lipids
•  from the greek word lipos for “fat.”
•  biomolecules that are soluble in organic solvents.
•  C-C and C-H bonds makes them very soluble in
organic solvents and insoluble in water.
lipids have many C-C and C-H bonds, but there is no
one functional group common to all lipids.
Lipids can be categorized as hydrolyzable or
nonhydrolyzable
Hydrolyzable lipids can be cleaved into smaller molecules by
hydrolysis with water.
Nonhydrolyzable lipids cannot be cleaved into smaller units by
aqueous hydrolysis.
Waxes
•  The simplest hydrolyzable lipids.
•  Esters (RCOOR') formed from a high molecular weight
alcohol (R'OH) and a fatty acid (RCOOH).
waxes are very hydrophobic. Triacylglycerols (triglycerides) •  the most abundant lipids •  triesters that produce glycerol and three molecules of fa9y acid upon hydrolysis. •  Simple triacylglycerols are composed of three identical
fatty acid side chains
•  mixed triacylglycerols have two or three different fatty
acids.
What are the characteristics of these fatty acids?
• All fatty acid chains are unbranched, but they may be
saturated or unsaturated.
• Naturally occurring fatty acids have an even number of
carbon atoms.
• Double bonds in naturally occurring fatty acids generally
have the Z configuration.
• The melting point of a fatty acid depends on the degree of
unsaturation.
Fats and oils are triacylglycerols (triesters of glycerol and
these fatty acids).
• Fats have higher melting points, making them solids at
room temperature.
• Oils have lower melting points, making them liquids at
room temperature.
The Most Common Fatty Acids in
Triacylglycerols
Three saturated side chains lie parallel to each
other, making a compact lipid.
One Z double bond in a fatty acid side
chain produces a twist so the lipid is
no longer so compact. Fatty Acid Composition of Some Fats and Oils
Hydrolysis of triacylglycerols
Hydrolysis of a triacylglycerol with water in the presence
of either acid, base, or an enzyme yields glycerol and three
fatty acids.
Hydrogenation of unsaturated fatty acids
Oxidation of unsaturated fatty acids
In the cell, the principal function of
triacylglycerols is energy storage.
Phospholipids
•  Hydrolyzable lipids that contain a phosphorus atom.
•  Two common types: phosphoacylglycerols and sphingomyelins.
•  Found almost exclusively in the cell membranes of plants and
animals
The phosphorus atom in a phosphodiester shares 10 electrons. Thirdrow elements (such as P and S) can be surrounded by more than eight
electrons.
Phosphoacylglycerols
•  Phosphoacylglycerols (or phosphoglycerides) are the second
most abundant type of lipid.
•  They form the principal lipid component of most cell
membranes.
There are two prominent types of phosphoacylglycerols. They
differ in the identity of the R’’ group in the phosphodiester.
• When R'' = CH2CH2NH3+, the compound is called a
phosphatidylethanolamine or cephalin.
• When R'' = CH2CH2N(CH3)3+, the compound is called
a phosphatidylcholine, or lecithin.
A phosphoacylglycerol has two distinct regions: two
nonpolar tails due to the long-chain fatty acids, and a
very polar head from the charged phosphodiester.
Sphingomyelins
Sphingomyelins, the second major class of phospholipids, are
derivatives of the amino alcohol sphingosine, in much the same way that
triacylglycerols and phosphoacylglycerols are derivatives of glycerol.
• A phosphodiester at C1.
• An amide formed with a fatty acid at C2.
Fat-Soluble Vitamins
Vitamins are organic compounds required in small quantities
for normal metabolism. The fat-soluble vitamins are lipids.
• Vitamin A (retinol) is obtained from fish liver oils and dairy
products, and is synthesized from β-carotene, the orange pigment in
carrots.
• In the body, vitamin A is converted to 11-cis-retinal, the
lightsensitive compound responsible for vision in all vertebrates
• A def ciency of vitamin A causes night blindness, as well as dry eyes
and skin.
• Vitamin D3 is the most abundant of the D vitamins. It can
be synthesized in the body from cholesterol.
• Vitamin D helps regulate both calcium and phosphorus
metabolism.
• A deficiency of vitamin D causes rickets, a bone disease.
• vitamin E refers to a group of structurally similar compounds,
the most potent being α-tocopherol.
• Vitamin E is an antioxidant, so it protects unsaturated side
chains in fatty acids from oxidation.
• A deficiency of vitamin E causes numerous neurologic problems.
• Vitamin K (phylloquinone) regulates the synthesis of
prothrombin and other proteins needed for blood to clot.
• A defi ciency of vitamin K leads to excessive and sometimes
fatal bleeding because of inadequate blood clotting.
• The electron density is distributed fairly evenly
among the carbon atoms of these vitamins due to
their many nonpolar C–C and C–H bonds.
Eicosanoids
Group of biologically active compounds containing 20 carbon
atoms derived from arachidonic acid. The prostaglandins and
the leukotrienes are two types of eicosanoids. Two others are
the thromboxanes and prostacyclins.
Dinoprostone, the generic name for PGE2 , is administered to
relax the smooth muscles of the uterus when labor is induced,
and to terminate pregnancies in the early stages.
Prostaglandins themselves are unstable in the body, often
having half-lives of only minutes, more stable analogues
have been developed that retain their important biological
activity longer.
•  A group of anti-infl ammatory drugs that block only the COX-2
enzyme was developed in the 1990s (rofecoxib, valdecoxib, and
celecoxib)
•  Especially effective NSAIDs for patients with arthritis.
•  Both rofecoxib and valdecoxib have been removed from the
market (increased risk of heart attack and stroke).
Terpenes
Lipids composed of repeating fi ve-carbon units called
isoprene units. An isoprene unit has fi ve carbons: four in
a row, with a one-carbon branch on a middle carbon.
• An isoprene unit may be composed of C–C s bonds only,
or there may be p bonds at any position.
• Isoprene units are always connected by one or more
carbon–carbon bonds.
• Each carbon atom is part of one isoprene unit only.
• Every isoprene unit has five carbon atoms. Heteroatoms
may be present but their presence is ignored in locating
isoprene units.
Classes of Terpenes
The Biosynthesis of Terpenes
Terpene biosynthesis is an excellent example of how
syntheses in nature occur with high efficiency.
[1] The same reaction is used over and over again to prepare
progressively more complex compounds.
[2] Key intermediates along the way serve as the starting materials
for a wide variety of other compounds.
[1] The two C5 diphosphates are converted to geranyl
diphosphate, a C10 monoterpene. Geranyl diphosphate is
the starting material for all other monoterpenes.
[2] Geranyl diphosphate is converted to farnesyl
diphosphate, a C15 sesquiterpene, by addition of a fivecarbon unit. Farnesyl diphosphate is the starting material
for all sesquiterpenes and diterpenes.
[3] Two molecules of farnesyl diphosphate are converted
to squalene, a C30 triterpene. Squalene is the starting
material for all triterpenes and steroids.
Biological Formation of Geranyl Diphosphate
Steps [1]–[2] Loss of the leaving group and nucleophilic attack
to form a new C–C bond. Step [3] Loss of a proton
Isomerization of Geranyl Diphosphate to Neryl Diphosphate
Steps [1]–[2] Isomerization of geranyl diphosphate to
linalyl diphosphate
Steroids
•  Composed of three six-membered rings and one five
membered ring.
•  Many steroids also contain two methyl groups, called
angular methyl groups, at the two ring junctions.
•  The steroid rings are lettered A, B, C, and D, and the 17 ring
carbons are numbered as shown. The two angular methyl
groups are numbered C18 and C19.
Cholesterol
•  Has the tetracyclic carbon skeleton characteris<c ofmsteroids. •  eight stereogenic carbons (seven on rings and one on a side chain), 28 = 256 possible stereoisomers. •  In nature, only the following stereoisomer exists: The biosynthesis of cholesterol
Two cholesterol-lowering drugs
Other Steroids
•  Sex hormones and the adrenal cortical steroids.
•  There are two types of female sex hormones, estrogens
and progestins.
•  The male sex hormones are called androgens.
The Female and Male Sex Hormones
Estradiol and estrone are estrogens synthesized in the
ovaries. They control the development of secondary sex
characteristics in females and regulate the menstrual
cycle.
Progesterone is often called the “pregnancy
hormone.” It is responsible for the preparation
of the uterus for implantation of a fertilized egg.
Testosterone and androsterone are androgens synthesized in
the testes. They control the development of secondary sex
characteristics in males.
Synthetic analogues of these steroids have found
important uses, such as in oral contraceptives
Adrenal cortical steroids
•  Cortisone, cortisol, and aldosterone. Synthesized in the outer
layer of the adrenal gland.
•  Cortisone and cortisol serve as anti-infl ammatory agents and
they also regulate carbohydrate metabolism.
•  Aldosterone regulates blood pressure and volume by controlling
the concentration of Na+ and K+ in body fluids.