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CHEM 3303.02 Biochemistry
Chapter 10:Lipids
Knowledge/ skillsEntire chapter- All except for most of
Lipids as signals, cofactors and pigments
Storage lipids Structural lipids in membranes Vitamins A and D are hormone precursors Vitamins E and K and the lipid quinones are
oxidation/reduction cofactors
General information Insoluble in water Principal energy storage ( fats and oils) Phospholipids and sterols are structural
elements of biological membranes Other as cofactors, electron carriers, light
absorbing pigments, hydrophobic anchors for proteins
Chaperones for membrane folding Emulsifying agents in digestion Many lipids play a passive role for energy
storage or as a barrier, some are active and play a role as messengers or signalling molecules such as hormones
Storage lipids Fatty acids-
Complete oxidation to CO2 and H2O Carboxylic acids with hydrocarbon
chains ranging from 4-36 carbons Chain can be branched/unbranched Chain can be saturated/ unsaturated May contain ring structures
naming Simplified
Chain length:number of double bonds Double bond position given by a Δ followed by
the number of the carbon which is participating in the double bond. Note that the carboxyl group carbon is C-1
E.g. 20:3(Δ3,9,12) denotes a carboxylic acid with 20 carbons and 3 double bonds between C-3 and C4; C-9 and C-10; C-12 and C-13
Most commonly occurring have even numbers with unbranched chains- results from the fact that in biological systems the chains are put together from two Carbon (acetate) units
Many double bonds are Δ9, 12,15 in the cis configuration
Almost never see alternating single and double bonds but separated by a methylene group
Trans fatty acids produced by fermentation in diary animals and hydrogenation of fish or vegetable oils- these increase the levels of LDL or bad cholesterol and decrease the levels of HDL or good cholesterol
Physical properties – solubility in water Largely determined by chain length and degree
of saturation of the hydrocarbon chain Longer fatty acyl chain- less soluble Fewer double bonds- less soluble Compare the solubility of lauric acid (12:0,
Mr200) 0.063mg/ml vs glucose (Mr 180) 1,100 mg/ml
Carboxylic acid group is polar and ionized at pH7 and accounts for the slight solubility of the shorter chain fatty acids in water.
Physical properties- melting point
Influenced by length and degree of unsaturation of the hydrocarbon chain At room temp the saturated fatty acids from 12:0 to
24:0 are waxy while the corresponding unsaturated fatty acids are oily liquids
Differences due to the degree of packing of the molecules
In fully saturated molecules, large degree of free rotation around the C-C backbone and very flexible and able to pack tightly into nearly crystalline arrays with atoms in contact all along their length
Unsaturated- cis double bond forces a kink in the hydrocarbon chain- result is that cannot pack as tightly as the fully saturated form. Overall van der Waal’s interactions less in unsaturated so less thermal energy to break bonds hence have lower melting points
In vertebrates, free fatty acids with a free carboxylate group are bound to a protein carrier called serum albumin
Fatty acid derivatives such as esters or amides lack the charged carboxylate group and are less soluble than free fatty acids.
Tri acyl glycerols
Composed of fatty acids linked to glycerol- by what linkage?
Ester linkage due to formation of a condensation bond
Simple if all acids are the same, e.g. tristerin
Most are mixed
Polar groups of both glycerol and fatty acid are linked so triacylglycerols are nonpolar, hydrophobic and insoluble
Lower specific gravity- float on water
Triacylglycerol function Energy storage in adipocytes as fat droplets Oils in plant seeds to provide energy and precursor
molecules for germination The carbon atoms of fatty acids are more reduced than
carbohydrates so that more than twice the energy (gram for gram) is yielded
Because the molecule is hydrophobic, do not need water of hydration associated with polysaccharides (2g per gram of polysaccharide)
Carbohydrates are a quick energy source and are readily soluble in water
Fats are better for long term storage Fats are good insulators against cold temperatures
especially in animals that live in the cold or hibernate Help with maintaining neutral buoyancy in diving
animals like whales
There are 4 adipocytes in this picture
Can you tell where they are? Storage lipids can make up more than 80% of the
adipocyte dry cell mass
The sperm whale Sperm whale’s head accounts for over 1/3 of its
body weight. About 90% made up of spermaceti organ that is filled with spermaceti oil. The oil helps the animal maintain neutral buoyancy at ocean depths where it hunts its food and is able to change density depending on temperature
Saturation of oils Many vegetable oils such as corn and olive oil
are composed of triacylglycerols with unsaturated fatty acids- liquid at room temp
Hydrogenation reduces some double bonds to single bonds and can give trans double bonds
Exposure of lipids to air results in oxidative cleavage of double bonds to produce aldehydes and shorter chain carboxylic acids that are more volatile and smell rancid
Waxes Esters of long chain (C14 to C36) saturated and
unsaturated fatty acids with long chain alcohols
Melting points (60ºC – 100ºC) Can be used as a metabolic fuel store Provide water repellent properties- help to waterproof Lubricate hair and skin Protect against parasites E.g. lanolin, beeswax, carnuba wax
Lipid types
This is an important overall picture of the lipids and how they are built
Lipids in membranes Membranes serve as a barrier to passage of
polar molecules and ions The molecules are amphipathic, one end is
hydrophobic and the other is hydrophilic The hydrophilic end interact with water on
either side of the membrane- can be a single OH or more complex as in the phospholipids, or sugars as in the glycolipids
The hydrophobic end interact with each other in a bilipid layer structure
Membrane lipids can make up 5 to 10% of the dry mass of cells
Glycerophospholipid Aka phosphoglyceraldehydes
Two fatty acids attached via ester linkage to C-1 and C-2 of glycerol
Polar or charged group attached via a phosphodiester linkage to C-3 with an alcohol
Di acyl-glycerol
Head group substituents
Ether lipids Alkyl ether lipid- saturated Plasmalogen- double bond between C-1 and C-2
An example of ester and ether linkage Molecular signal to stimulate platelet
aggregation
Galactolipids One or two galactose residues connected by
a glycosidic linkage to C-3 of a 1,2 diacylglycerol
Important in plant thylakoid membranes.
Sulfolipids Sulfonate group bears a negative charge
like a phosphate group
Glycerol dialkyl glycerol tetraether
Found in archaebacteria- live in extreme conditions of pH and temperature
Molecule is longer (32C) to make a membrane with a hydrophobic core and hydrophilic ends (glycerol)
Ether bond linkage more stable to hydrolysis than ester bond
Sphingolipids Polar head group but use the long chain amino
alcohol sphingosine instead of glycerol Has one long chain fatty acid Has a polar head group linked by a glycosidic
linkage or phosphodiester linkage
Breakdown of membrane lipids
Membrane lipids continuously made/broken
Specific enzymes attack each hydrolyzable bond
Defects in these enzymes lead to severe consequences (Tay-Sachs)
Diseases associated with problems in breaking down sphingolipids
Sterols- e.g cholesterol Have 4 fused Carbon
rings, three 6 carbon and one 5 carbon to form the steroid nucleus- rigid and planar
Present in eukaryotic cells
Cholesterol is ampiphatic
Are precursors for steroid hormones which regulate gene expression
Examples of sex hormones
Vitamin A, D, E and K Fat soluble vitamins All are isoprenoid compounds synthesized
by condensation of multiple isoprene units Vitamin A and D are hormone precursors
1,25-dihydroxycholecalciferol- a hormone that regulates calcium uptake in the intestine and Ca levels in kidney and bone; regulates gene expression to turn on synthesis of Ca 2+ binding protein
Vitamin A (retinol), hormone and visual pigment
Vitamin E- collective name for tocopherols Contain a substituted aromatic ring and long
isoprenoid side chain. Are hydrophobic and associate with
membranes, lipid deposits and lipoproteins Aromatic ring destroys free radicals to protect
fatty acids from being oxidized
Vitamin K Undergoes oxidation and reduction during
active prothrombin formation. Prothrombin splits the peptide bonds in
fibrinogen to fibrin an insoluble protein that holds blood clots together