Biochemistry I Spring Term
Lecture 25: Lipids
Lecture 25 - Lipids
March 22, 2013
Horton 9.1-9.3, Nelson & Cox 10.1
Proteins
O
O P2- O
Cholesterol
Biological Membranes
Fatty Acids
OH
Phospholipids
Waxes
Triglycerides
OH
Lipid Vesicles
A. Fatty Acids:
C18
O
A1. Structure:
O
A2. Nomenclature for fatty acids:
#
Name
Carbons
12
Lauric acid
14
Myristic acid
16
Palmitic acid
18
Stearic acid
20
Arachidic acid
Oleic
18:1 c9
(C9 = C10)
H
C18
O
O
H
C18
O
H
O
A3. Thermal Transitions in Fatty Acids:

Melting Temp of Fatty Acids
As the length of the chain
increases, so does TM.
The presence of a cis double bond
drops the melting temperature.
90
80
Melting Temperature (C)

H
70
60
50
40
30
20
10
0
10
12
14
16
18
20
Carbon Length
A4. Interaction with water
1. Fatty acids form micelles, aggregates of fatty acids with a polar (charged) surface and a hydrophobic,
waterless, interior. The assembly of micelles is spontaneous and driven by the hydrophobic effect.
2. The CMC (critical micelle concentration) is the highest concentration of monomeric fatty acids in solution.
3. The non-polar interior can dissolve hydrophobic compounds (oily 'dirt'), therefore, fatty acids and modified
fatty acids (e.g. SDS) are the principal components of soaps.
4. Free fatty acids are essentially toxic since they can dissolve normal cell membranes in the same way soap
would. Therefore, fatty acids are incorporated into triglycerides, phospholipids, or transported on proteins.
1
Biochemistry I Spring Term
Lecture 25 - Lipids
March 22, 2013
B. Waxes: Ester of a fatty acids and an alcohols, e.g. beeswax:
CH3(CH2)29O-CO-(CH2)14CH3
O
O
O
C. Triglycerides:
HO
C OH
C O
O
C OH
acyl chain
O
3 H2O
O
C O
HO
C OH
C O
O
ester
O
linkage
glycerol
HO
fatty acids
Trans/unsaturated-Fat:
Corn "oil" is a complex mixture of unsaturated
triglycerides that are extracted from corn. Corn oil, as
with most triglycerides from plants (e.g olive oil), is a
liquid at room temperature. To make solid margarine
from corn oil it is necessary to convert the double bonds
to single bonds by the addition of hydrogen - the product
is referred to as "hydrogenated" oil. This involves the
reaction of the triglyceride with hydrogen gas (H2) and a
catalyst, giving the products shown to the right.
i) Why is corn oil a liquid at room temperature?
O
O
R2
R3
O
O
H
H
O
H
R2
O
R3
H
R2
R3
corn oil
margarine
ii) Why does hydrogenation raise the melting temperature of the triglyceride, such that it forms a solid at
room temperature?
The health effects on consuming too much trans-fatty acids in triglycerides are:
 trans fats interfere with absorption of essential fatty acids.
 trans fats modify the properties of the lipid bilayer in cells.
 trans fats can change the ratio of blood lipo-proteins, potentially leading to coronary diseases.
2
Biochemistry I Spring Term
Lecture 25 - Lipids
March 22, 2013
Phospholipids:
Glycerophospholipids:
1. Head group + phosphate + glycerol + two fatty acids (acyl chains) of various types form a phospholipid.
2. Various head groups are attached to the phosphate, giving a diverse set of lipids.
3. The nomenclature indicates the type of fatty acid attached to the 1 and 2 positions of glycerol as well as
the type of head group on the phosphate.
Head group(-X)
none
Choline
(-C-C-N+(CH3)3)
Serine (linkage via sidechain)
Name of Phospholipid
phosphatidic acid (PA)
Phosphatidylcholine (PC), called lecithin
Phosphatidylserine (PS)
Net Charge
-1
0 (zwitterion)
-1
O
H3N
+
P
O
O
glycerol
O
O
O
O
O
P
1
OH
2
OH
C
O
C
O
O
3
HO C
Phosphate
O
Glycerol
2 Fatty acids(C12-Lauric)
O
3 H2O
O
O
1-lauroyl -2-lauroyl-3-phosphatidylcholine
O
O
(dilauroyl phophatidyl choline, DLPC)
H2O
O
1
C
H3C
N
+
O
2
C
3
O
H3C
O
HO
H3C
O
CH3
C
acyl chain
H3C
O
N
+
CH3
OH
O
P
O
O
Phosphatidic acid
O
head group
Choline
O
P
O
head group
Physical Properties of Pure Lipid Bilayers:
1.Phospholipids self-assemble in
water to form bilayers (two
opposing
layers
of
phospholipids). Bilayers are
formed instead of micelles
because the cross section of the
head group is roughly equal to
the cross section of the 2 fatty
acid
chains
found
in
phospholipids.
2. The bilayers form closed,
water filled, vesicles with a 4050 Å thick wall. The non-polar
acyl chain width is about 30 Å.
3. Permeability properties:
 Charged compounds do
not cross the bilayer.
 Polar ones infrequently.
 Non-polar ones readily.
O
O
P
O
O
O
O
O
O
=
Chemical
structure
Simplified
representation
Section of bilayer
Phospholipid
vesicle
3