Download 5.6. membrane lipids

UNIT 5.
LIPIDS
OUTLINE
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.
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.
5.2. FATTY ACIDS:
• 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).
conjugated
Not conjugated
• Ionizated at physiological pH.
• Usually, they are in the free, uncomplexed state.
5.2. FATTY ACIDS:
5.2. FATTY ACIDS:
• Fatty acid double bonds almost always have the cis configuration.
Hydrophobic chain
Polar group
Saturated fatty acid:
Stearic acid
Unsaturated acid:
Linoleic acid
5.2. FATTY ACIDS:
• Properties:
 Higher unsaturated grade: higher fluidity.
 Higher unsaturated grade (same chain length): lower melting point.
5.3. EICOSANOIDS:
• 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.
5.4. TRIACYLGLICEROLS:
• 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, 3palmitoyl 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
triestearina
weight of hydrated glycogen.
• They are hydrolysed by lipases or
G
L
Y
C
E
R
O
L
Fatty acid
Fatty acid
Fatty acid
G
L
NaOH or KOH Y
C
E
R
O
L
trioleina
alkali (saponification):
+
Fatty acid + Na+
+
Fatty acid + Na+
+
Fatty acid + Na+
Soap
Chemical composition of
three eatable fats.
5.5. WAXES:
• 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.).
Triacontanylpalmitate, major component of the bee wax.
5.6. MEMBRANE LIPIDS:
Amphipathic lipids
5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS:
• 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:
(Cefalina)
(Lecitina)
Phosphatidylcholine
5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS:
• Some phospholipids contain an alkyl group ether linked.
choline
Platelet activating factor: major mediator of hypersensivity, acute
inflammatory reactions, allergic responses and anaphylactic shock.
Plasmalogen (heart muscle)
5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS
• 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
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.
5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS
• The carbohydrate present in several sphingolipids define the human
being blood groups ( A, B and 0).
5.7. ISOPRENOIDS:
• They contain isoprene units (5 C).
• They precursor for the synthesis is
isopentenylpyrophosphate .
• The isoprenoids are grouped into
terpenes and steriods.
5.7. ISOPRENOIDS: TERPENES
• 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
Vitamin K:
- Blood clotting.
Ubiquinones and plastoquinones:
- Lipophilic electron transporters (redox reactions).
Vitamin E
Vitamin K1
Ubiquinone
5.7. ISOPRENOIDS: STERIODS:
• 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:
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:
5.8. LIPOPROTEINS:
• 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:
- 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: