Chapter 20
Lipids
Lipids
• 3 major roles in biochemistry
• Store E within fat cells (vs plants – cellulose)
• Membranes to separate compartments (e.g. lipid bilayer)
• Chemical messengers (e.g. hormones)
• Classification: (4 groups/types)
1. simple lipids (fats, waxes)
2. complex lipids (phospho/glyco-lipids)
3. steroids (cholesterol)
4. prostaglandins, thromboxanes, leukotrienes
(inflammation)
Structure of Triglyceride
• Triglyceride: an ester of glycerol with three (1-3) fatty acids (F.A.s).
2.
1.
+ 3 F.A.s 
3.
+ 3 H 2O
Confirming your knowledge:
Draw the structural formula of a triglyceride of:
myristic, palmitic and oleic acid and glycerol
See table 20.1 p 496
Properties of Triglycerides
• 1. Physical properties depend on the fatty acid components:
• Melting points, Oils and Fats
• Longer Carbon chain(s)  higher M.P.
• More d. bonds (unsaturated) lower M.P.
• RECALL
• Oils = liquid at room temp. b/c lots of d.bonds (as cis form)
• Fats = solid at room temp. b/c few d. bonds (stack easily)
NOTE: Natural form of d. bonds in fatty acids/ oils = always cis or trans?
e.g. the Essential Fatty Acid (EFA) linoleic acid
CHALLENGE QUESTION
Linoleic acid has M.P. of -5 *C.
What would happen to it’s M.P. if you converted the
cis d. bonds to trans and why?
Properties of Triglycerides
2. Hydrogenation
•
reduction of some or all d. bonds of an unsaturated triglyceride w/
H2/transition metal catalyst.
• Why? Easier kitchen use (e.g. Crisco oil etc)
• Better for baking, mixing, longer shelf life, cheaper
• Formation of mainly trans isomers (Trans fats)
• e.g. partially hydrogenated vegetable oil
• Drawbacks: unnatural,
• body does not recognize “trans” form
• Inflammation, weight gain, heart disease (higher cholesterol)
Properties of Triglycerides
3. Saponification
•
Hydrolysis of esters: using base
• Produces glycerol + mixture of fatty acids aka (SOAPS)
Polar Head
Note: Hard water disrupts soaps
Na+
Mg2+ or Ca2+
Binds to fatty acids
Na+
O
_ O
O
_ O
O
see chem conn. p.500
Na+
_ O
Greasy tail
Complex Lipids
1. Phospholipids
contain an alcohol, two fatty acids, and a phosphate ester.
a. in glycerophospholipids, the alcohol is glycerol.
(plasma membranes)
b. In sphingolipids, the alcohol is sphingosine.
(function in coatings neurons)
2. Glycolipids
Complex lipids that contain a carbohydrate.
((Water))
polar
Non polar
((Water))
glycerophospholipid
(traps dirt/grease )
Soap = “ Micelle”
20.5 Membranes
•
Complex lipids form the membranes around cells and small structures
within cells.
((water))
•
complex lipids spontaneously form into
a lipid bilayer with a back-to-back
arrangement of lipid monolayers.
2.
((water))
1.
1. Polar (hydrophilic) head groups are in contact with the aqueous environment.
2. Nonpolar (hydrophobic) tails are buried in bilayer shielded from environment.
- driving force of lipid bilayers = hydrophobic interaction.
• The arrangement of hydrocarbon tails in the interior can be
rigid (saturated fatty acids) or fluid (unsaturated fatty acids).
Fluid Mosaic Model
transport
Lipid types most likely found in membranes: glycolipids, & cholesterol
1a. Glycerophospholipids
• second most abundant group of naturally
occurring lipids.
• Occur in plant and animal membranes:
( 40% -50% phosphoacylglycerols)
(50% - 60% proteins)
• most abundant glycerophospholipids
derived from phosphatidic acid
The three most abundant fatty acids in phosphatidic
acids are palmitic (16:0), stearic (18:0), and oleic (18:1).
1a. Glycerophospholipids
• A phosphatidic acid
• 1. glycerol is esterified with: 2. two molecules of fatty acid
3. one phosphoric acid.
2.
H
H
1.
H
3.
1a. Glycerophospholipids cont.
3 other types:
-Ethanolamine
-Serine
-Inositols
Glycerophospholipids
Challenge Question (Homework)
Which Glycerophospholipids will have the greatest
solubility in Water:
ethanolamine?
choline?
serine?
N ame and Formula
choline
O
O
lin olen ic acid
O
O-
?
CH2
O CH2
palmitic acid
ch oline
+
HOCH2 CH2 N(CH 3 ) 3
+
P OCH2 CH2 N(CH?
3 )3
O CH
O
?
ethan olamine
HOCH2 CH2 NH2
glycerol
glycerol
serine
HOCH2 CHCOO
+
NH3
inositol
HO
HO
OH
OH
O
Sphingolipids
• Found in the coatings of nerve axons (myelin).
• Contain the long-chain aminoalcohol, sphingosine,
from which this class of compounds in named.
( CH2 ) 1 2 CH3
( CH2 ) 1 2 CH3
HO
HO
N H2
OH
Sphingosine
O
N HCR
OH
A ceramide
(an N-acylsphingosine)
( CH2 ) 1 2 CH3
HO
O
N HCR
O+
OPOCH 2 CH 2 N( CH3 ) 3
O
A s phingomyelin
Sphingolipids make up myelin
See chem. Connections 20E p.506, Swine flu, watch out!
Glycolipids
• Glycolipid: a complex lipid that contains a
carbohydrate.
• The carbohydrate is either glucose or galactose.
• The cerebrosides are ceramide mono- or
oligosaccharides.
(CH2 ) 1 2 CH3
a ceramide
a un it of
-D-glucop yranose
HO
O
NHCR
H OH
HO
HO
HO
O
H
H
OH
H
a-glycosidic bon d
Steroids
• Steroids: a group of plant and animal lipids that
have this tetracyclic ring structure.
C
A
Know this backbone
B
D
A. Cholesterol
• Cholesterol is the most abundant steroid in the human
body, and most important. Why?
1. It is a component in plasma membranes in all animal cells.
2. It is the precursor of all steroid hormones and bile acids.
HO
Gallstone of cholesterol
-CONFIRMING your KnowledgeHow many stereocenters exist in cholesterol?
B. Lipoproteins
• Cholesterol, along with fats, are transported by lipoproteins
Composition (% dry w eight)
Cholesterol Ph os phoTriLip op rotein
Proteins and esters lipids glycerides
High-dens ity
33
30
29
8
proteins
lipoprotein (HD L)
Low -dens ity
25
50
21
4
Cholest
lipoprotein (LD L)
Very-low den sity
10
22
18
50
Mostly fat lipoprotein (VLD L)
Chylomicron s
1-2
8
7
84
Lipoproteins
• Figure 20.5 Schematic of a low-density lipoprotein (LDL).
Cholesterol Transport
• Cholesterol made in Liver
• Transport of cholesterol from the liver starts out as VLDL.
• VLDL is carried in serum (blood) to fat or muscle tissues and . . .
deposits proteins and triglycerides. .
• As fat is removed, (diameter shrinks) its density increases
(i.e. VLDL  LDL)
•
LDL stays in the plasma for about 2.5 days.
Cholesterol Transport cont.
1. LDL carries cholesterol to cells, where specific LDL receptors bind
it.
2. After binding, LDL is taken into cells where enzymes liberate free
cholesterol and cholesteryl esters.
3. LDL receptor then recycled to “Pit”
1.
3.
2.
2.
3.
Challenge Question?
• What happens to patients who have limited LDL
receptors on there cell membranes?
•
(i.e. familial hypercholesterolemia)
Cholesterol Transport
1. High-density lipoproteins (HDL)
transport cholesterol: f/ tissues
liver and also transfer cholesterol to LDL.
• While in the serum,
free cholesterol in HDL is
HO
converted to cholesteryl esters.
2. In the liver, HDL binds to the
liver cell surface and transfers
its cholesteryl esters to the cell.
• These esters are used for the
synthesis of steroid hormones
(estrogen, testosterone)
and bile acids.
• After HDL has delivered its
cholesteryl esters to liver cells,
it reenters circulation.
2.
1.
Levels of LDL and HDL
• Most of the cholesterol is carried by LDL.
• If sufficient LDL receptors on the surface of cells, LDL is removed
from circulation and its concentration in blood plasma drops.
• number of LDL receptors is controlled by a feedback mechanism.
• When [cholesterol] inside cells is high, the synthesis of LDL
receptors is suppressed. (inside liver)
•Normal plasma levels: 175 mg/100 mL.
“hypercholesterolemia” plasma levels: 680 mg/100 mL!
Not enough LDL receptors
HO
HO
HO
HO
Levels of LDL and HDL
• These high levels of cholesterol can cause premature
atherosclerosis (myocardial infarction: MI) and heart attacks.
• High LDL means high cholesterol content in the plasma because
LDL cannot get into cells.
• Therefore, high LDL together with low HDL is a symptom of faulty
cholesterol transport and a warning of possible atherosclerosis.
• Get tested?!? . . Simple blood test, Planned parenthood, Longs
etc.
Treating High Cholesterol
• Reg. Exercise, Diets low in cholesterol and sat. fat reduce serum
cholesterol
• The commonly used statin drugs (Lipitor) inhibit the synthesis of
cholesterol by blocking HMG-CoA reductase in the liver
Steroid Hormones
• Androgens: male sex hormones
• synthesized in the testes
• responsible for the development of male secondary sex
characteristics
H3 C OH
H3 C
H3 C
O
H3 C
HO
Testosteron e
And rosterone
O