L.Noha Soliman

LIPIDS

Biochemistry(BMS 233)

2

OBJECTIVES

• Definition of lipids.• Functions of lipids.• Fatty acids

• Classifications of fatty acids.• Chemical reactions of fatty

acids.• Classification of lipids.

• Simple lipids.• Complex lipids.• Precursor and derived lipids.

Lipids are complex molecules composed of carbon , hydrogen , and oxygen.

Lipids are insoluble in water and soluble in nonpolar solvents.

Most lipids are non-polar and are hydrophobic because they contain hydrocarbon chains.

Definition

Functions of lipids

A.The Functions of Fat in the Body:1. Energy source esp. for muscles.

2. Serves as an energy reserve.

Lipid (TGL)Droplets InAdipose tissue

Functions of lipids

3. Cell Membrane Structural components

4. Cushion the vital organs to protect them from shock.

Functions of lipids

5. Thermal Insulator :

Subcutaneous layer of fat reduces heat loss in cold weather (helps maintain body temperature).

Functions of lipids

B. The Functions of Fat in Food:1.Provide calories (9 kcal per gram).

2.Provide satiety.

3.Carry Lipid-soluble vitamins (A,D,E, and K) & essential fatty acids.

4.Contribute aroma & flavor.

Adipose cells swollen with fat ! ! ! ! !

Humans can grow more fat cells, but can never get rid of them!!

As we gain and lose weight, they just swell and shrink.

…Makes you think twice about those French fries!!!

Bad Things About Fat

• Makes you fat!

• Heart disease, high blood pressure, cancer.

Fatty acids

Are long-chain monocarboxylic acids.

Generally contain an even number of carbon atoms.

General formula is CH3 (CH2)n COOH.

O

R C OH

#1 Carbon Acid Group

Classification of fatty acids

• Length of the Carbon Chain– Long-chain, medium-chain, short-chain

• Degree of Unsaturation– Saturated, Unsaturated (Mono, Poly)

• Location of Double Bonds– Omega-3 Fatty Acid, Omega-6 Fatty Acid

Classification of fatty acids

The Length of the Carbon Chain

Short-chain Fatty Acid

(l (less than 6 carbons)

s than Medium-chain Fatty Acid (6-10 carbons)

Long-chain Fatty Acid

(12 or more carbons)

Fat saturation

Saturation means “full”, so what could they be full of?

Full of H atoms! And wherever there is a chemical bond, there is stored energy.

General Formula: CH3 (CH2)n COOH.

Each C being “saturated” with H.

Contain only single C–C bonds.

long, straight chain.

Properties of saturated fatty acids

Properties of saturated fatty acids

Closely packed.

Strong attractions between

chains.

High melting points.

Solids at room temperature.

Properties of saturated fatty acids

Animal foods such as meat,

poultry and full-fat dairy

products.

Tropical oils such as palm and

coconut.

Properties of saturated fatty acids

Contributes to cardiovascular disease.

Properties of unsaturated fatty acids

Contain one or more double C=C bonds.

The kinks made by double bonded C prevent the molecules from packing tightly together.

Few interactions between chains.

Properties of unsaturated fatty acids

Low melting points

Liquids at room temperature

Plant & fish fats Vegetable oils

saturated unsaturated

Saturated VS Unsaturated

Fatty acids may be further subdivided as follows

1. Monounsaturated FA

2. Polyunsaturated FA

3. Eicosanoids

Fatty acids may be further subdivided as follows

1. Monounsaturated Fatty Acids (MUFA)

Containing one double bond.

Oleic (most common FA) 18:1;9

Fatty acids may be further subdivided as follows

2. Polyunsaturated Fatty Acids (PUFA)Containing two or more double bonds

Linoleic (corn, peanut, cottonseed) 18:2;9,12

– Are identified by position of the double bond nearest the methyl end (CH3) of the carbon chain; this is described as an omega number.

– If PUFA has first double bond 3 carbons away from the methyl end = omega 3 FA

– 6 carbons from methyl end = omega 6 FA

Polyunsaturated Fatty acids

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ω3, ω6, ω9 Fatty Acids

Cis-Fatty Acid

H’s on same side of the double bond; fold into a U-like formation; naturally occurring

Trans-Fatty Acid

H’s on the opposite side of the double bond; occur in partially hydrogenated food

Cis &Trans Fatty acids

Cis - Fatty acids

Trans - Fatty acids

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Essential & nonessential fatty acids

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Essential fatty acids

linoleic acid and linolenic acid are important for synthesizing arachidonic acid (20 C atoms)

Arachidonic acid in turn is the precursor for the synthesis of eicosanoids

Prostaglandins

Leukotrienes

Thromboxanes

Arachidonic acid

Chemical reactions of fatty acids

Esterification – Fatty acids react with alcohols to form esters and water

Acid Hydrolysis

Producing fatty acids from esters.Opposite of esterification.

Chemical reactions of fatty acids

Saponification:Alkaline hydrolysis produces glycerol and salts of fatty acids

(soaps).

CH 2 O

C HO

CH 2

C

C

O C

R1

R3

R2

O

O

OH2C OH

C HHO

H2C OH

ON aCR1

O

ON aCR3

O

+ ON aCR2

O

Triacylglycerol Glycerol Sodium salts of fatty acids (soap)

3 NaOH

Hydrogenation• Used in the food industry to convert polyunsaturated

vegetable oils into saturated solid fats.

Partial hydrogenation• Carried out to add hydrogen to some, but not all, double

bonds in polyunsaturated oils• In this way liquid vegetable oils are converted into solid

form (Crisco and margarine)

Reactions at the double bond of unsaturated fatty acids

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1. Simple Lipids

2. Complex Lipids

3.Precursor & Derived Lipids

Lipids

Classification of lipids

Simple lipids

Esters of fatty acids and alcohol. (trihydric or monohydric alcohol).

CH 2R

Fatty alcoholOH C R

Fatty acidHO

O

+

H2O

CH 2R O C R

O

Esterase (lipase) ester (lipid)

Simple lipids

FAT and OIL

Fats :esters of fatty acids with glycerol.

Ex. triglycerides.

Oils : are fats in liquid state.

Triglycerides

Triglycerides are the main dietary fats in human body.

It is a naturally occurring ester of three fatty acids and glycerol that is the chief constituent of fats and oils.

Glycerol forms the “backbone” of the fat.

Triglycerides

Fat VS Oil

Fat: • Is solid at room temperature.

• Is prevalent in meats, whole milk, butter, and cheese.

Oil:• Contain high proportion of

unsaturated fatty acids and saturated fatty acids.

• Is liquid at room temperature.

• Is prevalent in plants such as olive and safflower.

corn oil contains 86% unsaturated fatty acids and 14% saturated fatty acids

Waxes

Esters of long-chain fatty acids with higher molecular weight monohydric alcohols.

Has a weakly polar head group (ester linkage) and non-polar tails (hydrocarbon chain).

Fatty acids found in waxes are usually saturated

ex. stearic acid.

Alcohols may be saturated or unsaturated and may include sterols ex. cholesterol.

Complex lipids

Esters of fatty acids with various alcohols along with an additional group.

Simple lipid + Additional group = Complex Lipids

a. Phospholipids: Glycerophospholipids , Sphingophospholipids.

b.Glycolipids

c.Other Complex Lipids: Lipoproteins ,aminolipids and

sulfolipids.

Phospholipids

Glycerophospholipids (Phosphoglycerides): They are phosphate esters.

Structure: Glycerol + 2 Fatty acids + PO4 + polar molecule ( x).

Phospholipid structure

Phospholipid structure

It is an amphipathic molecule:• Fatty acid tails = hydrophobic• Polar region (glycerol + carbonyl o of fatty acids + PO4 + x)= hydrophilic • dual “personality”

interaction with H2O is complex & very important!

It likes water & also pushes it away!

Phospholipid behavior

Because of their bipolar nature , when placed in water phospholipids orient themselves in small spheres or “bubbles” with their nonpolar (hydrophobic) regions oriented away from water and their polar (hydrophilic) regions exposed to water.

These structure are called micelles and are similar in structure the cell membrane which is composed in part of a phospholipid bilayer.

Phospholipid & cells

• Phospholipids of cell membrane

double layer = bilayer

hydrophilic heads on outside in contact with aqueous solution outside of cell and inside of cell

hydrophobic tails on inside form core forms barrier between cell & external environment

Phospholipids

Sphingophospholipids (Sphingolipids): Are not glycerides no glycerol in them

However they are similar to phospholipids

These lipids are based on sphingosine:Long-chain Nitrogen-containing Alcohol

They can be modified by adding different “polar head” groups and non-polar fatty acid “tails”(via amide bond to N atom).

Fatty acids attach here

“polar head” groups

attach here

Phospholipids

Sphingophospholipids (Sphingolipids):

Phospholipids

Sphingophospholipids (Sphingolipids): Amphipathic, like phospholipids

Polar head group and Two non-polar fatty acid tail Structural component of cellular membranes.

Major categories:

Sphingomyelins Structural lipid of nerve cell membranes.Myelin sheath feature.

Glycosphingolipids Also important components of muscle & nerve membranes.

Phospholipids

Sphingophospholipids (Sphingolipids):

Precursor and derived lipids

They are compounds produced when simple and complex lipids undergo hydrolysis.

Fatty acids

Glycerol

Steroids

hormones

Vitamins (A,D,E,K)

Steroids are cyclic hydrocarbons usually composed of four rings.

A steroid nucleus consists of : three cyclohexane rings and one cyclopentane ring.

The rings are fused (4 fused C rings) different steroids created by attaching different functional groups to rings.

ex: cholesterol, sex hormones and vitamins.

Steroids

Cholesterol

It is the most common steroid! It is the building block for steroid hormones and also functions in cell membrane structure.

It is an amphipathic molecule (like phospholipids):• -OH group polar• Fused rings non-polar

High levels in blood may contribute to cardiovascular disease.

Importance of cholesterol in the body

1. Cell membrane component– Its fused ring portion is readily soluble in the hydrophobic region

of membrane– Polar -OH group sticks out of membrane

helps keep cell membranes fluid & flexible

Importance of cholesterol in the body

2-Precursor to bile salts: Bile salts made in liver but stored in gall bladder. Are emulsifying agents that aid in lipid digestion.

3-Vitamin D synthesis:

Importance of cholesterol in the body

3-Vitamin D synthesis:

Importance of cholesterol in the body

4. Male and female sex hormones

O

CH3

CH3

C

CH3

O

progesterone