Basic Biochemistry: Lipid Structure
Dr. Kamal D. Mehta
Department of Molecular & Cellular Biochemistry
Wexner Medical Center at the Ohio State University
Learning Objectives
Describe
• the structures and classification of lipids that play important
roles in controlling biological processes.
• the structure of some common lipids of biological importance.
Recognize
Identify
Describe
• components of lipids molecules, such as fatty acids, glycerol,
sphingosine, phosphate, organic bases, inositol, etc.
• how do amphipathic lipid molecules interact in aqueous
systems.
Learning Resources
 Principles of Medical Biochemistry, by Meisenberg &
Simmons
 Essential Cell Biology (ECB), by Alberts et al., Pages 55,
58-60
 Basic Medical Biochemistry, by Marks, Marks, & Smith,
Pages 58-62
1. What are lipids?
 Lipids are non-polar (hydrophobic) compounds.
They are not soluble in water but are soluble in
nonpolar solvents such as ether, choloroform,
and benzene.
 Lipids include fats, oils, steroids, and waxes.
 Lipids are related by their physical rather than
by their chemical properties.
2. Why do we need to study them?
 BIOMEDICAL IMPORTANCE:






Required for membrane formation
An efficient source of energy
Fat content of nerve tissue is particularly high
Signaling molecules
Hormones
Obesity, and Atherosclerosis
3. How are they classified and what are
their structures?
 CLASSIFICATION OF LIPIDS:







Long chain aliphatic carboxylic acids
Triacylglycerols- Esters of fatty acids with glycerol
Glycerophospholipids- Esters of glycerol with fatty acids
and phosphate derivatives
Glycolipids
Sphingolipids: Esters of fatty acids with sphingosine
Steroids- Polycyclic aliphatics
Prostaglandins and Leukotrienes-oxidized derivatives of
C-20 fatty acids
A. Fatty acids: CH3(CH2)n-COONon-polar Polar
 Fatty acids consist of a hydrocarbon chain with a carboxylic
acid at one end. About 40 different fatty acids are known
naturally.
 Most naturally occurring fatty acids have an even number of
carbon.
 In humans, fatty acids usually have 16 to 20 carbon atoms.
 Saturated fatty acids and unsaturated fatty acids
 Multiple sites of unsaturation are Polyunsaturated fatty acids,
whereas a single site of unsaturation are Monounsaturated
fatty acids.
 Their fluidity decreases with chain length and increases
according to degree of unsaturation.
Examples:
Examples of common fatty acids:
14:0
16:0
18:0
18:1 cisD9
18:2 cisD9,12
18:3 cisD9,12,15
20:4 cisD5,8,11,14
myristic acid
palmitic acid
stearic acid
oleic acid
linoleic acid
linonenic acid
arachidonic acid
Stearic
acid
(C18)
Palmitic
acid
(C16)
Oleic
acid
(C18)
Double bonds can be either cis or trans:
 Naturally occurring
unsaturated vegetable oils
have almost all cis bonds
but using oil for frying
causes some of the cis
bonds to convert to trans.
 Trans fatty acids are
carcinogenic
Double bonds creates a kink
Fatty acids bind to other molecules by amide
or ester linkage
O
Formation of an amide:
R’NH2+ HO-C-R”
O
R’-NH-C-R’’ + H2O
C
Example: fatty acids attached to proteins
O
Formation of an ester:
R’OH+ HO-C-R”
O
R’-O-C-R’’ + H2O
Example: triacylglycerol
B. Triacylglycerols:
 Animal Fats and Vegetable Oils:





Both are triacylglycerols (three fatty acids esterified with
glycerol) (triglycerides)
Animal fats- Saturated and monounsaturated fatty acids
Oils- Mono and polyunsaturated fatty acids
More unsaturation leads to lower melting point
Function of fat and oil is storage for energy
Triacylglycerols consist of 3 fatty acids
and glycerol
CH2OH
C
H
CH2OH
Glycerol
O
OH
+
3 fatty acids
C. Phospholipids (Glycerophospholipids or
Phosphoglycerides):
 Glycerol plus two fatty acyl
ester bonds- third position on
glycerol is linked to phosphate
which in turn esterified to
another small molecule.
 Plasma membrane is
composed of proteins and
lipids (50% by mass).
Phospholipids
Phosphatidylcholine (PC), Phosphatidylethanolamine,
Phosphatidyl-serine (PS), Sphingomyelin (SM), are the
major phospholipids in membrane.
D. Sphingolipids:
OH
H2C
 Esters of phosphate and
small molecules with
sphingosine.
 Sphingomyelin, a ceramide
with a phosphocholine or
phosphoethanolamine head
group, is a common
constituent of plasma
membranes.
OH
OH
H
C
CH
H3N+
CH
H2C
O
HC
Sphingosine
OH
H
C
CH
HN
CH
C
R
(CH2)12
CH3
CH3
N+
(CH2)12
Ceramide
CH3
H3C
HC
O
CH2
CH2
CH3
Phosphocholine
O
P
OH
O
H2C
Sphingosine
O
H
C
CH
HN
CH
C
fatty acid R
Sphingomyelin
HC
(CH2)12
CH3
Another major constituent of plasma
membrane: Cholesterol
 Amphipathic lipid and is an essential constituent of cell
membranes.
 Cholesterol is largely hydrophobic. It has one polar
group, a hydroxyl.
 It is the parent molecule from which all steroids in the
body are synthesized.
 Cholesterol modulates membrane fluidity. It restricts
phospholipid movement and affects membrane fluidity.
 Decreases the permeability of membrane to small watersoluble molecules.
Cholesterol
Cholesterol in Lipid Bilayer
Minor components of membrane:
Glycolipids
 Sphingosine based lipids
containing carbohydrate
instead of the phosphate
ester.
 They are found only in the
outer leaflet of the plasma
membrane.
Packing arrangements of lipid
molecules in an aqueous environment
Three views of a plasma membrane
Membrane Model
Cholesterol and the Lipid Bilayer
Nonpolar
tail
Lipid bilayer
Polar head of cholesterol
Cholesterol immobilize the first few hydrocarbon groups
of the phospholipid molecules. This makes the lipid bilayer less
deformable and decreases its permeability to small water-soluble
molecules. Without cholesterol a cell would need a cell wall.
The influence of cis-double bonds in
hydrocarbon chain
The assymetrical distribution of
phospholipids molecules in the lipid
bilayer of human red blood cells
Phosphpolipid mobility
 The types of movements possible for
phospholipid molecules in lipid bilayer.
PROPERTIES
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In Summary
 Lipids are hydrophobic and are related by physical
characteristic rather than by chemical structure.
 They perform different functions in the cell. For
example, triglycerides stores energy whereas
phospholipids are involved in membrane formation.
Another constituent of the membrane cholesterol
controls fluidity and block passage of small molecules.
 Membrane bilayers have polar groups of phospholipids
facing the aqueous environment whereas hydrophobic
fatty acid chain is buried in the hydrophobic environment.
 Proteins occupy this bilayer in an integral or peripheral
manner.
QUESTIONS?
Please direct questions to: mehta.80@osu.edu