UNIT 5. UNIT 5.

LIPIDSLIPIDS

5.1. Introduction.

5.2. Fatty acids.

5.3. Eicosanoids.

5.4. Triacylglycerols = Triglycerides.

5.5. Waxes.

5.6. Membrane lipids: glycerophospholipids and sphingolipids.

5.7. Isoprenoids (and steroids)

5.8. Lipoproteins.

OUTLINEOUTLINE

5.1. INTRODUCTION:

• Organic molecules highly hydrophobic and water insoluble.

• Biological roles:

- Energy reserve (i.e. triacylglicerols).

- Structural component of the biological membranes

(i.e. phospholipids).

- Protection (i.e. wax).

- Biochemical signals, cofactor or pigments • Lipid Classification:

- Fatty acids. - Phospholipids.

- Eicosanoids. - Sphingolipids.

- Triacylglicerols. - Isoprenoids.

- Waxes. - Conjugated lipids.

• Carboxylic acids with long-chain hydrocarbon side groups.

• They can be branched and saturated, unsaturated or polyunsaturated (contain

two or more double bonds).

• The double bonds tend to occur at every third carbon atom (not conjugated).

• Ionizated at physiological pH.

• Usually, they are in the free, uncomplexed state.

5.2. FATTY ACIDS:

conjugatedNot conjugated

5.2. FATTY ACIDS:

• Fatty acid double bonds almost always have the cis configuration.

5.2. FATTY ACIDS:

Polar group

Unsaturated acid:

Linoleic acidSaturated fatty acid:

Stearic acid

Hyd

roph

obic

cha

in

• Properties:

Higher unsaturated grade: higher fluidity.

Higher unsaturated grade (same chain length): lower melting

point.

5.2. FATTY ACIDS:

• They are derived from arachidonic acid, 20:4(5,8,11,14).

• Types:

Prostaglandins (they act as local hormones): they are involved in

- The production of pain and fever

- Inflammation processes

- Increase of the body temperature

- Regulation of blood pressure and blood coagulation.

- Control of the Ionic transport.

- Dream induction

5.3. EICOSANOIDS:

Thromboxanes (found within platelets):

Stimulate vasoconstriction and platelet aggregation.

Leukotrienes (present within leukocytes):

Involved in:

- Oedema production.

- Vasoconstriction.

- Bronchi constriction

- Anaphylactic processes.

5.3. EICOSANOIDS:

• Glycerol esterified with three fatty acids. Monoacylglicerols

and diacylglicerols are metabolic intermediates.

• Major energy reserve (stored in anhydrous form).

• Types:

Simple triacylglicerols (i.e. tristearoylglycerol(= tristearin);

trioleoylglycerol (=triolein).

Mixed triacylglicerols (i.e. 1-stearoyl, 2-linoleoyl, 3-

palmitoyl glycerol).

• Highly hydrophobic.

• Adipocytes and adipose tissue.

• Thermal insulation (important for warm-blooded aquatic

animals).

5.4. TRIACYLGLICEROLS:

• Provide about six times the

metabolic energy of an equal

weight of hydrated glycogen. • They are hydrolysed by lipases or

alkali (saponification):

Chemical composition of

three eatable fats.

trio

lein

a

trie

stea

rina

5.4. TRIACYLGLICEROLS:

NaOH or KOH

Soap

GLYCEROL

Fatty acid

Fatty acid

Fatty acid

GLYCEROL

Fatty acid + Na+

Fatty acid + Na+

Fatty acid + Na+

+

+

+

Triacontanylpalmitate, major component of the bee wax.

• Esters of long-chain monohydroxylic alcohols (16-30 C) with long-

chain fatty acids (14-36 C) (saturated or unsaturated)

• Properties:

- Energy reserve (plancton).

- Water-repellant surfaces (i.e. animals skin, leaves of

certain plants, bird feathers).

- Highly hydrophobic.

- Several industrial uses (lotions, cosmetics, etc.).

5.5. WAXES:

Amphipathic lipids

5.6. MEMBRANE LIPIDS:

• 1,2-diacylglycerol that has a phosphate group esterified at the carbon

atom 3 of the glycerol backbone. The phosphate group is linked to a

highly polar or charged group (X).• Usually they contain a saturated fatty acid (C16 or C18)C1-linked and

an unsaturated fatty acid (C16-C20) C2-linked.• Amphipathic molecules.• They are derived from phosphatidic acid.

Glycerophopholipids backbone structure

5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS:

5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS:

(Lecitina)

(Cefalina)

Phosphatidylcholine

• Some phospholipids contain an alkyl group ether linked.

choline

5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS:

Plasmalogen (heart muscle)

Platelet activating factor: major mediator of hypersensivity, acute inflammatory reactions, allergic responses and anaphylactic shock.

• One polar group and to unpolar chains (glycerol is not present)

They contain the amino alcohol sphingosine, one fatty acid

(long chain) and a polar group (alcohol or sugar)

Polar group

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS

Phospholipids

Glycolipids

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS

• Cerebrosides and globosides (ceramide oligosaccharides) are neutral

glycolipids. Gangliosides contain N-acetylneuraminic acid (sialic acid), with

negative charge at pH 7.

• The carbohydrate present in several sphingolipids define the

human being blood groups ( A, B and 0).

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS

• They contain isoprene units (5 C).

5.7. ISOPRENOIDS:

• They precursor for the synthesis is

isopentenylpyrophosphate .• The isoprenoids are grouped into

terpenes and steriods.

• Classification in the base of the number of isoprene units

5.7. ISOPRENOIDS: TERPENES

• Linear or cyclic structures.

• Present in plants, fungi and bacteria.

• Pigments, molecular signalling (hormones and pheromones) and

defence agents.

• Plant oils belong to this groups (aromas and flavours).

• They are precursor for fat-soluble vitamins synthesis.

5.7. ISOPRENOIDS: TERPENES

• Fat-soluble vitamins: they cannot be synthesised by human beings (diet).

• They are dissolved as fats and oils.

Vitamin A (retinol):

- Hormone and main pigment involved in vision.

- It is a carotenoid.

Vitamin D:

- It is derived from cholesterol.

- It increases the concentration of Ca2+ in physiological serum

Vitamin E (tocopherols):

- Biological antioxidant, It avoid food degradation.

- Prevent oxidation of the membrane lipids.

5.7. ISOPRENOIDS: TERPENES

5.7. ISOPRENOIDS: TERPENES

Vitamin K:

- Blood clotting.

Ubiquinones and plastoquinones:

- Lipophilic electron transporters (redox reactions).

Vitamin E

Vitamin K1

Ubiquinone

• Membrane lipids in eukaryotic cells.

• They are derived from triterpenes containing 4 rings: phenathrene

nucleus (3 rings containing 6 C), and a pentane ring (5 C):

Cyclopentanoperhydrophenanthrene.

• They are classified on the base of the number and position of the

double bonds, location of the radicals, etc.

5.7. ISOPRENOIDS: STERIODS:

5.7. ISOPRENOIDS: STERIODS:

Cholesterol: Main steriod in animals (cellular membranes) It is

amphipathic. It is the precursor in the synthesis of many steriods such as

hormones, vitamin D, bile acids

5.7. ISOPRENOIDS: STERIODS:Bile acids: they act as detergents in gut. They are more soluble

than cholesterol.

Steroid hormones: they promote metabolism and gene expression changes. Types:

- Glucocorticoids: i.e: cortisol: involved in proteins, lipids and carbohydrates metabolism.

- Aldosterone and mineralocorticoids:, they regulate water and salt excretion (kidneys).

- Androgens and estrogens: sexual development.

5.7. ISOPRENOIDS: STERIODS:

• Molecular complexes in blood plasma of mammals.

• They facilitate lipids transfer between tissues (triacylglycerols,

phospholipids and cholesterol).

• Apolipoproteins or apoproteins are synthesised in the liver.

• They have different density and lipids/proteins ratio.

5.8. LIPOPROTEINS:

5.8. LIPOPROTEINS:

- Chylomicrons: extremely low density. They transport

triacylglycerols and cholesterol esters (food) from gut to adipose tissue

and muscles.

- Very-low-density lipoproteins (VLDL): they transfer lipids

(triacylglycerols) to the tissues to be used for energy.

- Low-density lipoproteins (LDL): they transport cholesterol and

its esters to the tissues.

- High-density lipoproteins (HDL): Rich in cholesterol but poor in

triacyglycerol (they promote the excretion of the excess of cholesterol).

5.8. LIPOPROTEINS: