Introduction to Lipids

What are Lipids ??

• Lipids are organic compounds which are related to fatty acids

• Lipids are insoluble in water & soluble in nonpolar solvents (fat solvents) such as alcohol, ether, benzene, acetone, chloroform

classification of lipids

Simple Lipids Conjugated Lipids Derived Lipids

Natural fats waxes•Phospholipids•Galactolipids•lipoproteins

Substances obtained by

hydrolysis of lipids

Simple lipidsNatural fats

or: (Triglycerides, Triacylglycerols)

• They are esters of 3 fatty acids with the tri-hydroxy alcohol glycerol

• They include two types: Solid at ordinary temperature: FATs Liquid at ordinary temperature: OILs

• Occurrence of neutral fat:1- in plants: e.g. oils of cotton seed, olive, lin seed , palm oil, coconut ….etc2- in animals: e.g. butter, lard etc 3- in marin oils: e.g. oils of sea animals as cod liver

Chemistry of Triacylglycerols

Triacylglycerols are esters resulting from the reaction of fatty acids & glycerol

CH2- OH R1-COOH CH2-C-R1 I -3 H2O I CH2 – OH + R2-COOH CH - C - R2 I I CH2 – OH R3-COOH CH- C- R3

GLYCEROL + 3 molecules of fatty acids Triacylglycerol

O

O

O

Chemistry of Fatty Acids

• They are monocarboxylic acids (R- COOH) with: 1- Hydrocarbon chain (R): hydrophobic 2- Carboxyl group (–COOH ): that ionize to –COO- (hydrophilic) so, Fatty acid is an amphipathic molecule i.e. it have both hydrophilic & hydrophobic regions on the same molecule

1- Saturated Fatty Acids: contain even number of carbon atoms with no double bonds general formula Cn H2n+1 COOH important examples:

Butyric acid: (4C): CH3 - CH2 - CH2 - COOH 4:0 Palmitic acid (16 C): CH3 - (CH2)14 – COOH 16:0 Stearic acid (18C): CH3 - (CH2)16 - COOH 18:0

Classification of Fatty Acids in Nature (according to saturation)

2-

2- Unsaturated Fatty Acids:

They contain even number of carbon atoms and one or more double bonds

The most important:

Oleic acid (18C) : contains one double bond between C9 & C10 in the chain

CH3-(CH2)7-CH=CH-(CH2)7-COOH 18:1(9)

Linoleic acid (18 C): contains two double bonds between C9 & C10 and C12 & C13

CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COOH 18:2(9,12)

Linolenic acid (18 C): contains three double bonds between C9-10 , C12-13 & C15-16

CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH-(CH2)7-COOH 18: 3 (9,12,15), w3 ( from the methyl carbon end)

Classification of Fatty Acids according to saturation (cont.)

Essential fatty acids: can NOT be synthesized in human body. So, MUST be supplied with food.

Two fatty acids are dietary essential in humans:

Linoleic acid: precursor of arachidonic acid which is important for prostaglandin synthesis

a-Linolenic acid:

precursor for other w3 (omega3) fatty acids which are important for growth and development

Essential Fatty Acids

- Fatty acids are highly water-insoluble - So, a fatty acid must be transported in blood in with a protein

1- Fatty Acid Esters > 90% of blood fatty acids are esters i.e. included in triacylglycerol, in cholesterol esters & in phospholipids that are transported in blood with lipoprotein particles

2- Free Fatty Acids (Unesterified) < 10 of blood fatty acids are free (unesterified) i.e. not bound to any other molecule They are transported in blood with albumin.

Transport of Fatty Acids in Blood

Phospholipids

PL are conjugated lipidsPL are ionic polar structure formed of an alcohol that is attached by

bridge to a back bone either diacylglycerol or sphinogosine

A

hydrophilic head

Long hydrophobic Tail

Phospholipids & Glycolipids

Cholesterol

• It is the major sterol in animal tissues • It is available in cells of the body and in blood • Most plasma cholesterol is in esterified form (bound with a fatty acid

attached to C-3).

• Free cholesterol & esterified cholesterol in blood is transported with a protein in liopoproteins

• Blood cholesterol level is normally 150-250 mg/dl

• High cholesterol levels in blood (HYPERCHOLESTEROLEMIA) may lead to ATHEROSCLEROSIS (i.e. deposition of cholesterol on inner walls of blood vessels)

Cholesterol (cont.)

• Sources of cholesterol: 1- Diet: in animal sources 2- Synthesis: by liver cells & others

• Excretion of cholesterol: cholesterol is excreted in bile BUT NOT excreted in urine

Cholesterol (cont.)

Functions of Cholesterol

1- Cholesterol enters in the structure of cell membrane, brain, liver & other

organs

2- Cholesterol is the precursor for vitamin D3

3- Cholesterol is the precursor of steroid hormones 4- Cholesterol is the source of bile acids & salts which are secreted with the bile juice & essential for the digestion of lipids in the intestine

Plasma Lipoproteins

Plasma lipoproteins are spherical macromolecular complexes of lipids and proteins (apoproteins, apolipoproteins) They include:• chylomicrons,• very low density lipoproteins (VLDL), • low density lipoproteins (LDL) • high density lipoproteins (HDL) • They differ in lipid & protein composition, in size, density & site of origin

• lipoproteins function to keep their component lipids soluble to transport them in plasma

lipoproteins are composed of lipids & proteins:

1 - a neutral lipid core of hydrophobic lipid (containing triacylglycerol and cholesterol esters) 2 - surrounded by a shell of amphipathic lipids (phospholipids & nonesterified cholesterol)

3 - Then, the outermost layer is amphipathic apolipoprotein (protein) • Amphipathic compounds are oriented so that their polar portions are exposed on

the surface of the lipoprotein thus making the particle soluble in aqueous solution

• The triacylglycerol and cholesterol carried by the lipoproteins are obtained either from the diet (exogenous source) or from de novo synthesis (endogenous source)

Structure of Plasma Lipoproteins

Structure of Plasma Lipoproteins

Size & Density of Plasma Lipoprotein Particles

1- Chylomicrons• lowest in density & largest size of all lipoproteins • contains the highest percentage of lipid & the lowest percentage of protein 2- VLDLs & LDLs • are denser, having higher ratios of protein to lipid than chylomicrons

3- HDL • the densest as it contains the highest level of protein to lipid in all lipoproteind

Plasma lipoproteins can be separated - on the basis of their electrophoretic mobility (lipoprotein electrophoresis)

Plasma Lipoprotein Electrophoresis

• Chylomicrons: They carry dietary lipids (TG & Cholesterol) & fat soluble vitamins to blood.

• Very low density lipoproteins (VLDL) is to carry lipid from the liver to tissues.

• Low density lipoproteins (LDL) transfers cholesterol from blood to tissues (originated from VLDL in blood).

• High density lipoproteins (HDL): HDL takes cholesterol from peripheral tissues and transfers it to liver.

Functions of plasma lipoproteins