Lipids
Introduction to Lipids
• Lipids are biomolecules that are soluble in organic
solvents and insoluble in water.
• They are defined on the basis of a physical property
not by a particular functional group, thus they
have a variety of structures and functions.
• They contain many nonpolar C—C and C—H
bonds and few polar bonds resulting in their water
insolubility.
1
Lipids
Introduction to Lipids
Lipids can be categorized as:
1. Hydrolyzable lipids can be converted into
smaller molecules by hydrolysis with water.
2
Lipids
Introduction to Lipids
Lipids can be categorized as:
2. Nonhydrolyzable lipids cannot be cleaved into
smaller units by aqueous hydrolysis.
3
Fatty Acids
• Hydrolyzable lipids are derived from fatty acids.
• Fatty acids are carboxylic acids (RCOOH) with
long C chains of 12-20 C atoms.
• An example is CH3(CH2)14COOH (palmitic acid):
polar portion = hydrophillic
nonpolar portion = hydrophobic
4
Fatty Acids
• Naturally occurring fatty acids have an even
number of C atoms.
• Saturated fatty acids have no double bonds
in their long hydrocarbon chains.
• Unsaturated fatty acids have 1 or more double
bonds (generally cis) in their long hydrocarbon
chains.
• As the number of double bonds in the fatty acid
increases, the melting point decreases.
5
Fatty Acids
• Stearic acid (mp 71 oC) is a saturated fatty acid.
• Oleic acid (mp 16 oC) is an unsaturated fatty acid.
6
Fatty Acids
• Linoleic and linolenic acids are essential fatty
acids; they cannot be produced by the body
and must be consumed.
• Linoleic acid is called an omega-6 acid, because
of the position of the first C=C in the nonpolar chain.
7
Fatty Acids
• Linolenic acid is called an omega-3 acid, because
of the position of the first C=C in the nonpolar chain.
8
Waxes
• Waxes are esters (RCOOR’) formed from a fatty
acid and a high molecular weight alcohol.
• General structure of waxes:
• General formation of waxes:
9
Waxes
• For example, shown below is the formation of
spermaceti wax, isolated from the heads of sperm
whales.
10
Waxes
• Waxes form a protective coating on the feathers of
birds to make them water repellent, and on leaves
to prevent water evaporation.
• Beeswax contains the wax myricyl palmitate as a
major component.
Beeswax (myricyl palmitate):
O
CH3(CH2)14 C
O(CH2)29CH3
11
Waxes
• Like other esters, waxes are hydrolyzed with water
in the presence of acid or base to re-form the
carboxylic acid and alcohol they came from.
12
Triacylglycerols—Fats and Oils
• Triacylglycerols (triglycerides) are triesters
formed from glycerol and three molecules of
fatty acids.
• Below is a generalized block diagram of a
triacylglycerol:
13
Triacylglycerols—Fats and Oils
• The general reaction for the formation of a
triacylglycerol is:
14
Triacylglycerols—Fats and Oils
General Features
• Triacylglycerols may be composed of three
identical fatty acid side chains, or from two or three
different fatty acids.
• The fatty acids may be saturated or unsaturated.
• Animal fats and vegetable oils, the most abundant
lipids, are triacylglycerols with different physical
properties.
15
Triacylglycerols—Fats and Oils
General Features
• Fats have higher melting points; they are solids
at room temperature.
• Fats are derived from fatty acids with few double
bonds.
• Oils have lower melting points; they are liquids
at room temperature.
• Oils are derived from fatty acids having a larger
number of double bonds.
16
Triacylglycerols—Fats and Oils
General Features
• Solid fats have a relatively high percentage of
saturated fatty acids and are generally animal in
origin.
17
Triacylglycerols—Fats and Oils
General Features
• Liquid oils have a higher percentage of unsaturated
fatty acids and are generally vegetable in origin.
• In the unsaturated lipid, a cis double bond places a
kink in the side chain, making it more difficult to
pack efficiently in the solid state, thus leading to a
lower melting point.
18
Triacylglycerols—Fats and Oils
General Features
An
unsaturated
triacylglycerol:
19
Focus on Health & Medicine
Fats and Oils in the Diet
• Unsaturated triacylglycerols lower the risk of
heart disease by decreasing the level of cholesterol
in the blood.
• Triglycerols formed from omega-3 fatty acids are
very helpful in lowering the risk of a heart attack.
• However, if the double bond of the unsaturated
triacylglycerol is trans, the beneficial effect is lost.
• Trans fats, which are primarily synthesized
instead of naturally occurring, act like saturated
fats and increase the cholesterol levels in the blood.
20
Focus on Health & Medicine
Fats and Oils in the Diet
21
Hydrolysis of Triacylglycerols
• Triacylglycerols are hydrolyzed with water in the
presence of acid, base, or enzymes (in the body).
O
CH2—O
CH— O
C
O
(CH2)16CH3
C
(CH2)16CH3
+ 3 H2 O
H2SO4
O
CH2—O
C
(CH2)16CH3
The 3 bonds
that break
are drawn in red.
CH2—OH
O
CH— OH + 3 HO
C
CH2—OH
glycerol
(CH2)16CH3
3 stearic acids
22
Hydrolysis of Triacylglycerols
Soap Synthesis
• Soaps are metal salts of fatty acids prepared by
basic hydrolysis (saponification) of a
triacyl-glycerol.
polar head
ionic end
nonpolar tail
nonpolar end
23
Hydrolysis of Triacylglycerols
Soap Synthesis
O
CH2—O
CH —O
C
O
R
H2O
C
R
glycerol
O
CH2—O
C
+ 3 NaOH
R
3 soap
molecules
CH2—OH
O
CH —OH + 3 Na+ −O
C
R
CH2—OH
• The nonpolar tails dissolve grease and oil and
the polar head makes it soluble in water.
24
Phospholipids
• Phospholipids are lipids that contain a P atom.
• Phosphoacylglycerols are the most common
phospholipid, they are the principal component
of most cell membranes.
25
Phospholipids
Phosphoacylglycerols
• Structurally, they resemble a triacylglycerol, except
the third fatty acid has been replaced with
a phosphodiester bonded to an alcohol.
• There are two main types of phosphoacylglycerols
that differ in the identity of the R“ group in the
phosphodiester.
26
Phospholipids
Phosphoacylglycerols
• One of the main types of phosphoacylglycerols is
cephalin:
27
Phospholipids
Phosphoacylglycerols
• The second of the main types of phosphoacylglycerols is lecithin:
28
Focus on Health & Medicine
Cholesterol, the Most Prominent Steroid
• Steroids are a group of lipids whose carbon
skeletons contain several fused rings:
29
Focus on Health & Medicine
Cholesterol, the Most Prominent Steroid
• Cholesterol, the most prominent steroid, is
synthesized in the liver and found in almost all
body tissues.
• It is obtained in the diet from many sources,
including meat, cheese, butter, and eggs.
30
Focus on Health & Medicine
Cholesterol, the Most Prominent Steroid
• Elevated levels of cholesterol in the bloodstream
lead to coronary artery disease, heart attack, etc.
• Cholesterol is insoluble in the aqueous medium
of blood.
• It is transported through the bloodstream by
lipoproteins, aggregates of phospholipids and
proteins.
• Low-density lipoproteins (LDLs) transport
cholesterol from the liver to the tissues.
• High-density lipoproteins (HDLs) transport
cholesterol from tissues back to the liver.
31
Focus on Health & Medicine
Cholesterol, the Most Prominent Steroid
• LDLs deposit cholesterol on the walls of arteries
when they carry more than is needed to form
cell membranes.
• This forms plaque, which restricts blood flow; thus,
LDL cholesterol is called “bad” cholesterol.
• HDLs reduce the level of cholesterol in the bloodstream by bringing excess back to the liver; HDL
cholesterol is called “good” cholesterol.
• Recommended levels are: HDL > 40 mg/dL,
LDL < 100 mg/dL, total serum cholesterol
< 200 mg/dL.
32
Steroid Hormones
• A hormone is a molecule that is synthesized in
one part of an organism, which then elicits a
response at a different site.
• Two important classes of steroid hormones include
sex hormones and adrenal cortical steroids.
• The female sex hormones are estrogens and
progestins.
• The male sex hormones are called androgens.
33
Steroid Hormones
Female Sex Hormones: Estrogens
• The estrogens estradiol and estrone control
development of secondary sex characteristics,
regulate the menstrual cycle, and are
made in the ovaries.
34
Steroid Hormones
Female Sex Hormones: Progestin
• The progestin progesterone is called the
“pregnancy hormone”; it is responsible for the
preparation of the uterus for implantation of a
fertilized egg.
35
Steroid Hormones
Male Sex Hormones: Androgens
• Testosterone and androsterone are androgens
made in the testes.
• They control the development of secondary sex
characteristics in males.
36
Steroid Hormones
Anabolic Steroids
• Synthetic androgen analogues, called anabolic
steroids, promote muscle growth.
• They have the same effect as testosterone, but
are more stable, so they are not metabolized as
quickly.
• They have come to be used by athletes and body
builders, but are not permitted in competitive
sports.
• Prolonged use of anabolic steroids can cause
physical and psychological problems.
37
Steroid Hormones
Anabolic Steroids
• Some examples of anabolic steroids:
38
Steroid Hormones
Adrenal Cortical Steroids
• Three examples of adrenal cortical steroids are:
aldosterone
cortisone
cortisol
39
Steroid Hormones
Adrenal Cortical Steroids
• Aldosterone regulates blood pressure and volume
by controlling the concentration of Na+ and K+
in body fluids.
• Cortisone and cortisol serve as anti-inflammatory
agents, which also regulate carbohydrate
metabolism.
• Prolonged use of these steroids can have
undesired side effects, including bone loss and
high blood pressure.
• Prednisone, a synthetic alternative, has similar
anti-inflammatory properties.
40
Focus on Health & Medicine
Fat-Soluble Vitamins
• Vitamins are organic compounds required in
small quantities for normal metabolism and
must be obtained from the diet.
• Vitamins are either water soluble or fat soluble.
• The four fat-soluble vitamins (A, D, E, and K)
are found in fruits, vegetables, fish, liver, and
dairy products.
• They are stored in adipose cells to be used when
needed.
41
Focus on Health & Medicine
Fat-Soluble Vitamins
• Vitamin A is found in liver, fish, and dairy products,
and is made from β-carotene.
• It is needed for vision and for healthy mucous
membranes.
• Vitamin A deficiency causes night blindness and
dry eyes and skin.
42
Focus on Health & Medicine
Fat-Soluble Vitamins
• Vitamin D can be synthesized from cholesterol.
• It can be obtained in the diet from many foods,
especially milk, and helps regulate Ca+ and K+
metabolism.
• A deficiency of vitamin D causes rickets (bone
malformation).
43
Focus on Health & Medicine
Fat-Soluble Vitamins
• Vitamin E is an antioxidant, protecting unsaturated
side chains in fatty acids from unwanted
oxidation.
• Deficiency of vitamin E causes numerous
neurological problems, although it is rare.
44
Focus on Health & Medicine
Fat-Soluble Vitamins
• Vitamin K regulates the synthesis of clotting
proteins (prothrombin), and deficiency of this
leads to excessive or fatal bleeding.
45