluble in fat solvents. They are found in the brain.
Derived Lipids
Substances include the following classes:-
1- Fatty acids
2- Alcohols:- - Glycerol, sometimes not included, being miscible with water
- Long straight chain alcohols such as myristyl alcohols found in bees wax
- Sterols such as cholesterol and ergosterol.
- Vitamins A & D are alcohols.
- Sphingosine.
3- Substances associated with lipids in nature:- such as
- Squalene: which is C30 polyunsaturated hydrocarbon found in the liver and in olive oil in high concentrations. It is one of the intermediate in the biosynthesis of cholesterol.
- Steroids: which include sterols, bile acids and steroid hormones.
- Carotenoids: include carotenes and xanthophylls.
- Vitamins E and K which are fat soluble vitamins.
Steroids
Steroids are derivatives of the C17 cyclopentanoperhydro-phenanthrene ring (steroid nucleus).
Steroids may be classified into 3 groups, sterols, bile acids and steroid hormones.
Sterols:- these are steroid alcohols, they contain –OH group C3 and methyl groups at C10 & C13 in addition to side chain at C17.
Sterols includes:-
1- Phytosterols which are present in higher plants, the most important one is sitosterol.
2- Mycosterols: are present in fungi, the most important is ergosterol, which found in yeast and can be converted into vitamin D by irradiation with U.V. light.
3- Zoosterols: which present in animals, the most important is cholesterol.
- Cholesterol is reduced in the body into dihydroxycholesterol (cholestanol) and reduced also by intestinal bacteria into coprostanol.
- Oxidation of cholesterol occurs in the liver, intestine and skin forming 7-dehydrocholesterol, which is converted into vitamin D by exposure to U.V.
- Cholesterol is synthesized from acetyl-CoA. It is the precursor of bile acids and all steroid hormones.
- Bile acids are conjugated with glycine or taurine and are secreted in bile in the form of their sodium salts.
- Bile salts are important in digestion and absorption of lipids.
Eicosanoids
Eicosanoids are compounds derived from polyunsaturated fatty acid that contain 20 carbon atoms (Arachidonic acid) and include:
1- Prostanoids:- such as prostaglandins, prostacyclins and thromboxan.
2- Leukotriens: present in leukocytes
- Prostaglandins:
They are synthesized from arachidonic acid and possess a 5-membered ring and contain 20 carbon atoms. It has protective function in GIT thus prevent peptic ulcer, also can be used in treatment of bronchial asthma and induction of labour.
- Prostacyclins:
It have vasodilator effect and inhibitory effects on blood clotting
- Thromboxan:
Stimulate platelet aggregation and blood clotting.
Definition:-
Lipids are heterogeneous group of compounds related to fatty acids and include fats, oils, waxes and related compounds.
Function of lipids:-
1- High source of energy when stored in adipose tissues
2- Acts as medium for fat soluble vitamins
3- It serves as insulating material in the subcutaneous tissues
4- The fat contents of the nerve tissues acts as pad for the nerve
Classification of lipids:
A)-Simple lipids: these are esters of fatty acids with alcohols. According to the alcohol, they are subclassified into:-
1- Neutral fats: these are esters of fatty acids with glycerol. They are also called triglycerides or triacylglycerols.
- Neutral fats are either fats or oils.
- Fats are solid triacylglycerols due to high content of saturated fatty acids.
- Oils are liquid triacylglycerols due to high contents of unsaturated fatty acids.
- Glycerol is trihydric alcohol can be esterified with one, two or three fatty acids forming monoglycerides, diglycerides or triglycerides respectively.
2- Waxes:- These are esters of fatty acids with alcohol higher than glycerol.
B)- Compound lipids:-
These are esters of fatty acids with alcohols containing other groups and according to these groups they are subclassified into:-
1- Phospholipids:- these contain in addition to fatty acids and alcohol, a phosphate radical. They also have nitrogen containing bases. The alcohol of phospholipids is usually glycerol.
2- Glycolipids:- contain in addition to fatty acids and alcohol, a carbohydrate radical.
3– Proteolipids:- contain protein radical in addition to fatty acids and alcohols.
C)- Derived lipids:-
These include substances obtained by the hydrolysis of the above groups. They also include substances associated with them in nature and related to them in properties and metabolism.
Fatty Acids
- Fatty acids (F.A.) are momocarboxylic acids obtained from the hydrolysis of fats.
- They are either saturated (containing no double bonds) or unsaturated (containing one or more double bonds).
- F.A. usually containing an even number of carbon atoms (because they are synthesized from two carbon units).
- In F.A. the carbon atoms are numbered from the carboxylic end, which is known as carbon No. 1
- The carbon atom adjacent to the carboxyl group is known as carbon No. 2 or α-carbon, and the next carbon as carbon 3 or β- carbon and the next carbon 4 or γ-carbon, …etc.
- The methyl carbon at the other end of F.A. is known as omega carbon (ω-carbon).
Saturated fatty acids:-
Saturated fatty acids do not contain any double bonds or other functional groups along the chain. The term "saturated" refers to hydrogen, in that all carbons (apart from the carboxylic acid [-COOH] group) contain as many hydrogens as possible. In other words, the omega (ω) end contains 3 hydrogens (CH3-) and each carbon within the chain contains 2 hydrogens (-CH2-). The first member of saturated F.A. is acetic acid and the most common being palmitic acid and stearic acid.
Some saturated fatty acids are:
• Butyric: CH3(CH2)2COOH
• Lauric: (dodecanoic acid): CH3(CH2)10COOH
• Myristic: (tetradecanoic acid): CH3(CH2)12COOH
• Palmitic: (hexadecanoic acid): CH3(CH2)14COOH
• Stearic: (octadecanoic acid): CH3(CH2)16COOH
• Arachidic: (eicosanoic acid): CH3(CH2)18COOH
Unsaturated F.A.:-
These contain double bonds; they can be sub-divided according to the number of unsaturation into:-
A- Monoethenoid (Monounsaturated F. A.):-
These contain one double bond:
e.g.:- Palmitoleic acid (C16):
CH3 - (CH2)5 – HC9 = 10CH - (CH2)7 - COOH
- Oleic acid (C18):
CH3 - (CH2)7 - HC9 = 10CH - (CH2)7 – COOH
B- Polyethenoid (Polyunsaturated) F. A.:- These contain two or more double bonds:-
1- Tow double bonds: e.g. Linoleic acid 18:2;9,12
18 carbon atoms; 2 double bonds (number of unsaturation), the first between carbon 9–10 and the other between 12–13.
2- Three double bonds: Linolenic acid 18:3;9,12,15.
3- Four double bond: e.g.: Arachidonic acid 20:4;5,8,11,14.
Arachidonic acid found in small quantity with linoleic acid and linolenic acid mainly in peanut oil.
:sm255:
Essential fatty acids:
- The above mentioned three polyunsaturated fatty acids (Linoleic, Linolenic and Arachidonic acids) can't be synthesized by the body at sufficient rates; therefore they must be supplied in our diet.
- They protect against atherosclerosis as they form esters with cholesterol.
- Vegetable oils as corn and olive oils are rich in essential fatty acids and are beneficial in treatment of hypertension and heart diseases.
- Arachidonic acid is an important precursor of a group of biologically active compounds (Eicosanoids) known as prostaglandins and related compounds.
Alcohols:
Alcohols found in lipids are glycerol, cholesterol and higher alcohols (e.g.; cetyl alcohol found in waxes). Glycerol is colourless, odourless, sweet liquid which is miscible with water.
Compound LipidsI-
Phospholipids:
Phospholipids or phosphatides are compound lipids containing phosphate radicals, they include:-
1- Phosphatidic acid (Diacylglycerol phosphate):-
Consists of glycerol which is esterified with saturated F.A. at position (1) and unsaturated F.A at position (2) and phosphoric acid at position (3).
:sm255:
2- Cardiolipin:
Consists of 2 molecules of phosphatidic acid connected together by phosphate groups through one molecule of glycerol.
:sm255:
3- Lecithin (Phosphatidyl choline):-
It consists of phosphatidic acid, the phosphate of which is esterified with nitrogenous base (choline). By hydrolysis lecithin produce glycerol + saturated F.A. + unsaturated F.A. + choline base.
4- Cephalin (Phosphatidyl ethanolamine):-
5- Lipositol (Phosphatidyl inositol): inositol attached to phosphatidic acid
6- Phosphatidyl serine:- It is a cephaline like phospholipids but amino acid serine rather than ethanolamine found
7- Lysophospholipids (Lysophosphatides):
If the fatty acid in position 2 of lecithin or cephalin was removed, we get lysolecithin and lysocephalin respectively.
:extra170:
8- Plasmalogens:-
The compounds constitute as much as 10% of the phospholipids of brain and muscles. They are similar to lecithin and cephalin but amino acid in position (1) is replaced by enol form of fatty aldehyde in ether linkage instead of ester linkage of F.A.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة] 9- Sphingomylein:
Differ from all other types of phospholipids in that they contain amino alcohol sphingosine instead of glycerol. A fatty acid usually saturated or monounsaturated is attached in amide linkage to the amino group of sphingosine forming ceramide. A phosphate is esterified to the one position of sphingosine and choline is esterified to the phosphate. Sphingomyleins are found in large quantities in brain and nerve tissues.
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
Distribution of phospholipids:-
Lecithins, cephalins, plasmalogens and sphingomyleins are found in almost all tissues, being essential components of cell membranes and in the blood plasma. High concentrations are found in nerve tissue, egg yolk and liver. In addition, sphingomyleins is found in the lungs, plasmalogens in muscles and cardiolipin in the heart. Phosphatidic acid is formed as intermediate in the biosynthesis of phospholipids and triglycerides, but not found in tissues to any sufficient extent. Lysolecithin is found in very small quantities in the blood plasma.
II- Glycolipids:
These are compound lipids containing a carbohydrate radical. They also contain sphingosine and therefore they can be classified with sphingomyleine as sphingolipids.
Glycolipids include:-
1- Cerebrosides:- these consist of sphingosine and galactose or rarely glucose. The F.A connected to the amino group of sphingosine in amide linkage and the sugar is connected to the primary alcohol group of sphingosine in glucosidic linkage.
2- Sulfolipids (sulfatides):- These consist of cerebrosides in which the galactose is esterified at C3 with sulfuric acid. They are found in the brain.
3- Gangliosides:- Therse are similar to cerebrosides but contain a more complex carbohydrate radical and F.a is usually stearic or oleic acids. They are also found in the brain.
III- Proteolipids:-
This is a group of lipid surrounds the protein, making the molecule insoluble in water and soluble in fat solvents. They are found in the brain.
Derived Lipids
Substances include the following classes:-
1- Fatty acids
2- Alcohols:- - Glycerol, sometimes not included, being miscible with water
- Long straight chain alcohols such as myristyl alcohols found in bees wax
- Sterols such as cholesterol and ergosterol.
- Vitamins A & D are alcohols.
- Sphingosine.
3- Substances associated with lipids in nature:- such as
- Squalene: which is C30 polyunsaturated hydrocarbon found in the liver and in olive oil in high concentrations. It is one of the intermediate in the biosynthesis of cholesterol.
- Steroids: which include sterols, bile acids and steroid hormones.
- Carotenoids: include carotenes and xanthophylls.
- Vitamins E and K which are fat soluble vitamins.
Steroids
Steroids are derivatives of the C17 cyclopentanoperhydro-phenanthrene ring (steroid nucleus).
Steroids may be classified into 3 groups, sterols, bile acids and steroid hormones.
Sterols:- these are steroid alcohols, they contain –OH group C3 and methyl groups at C10 & C13 in addition to side chain at C17.
Sterols includes:-
1- Phytosterols which are present in higher plants, the most important one is sitosterol.
2- Mycosterols: are present in fungi, the most important is ergosterol, which found in yeast and can be converted into vitamin D by irradiation with U.V. light.
3- Zoosterols: which present in animals, the most important is cholesterol.
- Cholesterol is reduced in the body into dihydroxycholesterol (cholestanol) and reduced also by intestinal bacteria into coprostanol.
- Oxidation of cholesterol occurs in the liver, intestine and skin forming 7-dehydrocholesterol, which is converted into vitamin D by exposure to U.V.
- Cholesterol is synthesized from acetyl-CoA. It is the precursor of bile acids and all steroid hormones.
- Bile acids are conjugated with glycine or taurine and are secreted in bile in the form of their sodium salts.
- Bile salts are important in digestion and absorption of lipids.
Eicosanoids
Eicosanoids are compounds derived from polyunsaturated fatty acid that contain 20 carbon atoms (Arachidonic acid) and include:
1- Prostanoids:- such as prostaglandins, prostacyclins and thromboxan.
2- Leukotriens: present in leukocytes
- Prostaglandins:
They are synthesized from arachidonic acid and possess a 5-membered ring and contain 20 carbon atoms. It has protective function in GIT thus prevent peptic ulcer, also can be used in treatment of bronchial asthma and induction of labour.
- Prostacyclins:
It have vasodilator effect and inhibitory effects on blood clotting
- Thromboxan:
Stimulate platelet aggregation and blood clotting.
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