Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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LIPIDS

Lipids are a class of biological molecules defined by low solubility in water and high solubility in

nonpolar solvents like ether and chloroform.

FUNCTIONS OF LIPIDS

1- Storage molecules for ENERGY (fats and oils)

o Can get lots of energy from a fat

o Stored in adipose tissue

2- Structural components of cellular membranes

3- Protective molecules (waxes)

4- Hormones and vitamins

5- Intracellular messengers

6- Pigments

7- Insulation

Classes of Lipids

In the year 1943 Bloor proposed the following classification of lipids based on their chemical

composition.

FATTY ACIDS (FA)

o composed of a long hydrocarbon chain (“tail”) and a terminal carboxyl group (or “head”)

o No hydrogen bonds form between the carboxylic acid functional group

Fatty Acids interact through HYDROPHOBIC INTERACTIONS

o By nature, fatty acids are AMPHIPATHIC – have both hydrophilic and hydrophobic parts

o Often have double bonds

• They are naturally occurring monocarboxylic acids that tend to have even numbers of

carbon atoms, and may be classified as:

•

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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– long-chain (C12 to C26)

– medium-chain (C8 and C10)

– short-chain (C4 and C6)

TYPES

Saturated fatty acids are long chain carboxylic acids and do not have double bonds.

Pack close together

Less fluid (FAs can’t move as freely)

Higher melting temperature because it takes more energy to break interactions

Likely to be solids at room temperature

Example: Arachidic acid, Palmitic acid, etc.

Unsaturated fatty acids have one or more double bonds between carbon atoms. The two

carbon atoms are bound to each other through double bonds and can occur in cis or trans

configuration. Double bonds are “cis” configuration cause a bend in the chain

Do NOT pack as closely

More fluid than saturated

Lower melting temperature than saturated

Likely to be liquid at room temperature

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Saturated Unsaturated

FATTY ACIDS

o Lipids can contain many different fatty acids

o Fatty acid chain length and degree of unsaturation affect:

Melting point of lipids

Fluidity of lipids

NOMENCLATURE OF FA’s

- Referred to as a system of numbers

- # of carbons: # double bonds ∆x, y, z (position of double bonds)

- For example: oleic acid 18:1∆9

18:1Δ9

=

polar end non-polar; hydrophobic

Palmitic acid (26% of human fat)

Oleic Acid (45% of human fat)

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Common Name Systematic Name Structural Formula

Butyric acid Butanoic acid CH3(CH2)2COOH

Caproic acid Hexanoic acid CH3(CH2)4COOH

Caprylic acid Octanoic acid CH3(CH2)6COOH

Capric acid Decanoic acid CH3(CH2)8COOH

Lauric acid Dodecanoic acid CH3(CH2)10COOH

Myristic acid Tetradecanoic acid CH3(CH2)12COOH

Palmitic acid Hexadecanoic acid CH3(CH2)14COOH

Stearic acid Octadecanoic acid CH3(CH2)16COOH

Arachidic acid Eicosanoic acid CH3(CH2)18COOH

Common name Chemical structure C:D =

Myristoleic acid CH3(CH2)3CH=CH(CH2)7COOH 14:1 cis-Δ9

Palmitoleic acid CH3(CH2)5CH=CH(CH2)7COOH 16:1 cis-Δ9

Sapienic acid CH3(CH2)8CH=CH(CH2)4COOH 16:1 cis-Δ6

Oleic acid CH3(CH2)7CH=CH(CH2)7COOH 18:1 cis-Δ9

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Elaidic acid CH3(CH2)7CH=CH(CH2)7COOH 18:1 trans-Δ9

Vaccenic acid CH3(CH2)5CH=CH(CH2)9COOH 18:1 trans-Δ11

Linoleic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH 18:2 cis,cis-

Δ9,Δ

12

Linoelaidic acid CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH 18:2 trans,trans-

Δ9,Δ

12

α-Linolenic acid CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH 18:3 cis,cis,cis-

Δ9,Δ

12,Δ

15

Arachidonic acid

CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOHNIS

T

20:4

cis,cis,cis,c

is-

Δ5Δ

8,Δ

11,Δ

1

4

Essential fatty acids are those that cannot be made through any chemical pathways, known

to happen in humans. They must be obtained from the diet. Linoleic acid and linolenic acid are

the essential fatty acids.

Non-essential fatty acids are those which are not necessary to be taken through diet, they

are synthesized through chemical pathways.

Simple Lipids or Homolipids

Simple lipids are the esters of fatty acids with various alcohols.

Fats and Oils (triglycerides and triacylglycerols) - These are esters of fatty acids with a

trihydroxy alcohol, glycerol. A fat is solid at ordinary room temperature and usually from

animals and do not contain unsaturated fatty acids, an oil is liquid and usually from plants and

Contain more unsaturated fatty acids.

Simple Triglycerides - Simple triglycerides are one in which three fatty acids are similar or

are of the same type. Example: Tristearin, Triolein.

Mixed Triglycerides are one in which the three fatty acids radicles are different from each

other. Example: distearo-olein, dioleo-palmitin.

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Waxes

Wax is a simple lipid which is an ester of a long-chain alcohol and a fatty acid. The alcohol

may contain from 12-32 carbon atoms. Waxes are found in nature as coatings on leaves and

stems. The wax prevents the plant from losing excessive amounts of water. . Example:

Beeswax, Carnaubawax.

WAXES

Wax Alcohol Fatty Acid

Carnuba CH3(CH2)28CH2-OH CH3(CH2)24COOH

Beeswax CH3(CH2)28CH2-OH CH3(CH2)14COOH

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Reactions of triglyceride

1- Hydrolysis

2- Saponification

Saponification number: is the number of milligrams of potassium hydroxide

required for the complete saponification of one gram of substance—called

also saponification value. It points to the degree of hydrocarbon chain length of lipid fatty

acids (negative relation).

3- Hydrogenation

It is the reaction by which the oil is converted to fat ( unsaturated fatty acids are converted

to saturated)

Triolein Tristearin

4- Halogenation

Unsaturated fatty acids in triglyceride accept halogen such as I2 at the double bonds.

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Iodine Number: is the number of grams of iodine or equivalent halogen consumed by 100

grams of the substance <the iodine numbers of linseed oil, olive oil, and coconut oil are

approximately 175–201, 77–91, and 8–9.5 respectively>

5- Formation of Acrolein (heat reaction)

6- Rancidity

rancidity, is the chemical decomposition of fats, oils and other lipids. When this process

occurs in food, undesirable odors and flavors can result. In some cases, however, the

flavors can be desirable (as in aged cheeses):

Hydrolytic rancidity

Hydrolytic rancidity occurs when water splits fatty acid chains away from the glycerol backbone

in triglycerides (fats). The chemical term is ester hydrolysis. Usually this hydrolysis process

goes unnoticed, since most fatty acids are odorless and tasteless. A particular problem arises

with butter, which contains triglycerides with a high content of butyric acid derivatives and

acetic acids(volatile short chain fatty acids).

Oxidative rancidity

Oxidative rancidity is associated with the degradation by oxygen in the air. Via a free radical

process, the double bonds of an unsaturated fatty acid can undergo cleavage, releasing

volatile aldehydes and ketones. This process can be suppressed by the exclusion of oxygen or

by the addition of antioxidants. Oxidation primarily occurs with unsaturated fats.

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Microbial rancidity

Microbial rancidity refers to a process in which microorganisms, such as bacteria, use their

enzymes such as lipases to break down fat. This pathway can be prevented by sterilization.

Compound Lipids or Heterolipids

Heterolipids are esters of fatty acids with alcohol and possess additional groups also. Phospholipids or Phosphatides are Very similar in structure to triacylglycerols except one of the alcohols of glycerol is esterified by phosphoric acid instead of a fatty acid = phosphatidic acid (PA). They usually possess one hydrophilic head and two non-polar tails. They are called polar lipids and are amphipathic in nature.

16-18 FAs most common Position 1 favors SATURATED FAs Position 2 favors UNSATURATED FAs

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Phospholipids are MUCH MORE amphiphilic than triacylglycerols due to CHARGED groups at neutral pH o Has both hydrophilic and hydrophobic regions phospholipids have One POLAR HEAD and TWO NON-POLAR TAILS

Phosphoglycerides are major phospholipids, they are found in membranes. It contains fatty

acid molecules which are esterified to hydroxyl groups of glycerol. The glycerol group also

forms an ester linkage with phosphoric acid. Example: Lecithin , Cephalins.

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Lecithin(Phosphotidyl Cholin) Cephalins (Phosphotidyl ethanol amine)

Phosphotidyl serine

Phosphoinositides are said to occur in phospholipids of brain tissue and soybeans. They

play important role in transport processes in cells.

Phosphotidyl inositol Phosphotidyl glycerol

Cardiolipin: Cardiolipin is known as the signature phospholipid of mitochondria. It is

responsible for a wide range of mitochondrial functions, including but not limited to ATP

synthesis

Plasmalogens: are a specific glycerophospholipid class containing a vinyl ether moiety at the

sn-1-position of the glycerol backbone. It make up 18% of phospholipids mass of human.

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Note: Phospholipids can be degraded to their component parts by a family of enzymes called PHOSPHOLIPASES.

Some snake poisons are phospholipase such as Phospholipase A2 that catalyzes the hydrolysis of fatty acids at the C2 Position that lead to formation of lysolecithin that is “one legged” phospholipids, acts as a detergent, and dissolved membranes in red blood cells causing them to rupture

Phosphosphingosides are commonly found in nerve tissue. Example: sphingomyelins.

Sphingosine unit

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Glycolipids are the compounds of fatty acids with carbohydrates and contain nitrogen but no

phosphoric acid. The glycolipids also include certain structurally related compounds

comprising the groups gangliosides, and cerebrosides.

Derived Lipids

Derived lipids are the substances derived from simple and compound lipids by hydrolysis.

These includes fatty acids, alcohols, monoglycerides and diglycerides, steroids, terpenes,

carotenoids.

The most common derived lipids are steroids, terpenes and carotenoids.

Steroids do not contain fatty acids, they are nonsaponifiable, and are not hydrolyzed on

heating. They are widely distributed in animals, where they are associated with physiological

processes. Example: Estranes, and rostranes, etc.

Function of Steroids

Glucose+ Oligosaccharides Gangliosides

β-galactocerebroside

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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-Source of some important biological molecules (Cholesterol, bile salts, vit. D, and sex

hormones)

- components of cell membranes.

- Energy storage

Bile acids

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Terpenes in majority are found in plants. Example: Natural rubber. Geranoil, Camphor, etc.

It is built up from isoprene, a hydrocarbon(diene) consisting of five carbon atoms attached to

eight hydrogen atoms (C5H8).

The term is often extended to the terpenoids, which are oxygenated derivatives of these

hydrocarbons.

The true terpenes are usually grouped according to the number of isoprene (C5H8) units in the

molecule: monoterpenes (C10H16) contain two such units;

sesquiterpenes (C15H24);

diterpenes (C20H32),

triterpenes (C30H48),

tetraterpenes (C40H64),.

Squalene is a triterpene by which cholesterol molecule is built up in liver.

Professor Dr. Raid M. H. Al-Salih Biochemistry / Lipids

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Carotenoids are tetraterpenes. They are widely distributed in both plants and animals. They

are exclusively of plant origin. Due to the presence of many conjugated double bonds, they are

colored red or yellow. Example: Lycopreene, carotenes, Xanthophylls.

Rubber is polyterpenes in which 1,000–5,000 isoprene units are joined in a long chain