Lipids chemistry

• Definition:-

• Lipids are a heterogeneous group of water-

insoluble (hydrophobic) organic molecules.

• They can be extracted from tissues by non

polar solvents such as ether, chloroform,

benzene.

Classification of lipids:-

• 1) Simple lipids:-

• 1-Fats (esters of fatty acids with glycerol).

• 2-Waxes(esters of fatty acids with higher

molecular weight monohydric alcohols).

• 2)Complex lipids:-

• 1-Phospholipids:-

• *Glycerophospholipids

• *Sphingophospholipids.

• 2-Glycolipids.

• 3-Other complex lipids:-

• *Sulfolipids.

• *Aminolipids.

• *lipoproteins.

• 3)Precursor or derived lipids:-

• 1-Fatty acids.

• 2-Glycerol.

• 3-Steroids.

• 4-Hydrocarbons.

• 5-lipids-soluble vitamins.

• 6-Hormones.

• 7-Ketone bodies.

Fatty acids:-

• *Roles in the body:-

• 1-as the components of more complex

membrane lipids.

• 2- as the major components of stored fat in the

form of triacylglycerol.

• *Fatty acids are straight aliphatic chains with a

methyl group at one end a carboxyl group at

other end.

• *Most fatty acids in the human have an even

number of carbon atoms(16-20).they may be

saturated or Un-saturated.

• *Saturated fatty acid:-

• -Straight chains,even number of carbon

atoms,in soluble in water.

• Examples:-

Fatty Acids

Saturated

Formula Common Name Melting Point

CH3(CH2)10CO2H lauric acid 45 ºC

CH3(CH2)12CO2H myristic acid 55 ºC

CH3(CH2)14CO2H palmitic acid 63 ºC

CH3(CH2)16CO2H stearic acid 69 ºC

CH3(CH2)18CO2H arachidic acid 76 ºC

Unsaturated

Formula Common Name Melting Point

CH3(CH2)5CH=CH(CH2)

7CO2H palmitoleic acid 0 ºC

CH3(CH2)7CH=CH(CH2)

7CO2H oleic acid 13 ºC

CH3(CH2)4CH=CHCH2C

H=CH(CH2)7CO2H linoleic acid -5 ºC

CH3CH2CH=CHCH2CH

=CHCH2CH=CH(CH2)7C

O2H

linolenic acid -11 ºC

CH3(CH2)4(CH=CHCH2)

4(CH2)2CO2H arachidonic acid -49 ºC

• Un-saturated fatty acid:-

• -Are containing in their hydrocarbon skeleton one or more

double bonds.

• -Monounsaturated fatty acids contain one double bonds&

polyunsaturated acids contain two or more double bonds.

• Ex:-

• Oleic acid:-18:1∆9,:-or 18:1(9):-

• 18 indicates the number of carbon atoms,1 indicates the

number of double bonds,indicate the position of double

bonds.

• Arachidonic acid:-20:4,∆⁵̓̓̓ ⁸ ´¹¹ ¹4 :-or 20:4(5,8,11,14):-

• -The double bonds in most naturally occurring fatty

acids are in the cis configuration(cis means that the

acyl chains are on the same side of the double bonds

• Melting point of fatty acids:-

• -The melting point of fatty acids increases with chain

length and decreases with the degree of

unsaturation(shorter chain length and increasing

unsaturation result in fatty acids of lower melting point)

• *The fatty acid composition of the membrane phospholipids

determines the fluidity of membranes at body temperature.

• Essential fatty acids:-linoleic,linolenic and arachidonic

acids(polyunsaturated), they cannot synthesized by the

animal and must therefore be provided in the diet.

• Acylglycerols:-

• Fatty acids react with alcohol(hydroxyl group) to form

esters.the three hydroxyl groups of glycerol can react with

fatty acids to form monoacylglycerols,diacylglycerol and

triacylglycerol(TG),which contain 1,2 or 3 fatty acids

esterified to glycerol, respectively.

• Phosphoacylglycerols:-

• -contain fatty acids esterified to position 1 and 2 of glycerol

and aphosphoryl group at position 3.if only a phosphate group

is attached to position 3 ,the compound is known as a

phosphatidic acid,phosphatidic acid are intermediates in the

synthesis of triacylglycerols and phosphoacylglycerols.

• -compounds derived from phosphatidic acid include the most

common

phosphoacylglycerols:phosphatidylcholine(lecithin),phosphati

• dylethanolamine(cephalin), and phosphatidylserine. These

compounds are major components of cellular

membranes.removal of a fatty acid from these compounds

produces a lysophosphoacylglycerol, e.g., lysolecithin.

• -phosphatidylinositol is another important membrane

component.itis phosphoryled and cleaved in response to

extracellular signals to form diacylglycerol and inositol

triphosphate, which serves as intracellular signals known as

second messengers.

Functions of phospholipids:-

• 1-As structural components of membranes of cell surface and sub cellular organelles.

• 2-As activator of certain enzymes . for example,β-hydroxy butyrate dehydrogenase has an absolute requirement for lecithin .also lipoprotein lipase is activated by phospholipids.

• 3-As surfactant: dipalmitoyl lecithin constitutes more than80% of the phospholipids in the extracellular liquid layer that line alveoli of normal lung . it is called surfactant due to its powerful capacity to decrease the surface tension of the aqueous surface layar of the lung,it prevents atelectasis at the end of expiration phase of breathing.

• 4-As detergents:phospholipids and especially

lecithin play an important role in bile where they

function to solubilize cholesterol.impairment in

phospholipids production and secretion into bile

can result in the formation of cholesterol and bile

pigment gallstones.

• 5-Phosphatidylinositol and lecithin serve as

donors of arachidonic acid for synthesis of

eicosanoids and related compounds.

• Sphingolipids:-

• *Serve in intercellular communication and as the

antigenic determinants of the ABO blood

group.they are composed of a polar head group

and two non polar tails.

• *The core of sphingolipids is sphingosine, which

is synthesized from palmitoyl CoA and serine.

• *Amino acylation ,with along chain fatty acid, at

carbon 2 of sphingosine yields a ceramide.

• *The sphingolipids includes:-

• 1-sphingomyelins:-

• *are the only sphingolipids that are phospholipids.

• *contain palmitic or stearic acid N-acylated at carbon 2 of sphingosine.

• 2-Glycosphingolipids(glycolipids):-

• *Are composed of a ceramide backbone with carbohydrate (mono or oligosaccharides) attached to carbon 1 of sphingosine.

• *The four classes of glycolipids:-

• 1)Cerebrosides:

• Cereamide+Glucose =glucocerebrosides

• Cereamide+Galactose=galactocerebrosides.

• 2)Sulfatides:

• *Sulfuric acid esters of galactocerebrosides.

• *Cereamide+glucose+sulfat group=sulfatides.

• *Brain.

• 3)Globosides:

• Cereamide+glucose , galactose+N-acetylglactosamine

• *Lactosyl cereamide is found in erythrocyte plasma membranes.

• *liver,RBCs & spleen.

• 4)Gangliosides:

• *Similar to globosides except that they also contain N-acetylneuraminic acid (NANA).

• Plasmalogens(ether glycerolipids):-

• *Are glycerolether phospholipids.

• *They are two types:

• 1)alkyl ether(-O-CH₂-). • 2)Alkenyl ether(-O-CH=CH-).

• *Three major classes:choline,ethanolamine &serine plasmalogens.

• *Ex:platelet activating factor(PAF):is one of choline alkyl ether plasmalogen.

• PAF functions as a mediator of hypersensitivity , acute inflammatory reactions, produces responses in liver, heart, smooth muscle &uterine and lug tissues.

• Eicosanoids:

• *Are agroup of hormone-like compounds produced by many cells in the body.

• *Synthesized from polyunsaturated fatty acids containing20 carbon atoms(eicosanoic acids) with 3 ,4 ,5 double bonds.

• *Groups:prostaglandins,thromboxanes and leukotrienes.

• Steroids:

• *Are compounds contain astructure with four ring(steroid nucleus).

• *Ex:Cholesterol ,Ergosterol.