Chemistry of Lipids

What is lipids? Lipids are a broad group of naturally occurring molecules  which includes fats, waxes,  sterols, fat-soluble vitamins, monoglycerides, diglycerides,  phospholipids, and others.

It is soluble in water. It is soluble in nonpolar organic solvents such as ether, acetone, CCl4. It contains C, H, O, sometimes N or K It yields fatty acids on hydrolysis.

What are the functions of lipids? The importance of lipids can be seen from their varied functions. They are:

What are the functions of lipids? As membrane structural components As intracellular storage depots of metabolic fuel As transport form of metabolic fuel As regulatory substances (some hormones) As protective form of the cell walls of many bacteria, of the leaves of higher plants, of the exoskeleton of insects and the skin of vertebrates

What are the functions of lipids? As enzymes cofactors (some vitamins) As transport form of some neurotransmitters (acetylcholine) As receptors in nerve ending membranes As determinants of immunological specificity As part of the outer coating between the body of the organism and the environment to prevent infection and excessive loss or gain of water

BIOMEDICAL IMPORTANCE Stored as a source of energy in the body. 1 gram of lipid contains 2.25 as much energy as 1 gram of Carbohydrate Lipid (TGL) Droplets In Adipose tissue

2. Structural components of biomembranes.

Provides insulation against changes in external temperature. 3. Thermal Insulator : Provides insulation against changes in external temperature.

4. Lipids act as electric insulators in neurons.

5. Act as metabolic regulators (steroid hormones and prostaglandins). 6. Act as surfactants and prevents collapse of lungs during expiration. 7. Lipids are used as detergents. 8. Lipids used in emulsification and intestinal absorption of non polar nutrients like fatty acids and fat soluble vitamins. 9. Associated with diseases such as atherosclerosis, diabetes mellitus and obesity. 10. Gives shape and contour to the body.

Lipid Classification Lipids Nonsaponifiable Saponifiable Steroids 17.1 Biological Functions of Lipids Steroids Prostaglandins Simple Complex Sphingolipids Phosphoglycerides Waxes Triglycerides

Classification Simple Lipids a) Fats and Oils b) Waxes 2. Compound Lipids a) Phospholipids b) Glycolipids 3. Derived Lipids a) Fatty acids b) Alcohol c) Sterols

Second Classification of Lipids Four Main Groups Fatty Acids Saturated Unsaturated Glycerides glycerol-containing lipids Nonglyceride lipids Sphingolipids Steroids Waxes Complex lipids lipoproteins 17.1 Biological Functions of Lipids

A Scheme to Classify Lipids 17.1 Biological Functions of Lipids

LIPIDS 1. Simple Lipids 2. Compound Lipids

Simple and Complex Lipids Simple: an ester-containing lipid with just two types of components An alcohol One or more fatty acids Complex: an ester-containing lipid with more than two components Fatty acids Plus others 17.1 Biological Functions of Lipids

1. Simple Lipids eg: triglycerides Esters of fatty acids with various alcohols. a. Fats :esters of fatty acids with glycerol. eg: triglycerides b. Waxes: esters of fatty acids with higher molecular weight monohydric alcohols. eg: beeswax

Triglycerides are most common 3 fatty acids attached to glycerol held together by ester linkage

Triglycerides are lipids with three fatty acids attached to a glycerol

© 2009 Cengage - Wadsworth

At 20°C, triglycerides can be classified as fats or oils Solid = fats Liquid = oil

Fats and oils are triglycerides that are formed by condensation reactions of propane- 1,2,3-triol (glycerol) and fatty acids (long-chain carboxylic acids).

Triglycerides are the commonest lipids in living organisms.

Chemical properties

Chemical Properties Triglycerides have typical ester and alkene chemical properties as they are composed of these two groups: Saponification: replace H with salt from a strong base Hydrolysis: produces the fatty acids and glycerol, a reverse of formation Hydrogenation: saturates the double bonds 17.3 Glycerides

Triglyceride Reactions Triglycerides undergo three basic reactions These reactions are identical to those studied in carboxylic acids Triglyceride H2, Ni H2O, H+ NaOH More saturated triglyceride Glycerol Fatty Acids Glycerol Fatty Acid Salts

Saponification reaction

Hydrolysis Chemical (Autocatalytic) Enzymatical (Lipase) +3H20 O C triacylglycerol = - R2 - R3 HC H2C OH glycerol HC H2C HO - C O - R1 3 fatty acids + - R3 - R2 +3H20

Hydrolysis reaction

Addition reaction Hydrogenation reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H2/metal catalyst.

Hydrogenation of Unsaturated Fats Oils can be converted to semi-solids through hydrogenation that converts the double bonds to single bonds In the process, some double bonds are converted to trans form

Estimation of Iodine Value of Fats and Oils Fatty acids react with a halogen [ iodine]  resulting  in the addition of the halogen at the C=C double bond site.  In this reaction, iodine monochloride  reacts with the unsaturated bonds to produce a di-halogenated single bond, of which one carbon has bound an atom of iodine. 

Estimation of Iodine Value of Fats and Oils

Oxidation of Oils and Fats

Oxidation of Oils and Fats

2. Compound Lipids Schematic diagram of simple and complex lipids.

Simple lipid + Additional group = = Compound Lipids Esters of fatty acids with various alcohols along with an additional group. Simple lipid + Additional group = = Compound Lipids a. Phospholipids: b. Glycolipids: c. Other Complex Lipids:

COMPOUND LIPIDS Esters of fatty acids with various alcohols along with an additional group. Phospholipids Glycolipids Others Glycerphospholipids -Phosphatidyl choline - Phosphatidyl ethanolamine - Phosphatidyl serine - Phosphatidyl inositol - Cardiolipin - Plasmalogens Sphingophospholipids Lipoproteins Aminolipids Cerebrosides Gangliosides Globosides

Phospho lipids

SIGNIFICANCE OF PHOSPHOLIPIDS Structural components of cell membrane. Enable enzyme action (mitochondrial enzyme system). Required for blood coagulation (prothrombin to thrombin, activation of factor 8 by factor 9).

SIGNIFICANCE OF PHOSPHOLIPIDS Transports lipids from intestine and liver. Choline acts as a lipotropic agent since it prevents the formation of fatty liver. Phospholipids of myelin sheath provides insulation around nerve fibers.

PHOSPHOLIPIDS Phospholipids are similar in structure to triglycerides but one of the fatty acid groups is replaced with a phosphate group, and are hence known as diglycerides.

PHOSPHOLIPIDS

PHOSPHOLIPIDS The phosphate group is ionized and negatively charged, therefore water molecules are attracted to this polar part of the molecule, this end is then hydrophilic and soluble in water. The two fatty acid chains are insoluble and nonpolar.

In cells, these phopholipids occur with proteins and cholesterol in a bilayer in the form of a cell membrane. Bilayers occur when the hydrophobic tails line up with one another with the hydrophilic heads exposed

Membranes Complex lipids form the membranes around cells and small structures within cells. In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers. Polar (hydrophilic) head groups are in contact with the aqueous environment. Nonpolar (hydrophobic) tails are buried within the bilayer The arrangement of hydrocarbon tails in the interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids).

Phosphatidyl Phosphatidyl Glycerol

Phosphatidyl Choline (LECITHIN) Phosphatidic acid + Choline Most abundant phospholipid of biomembranes

Phosphatidyl Choline (LECITHIN) Dipalmitoyl Lecithin – Lung surfactant. Synthesized by Alveolar type - II cells. Rich in alveolar fluid lining the alveoli Reduces surface tension of alveolar fluid & prevents collapse during expiration Deficiency in premature infants leads to collapse - Respiratory Distress syndrome

Below is Lecithin, or phosphatidycholine, a major component of biological membranes and can be isolated from egg yolk or soybeans Focus on the “tails” and “heads” which form a membrane

Lipids in Membranes: Fluid Mosaic Model Polar heads and nonpolar tails Double layer of lipids amphipathic

GLYCOLIPIDS Lipids containing carbohydrate moiety - Glycolipids Alcoholic component – Sphingosine Ceramide – Common group of all Glycolipids Occur in brain, spinal cords and other tissues. Predominant in outer leaflet of biomembranes

Types of Glycolipids 1. Cerebrosides 2. Sulfatides 3. Globosides 4. Gangliosides

STEROIDS Lipids containing Cyclo Pentano Perhydro Phenanthrene (CPPP) ring 18 12 17 11 16 13 D C 19 1 15 9 14 2 10 8 A B 3 7 5 4 6

STEROLS Steroids containing alcoholic group - Sterols Plant Sterols : Ergosterol, Stigmasterol, Sitosterol Animal Sterols : Cholesterol

STEROIDS lipids often found in cell membrane (ex cholesterol) consist of 4 fused hydrocarbon rings and several different functional groups

Double bond between 5th & 6th carbons CHOLESTEROL Chemistry: Has Steroid Nucleus OH group at 3rd position Double bond between 5th & 6th carbons 8-Carbon containing side chain at 17th position. 17 3 5 6

CHOLESTEROL Chemical Properties 1. Undergoes rapid oxidation to form cholestenones. 2. Hydroxyl group forms esters with acids to form Cholesterol Esters (cholesterol acetate, palmitate and propionates) 3. Presence of double bond gives hydrogenation reactions (similar to unsaturated fatty acids). 4. Colour reactions: LIEBERMANN BURCHARD, SALKOWSKY, ZAKS.

Biomedical Importance CHOLESTEROL Biomedical Importance Structural role – Biomembranes, Lipoproteins Occur in large amounts in brain and nerve tissues. Act as insulator against nerve impulses which discharge electric charges. Biomolecules synthesized from cholesterol Bile acids Vitamin D Steroid hormones : Androgens, Estrogens, Progesterone, Aldosterone etc