LECTURES 25 – 27
The fatty acids are hydrocarbon derivatives, at the same low oxidation state as
the hydrocarbons in fossil fuels. The cellular oxidation of fatty acids (to CO2 and
H2O), like the controlled, rapid burning of fossil fuels in internal combustion
engines is highly exergonic.
They are carboxylic acids with hydrocarbon chains ranging from 4-36 carbons
long. In some fatty acids, this chain is unbranched and fully saturated (contains
no double bonds); in others the chain contains one or more double bonds. A few
contain three-carbon rings, hydroxyl groups, or methyl-group branches.
The physical properties of the fatty acids, and of compounds that contain them,
are largely determined by the length and degree of unsaturation of the
hydrocarbon chain. The nonpolar hydrocarbon chain accounts for the poor
solubility of fatty acids in water.
Melting points are strongly influenced by the length and degree of unsaturation
of the hydrocarbon chain. The difference in melting points is due to different
degrees of packing of the fatty acid molecules.
In the fully saturated compounds, free rotation around each carbon-carbon bond
gives the hydrocarbon chain great flexibility; the most stable conformation is the
fully extended form, in which the steric hindrance of neighbouring atoms is
minimized. These molecules can pack together tightly in nearly crystalline arrays,
with atoms all along their lengths in van der Waals contact with the atoms of
In unsaturated fatty acids, a cis double bond forces a kink in the hydrocarbon
chain. Fatty acids with one or several such kinks cannot pack together as tightly
as fully saturated fatty acids, and their interactions with each other are therefore
weaker. Because less thermal energy is needed to disorder these poorly ordered
arrays of unsaturated fatty acids, they have markedly lower melting points than
saturated fatty acids of the same chain length.
Polyunsaturated fatty acids (PUFAs):
The family of polyunsaturated fatty acids with a double bond between the third
and fourth carbon from the methyl end of the chain are of special importance in
human nutrition. Because the physiological role of PUFAs is related more to the
position of the first double bond near the methyl end of the chain than to the
carboxyl end, an alternative nomenclature is sometimes used for these fatty
They are the simplest lipids constructed from fatty acids. Triacylglycerols are
composed of three fatty acids each in ester linkage with a single glycerol. Those
containing the same kind of fatty acid in all three positions are called simple
triacylglycerols and are named after the fatty acid they contain.
Because the polar hydroxyls of glycerol and the polar carboxylates of the fatty
acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic
molecules, essentially insoluble in water. Lipids have lower specific gravities
than water, which explains why mixtures of oil and water have two phases: oil,
with the lower specific gravity, floats on the aqueous phase.
There are two significant advantages to using triacylglycerols as stored fuels,
rather than polysaccharides such as glycogen and starch. First, the carbon atoms
of fatty acids are more reduced than those of sugars, and oxidation of
triacylglycerols yields more than twice as much energy as the oxidation of
Second, becayse triacylglycerols are hydrophobic and therefore unhydrated, the
organism that carries fat as fuel does not have to carry the extra weight of water
of hydration that is associated with stored polysaccharides.
The hydrophobic moieties in these amphipathic compounds may be as simple as
a single –OH group at one end of the sterol ring system, or they may be much
more complex. In glycerophospholipids and some sphingolipids, a polar head
group is joinsed to the hydrophobic moiety by a phosphodiester linkage; these
are the phospholipids. Other sphingolipids lack phosphate but have a simple
sugar or complex oligosaccharide at their polar ends; these are the glycolipids.
They are membrane lipids in which two fatty acids are attached in ester linkage
to the first and second carbons of glycerol, and a highly polar or charged group is
attached through a phosphodiester linkage to the third carbon. Glycerol is
prochiral; it has no asymmetric carbons, but attachment of phosphate at one end
converts it into a chiral compound, which can be correctly named either L-
glycerol 3-phosphate, D-glycerol 1-phosphate, or sn-glycerol 3-phosphate.
Glycerophospholipids are named as derivatives of the parent compound,
phosphatidic acid. Sphingolipids:
They have a polar head group and two nonpolar tails, but unlike
glycerophospholipids and galactolipids, they contain no glycerol. Sphingoipids
are composed of one molecule of the long-chain amino alcohol sphingosine or
one of its derivatives, one molecule of a long-chain fatty acid, and a polar head
group that is joined by a glycosidic linkage in some cases and a phosphodiester
Carbons C-1, C-2 and C-3 of the sphingosine moelcules are structurally analogous
to the three carbons of glycerol in glycerophospholipids. When a fatty acid is
attached in amide linkage to the –NH2 on C-2, the resulting compound is
ceramide, which is structurally similar to a diacylglycerol. Ceramide is the
structural parent of all sphingolipids.
Phosphatidylinositols act at several levels to regulate cell structure and
metabolism. Phosphatidylinositols 4,5-bisphosphate in the cytoplasmic (inner)
face of plasma membranes serves as a reservoir of messenger molecules that are
released inside the cell in response to extracellular signals interacting with
specific surface receptors.
Extracellular signals suc