Lipids

Presented by

MICHAEL KANHAI

lipid

an oily organic compound insoluble in water but soluble in organic solvents; essential structural component of living cells (along with proteins and carbohydrates)wordnetweb.princeton.edu/perl/webwn

Classification of Lipids

Lipids

Complex lipids

Non-glyceride

lipids

GlyceridesFatty acids

Fatty Acids Long straight-chain carboxylic acids

• no branching• Most common chains range from 10–20 C in length• Usually, an even number of carbons in the chain,

including the carboxyl carbon

General formula is CH3(CH2)nCOOH, where n= even integer

Can be saturated or unsaturated, but usually no other functional groups present

Saturated Fatty Acidshave no double bondsMost common saturated fatty acids in nature are:

• Stearic acid

• Palmitic acid

Stearic acid

Unsaturated Fatty AcidsAn unsaturated fatty acid has one or more C=C

bonds in the chainC

O

OCH2CH2

C C

CH2

CH2

CH2

CH2

CH2

CH2

HH

CH2

CH2

CH2

CH2

CH3

Palmitoleic acid

– The 1st double bond is usually at the 9th carbon

– The double bond is normally in a cis configuration

– Monounsaturated one C=C– Polyunsaturated more than

one C=C bond

Oleic Acid, 18:1(9)is the most common unsaturated fatty acid in nature

Naming system for fatty acids There are four common naming systems; three of them attempt

to denote the chain length and the number and positions of any double bonds.     

         The first two columns show systems based on complete names,

and the last two columns show systems for denoting compounds with abbreviations.

Numbering system for fatty acids

The carboxyl-reference system indicates• the number of carbons,

• the number of double bonds,

• the positions of the double bonds, counting from the carboxyl carbon (which is numbered 1, as in the IUPAC system).

• E.g. oleic acid is: cis 18:1 9

fatty acid has 18 C atoms ( including the carboxylate C i.e. COOH)

1 double bond at C-9 Orientation around double bond is cis

The omega-reference system

• indicates the number of carbons,

• the number of double bonds

• and the position of the double bond closest to the omega carbon, (the CH3 end)

Numbering system for fatty acids

•This system is useful in physiological considerations because of the important physiological differences between omega - 3 and omega - 6 fatty acids, and the impossibility of interchanging them in the human body.

Fatty Acid PropertiesMelting point increases with increasing carbon number

Melting point of a saturated fatty acid is higher than an unsaturated fatty acid with the same number of carbons• they always form straight, rigid chains• they have no kinks and are able to pack tightly• solid at room temperature

Double bonds lower melting point relative to saturated acid• cis double bonds give them a kink along their chain length• prevent good alignment of molecules in unsaturated fatty acids

leading to poor packing• Have lower melting temperatures• Liquid at room temperature

Essential Fatty Acids

Body can make most fatty acids from carbohydrate, protein and other fats• Any fatty acid that cannot be synthesized by the body

is called an essential fatty acid • Must be obtained from the diet

Exceptions: linoleic acid and linolenic acid• Polyunsaturated fatty acids that must be obtained

from diet• Cannot be made from other materials• Cells cannot convert one into the other

Essential Fatty Acids

Sources of linoleic acid:• Leafy vegetables, nuts. Vegetable oils (seasame, corn oil,

sunflower, soybean), poultry fat

Sources o f linolenic acid:• Nuts and seeds (soybean, walnuts, flaxseed), oils (soybean,

canola)

Both play an important role in the health of the heart• Reduce plasma triglycerides

• Reduce blood clotting

• Reduce inflammatory responses in the body

Essential Fatty Acidslinoleic 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

COO-

arachadonic acid

Biological Processes Regulated by Eicosanoids

Prostaglandins• Mediate aspects of inflammatory response e.g. pain

and fever

• Stimulation of smooth muscle during labour (PGE2)

• Inhibit gastric secretion

• Increase secretion of protective mucus in GI tract

• Inhibition of hormone-sensitive lipases in GI tract

• Dilate renal blood vessels Results in increased water and electrolyte excretion

Biological Processes Regulated by Eicosanoids

Leukotrienes• promote the constriction of bronchi

Thromboxane • stimulates constriction of blood vessels and platelet

aggregation

Triglycerides A triglyceride (TG) places fatty acid chains at each alcohol

group of the glycerol Principal function is storage of energy Excess energy-rich nutrients stored as TG in adipose tissue When energy needed TGs broken down energy released

Fatty acid chains

Glycerolpart

Phosphoglycerides Phospholipid is a more general term

• Any lipid containing phosphate group

Phosphoglycerides contain: • Glycerol

• Fatty acid

• Phosphoric acid with an amino alcohol

Replace an fatty acid on C-3 of a TG with phosphoric acid

Because the phosphate group can ionize in solution charged lipid

Glycerol

Fatty Acid

Fatty Acid

Phosphoric Acid

Alcohol

PO42- (phosphate

group)

Phosphoglycerides

Have hydrophobic and hydrophilic domains• Polar “heads” and non-polar “tails”

Structural components of membranes

Emulsifying agents

Suspended in water, they spontaneously rearrange into ordered structures• Hydrophobic group to center

• Hydrophilic group to water

• Basis of membrane structure

Basic composition of a phospholipid. X can be a number of different substituents

Types of Phosphoglycerides

The phospho-amino-alcohol is highly hydrophilic

They are used in:• Cell membranes • Emulsifying • Micelle-forming agents in the

blood

Two types• Ones made with choline are

called lecithin• Those made with either

ethanolamine or serine are called cephalins

Phosphoglycerides are amphipathic have both polar and non-polar regions

Representation of a phospholipid

Nonglyceride LipidsSphingolipids 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

Sphingolipids

Amphipathic, like phospholipids• Polar head group• 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

Nonglyceride Lipids - SteroidsContain the steroid nucleus

• A collection of 4 fused carbon rings

• Many steroids have methyl groups attached to C-10 and C-13 as well as alkyl, alcohol or ketone groups attached to C-17

SteroidsCholesterol is a common steroid

CH CH2 CH2CH2CH(CH3)2

OH

H

CH3 H

CH3

H H

H

CH3

Cholesterol

It is an amphipathic molecule (like phospholidpids)• -OH group polar

• Fused rings non-polar

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

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

4. Male and female sex hormones

• Through a series of reactions cholesterol can be converted to progesterone

During pregnancy progesterone suppresses further ovulation

Maintains lining of uterus to accept fertilized egg

• Progesterone can be chemically modified to give testosterone and estradiol

Involved in development of secondary sexual characteristics

O

CH3

CH3

C

CH3

O

progesterone

O

CH3

CH3

OH

testosterone

Importance of cholesterol in the body

5. Adrenal hormones• E.g. cortisol (& derivative cortisone)

enhances carbohydrate metabolism by increasing glucose and glycogen in the body

• E.g. aldosterone secreted form adrenal cortex when sodium ion levels are low in blood

• Aldosterone cause the kidneys to maximally reabsorb water and sodium ions and return to them blood.

• When sodium levels high aldosterone not secreted and so sodium ions are filtered by kidney and lost in urine

WaxesEsters of long-chain alcohols with

long-chain fatty acidsHas a weakly polar head group

(ester linkage) and non-polar tails (hydrocarbon chain)• Insoluble in water• Therefore confers water-repellant to

skin, bird feathers, leaves

Fatty acids found in waxes are usually saturated e.g. stearic acid

Alcohols may be saturated or unsaturated and may include sterols e.g. cholesterol

WaxesLinolin (woolwax) -obtained from the wool

of sheep during the cleaning or refining process • Base for pharmaceutical & cosmetic products

because it is rapidly assimilated by human skin

Spermaceti or sperm whale oil• was once in great demand as a lubricant but now is

prescribed.

• Many marine animals from invertebrates to whales contain appreciable amounts of waxes Have a variety of functions in fish, from serving as an

energy source to insulation, buoyancy and even echo location.

Biological Functions of Lipids

As an energy source, lipids provide 9 kcal of energy/gram

Triglycerides provide energy storage in adipocytes

Phosphoglycerides, sphingolipids, and steroids are structural components of cell membranes

Steroid hormones are critical intercellular messengers• See functions of cholesterol

Emulsifiers

Uses of fats in the body

Provide padding• Adipose tissue pads our body and

protects organs e.g. liver & kidney

Provide insulation• Fat under skin insulates our body

to help us retain heat

Enable the transport of fat-soluble vitamins (A,D, E, into cells of small intestine

References

wordnetweb.princeton.edu/perl/webwnwww.chemguide.co.ukhttp://www.mcgraw-hill.com/http://www.chemcases.com/olestra/olestra05.htm

The end

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