A.
Introduction
Lipid-
a class of biomolecules that exhibit poor solubility in water
1.
Biological function of lipids
a.
Energy storage: Lipids represent
highly reduced forms of carbons and upon oxidation
in metabolism release large amounts of energy. (Triacylglyercol)
b.
Cell membrane components: Maintain
a hydrophobic medium that separate intracellular components from extracellular
media. (Phospholipids & glycolipids)
c.
Chemical messengers (hormones): Cellular
responses are mediated by many lipid chemicals
2.
Description
a.
Hydrophobic- consist of only nonpolar groups, typically insoluble
b.
Amphipathic- consist of both polar and nonpolar groups
B.
Classes of Lipids
1.
Fatty acids- long hydrocarbon chain (tail) and a terminal carboxyl group (head)
-
carboxyl is usually ionized (carboxylate) in physiological conditions
-
abundant in biological systems, but not usually in free state (fats)
-
most fatty acids are composed of even numbers of carbons (12-24). Due to the way
they are synthesized from acetate (2-C).
a.
saturated- all carbons are sp3 hybridized with 4 single bonds
1.
each carbon-carbon bond (sigma bond) exhibits free rotation therefore
saturated fatty acids are extremely flexible
2.
fully extended conformation is the most stable
|
Table 1: Saturated fatty acids |
||||||
|
Systematic Name |
Common
Name |
Notation |
Systematic Name |
Common
Name |
Notation |
|
|
Ethanoic |
Acetic |
2:0 |
Tetradecanoic |
Myristic |
14:0 |
|
|
Butanoic |
Butyric |
4:0 |
Hexadecanoic |
Palmitic |
16:0 |
|
|
Hexanoic |
Caproic |
6:0 |
Octadecanoic |
Stearic |
18:0 |
|
|
Octanoic |
Caprylic |
8:0 |
Eicosanoic |
Arachidic |
20:0 |
|
|
Decanoic |
Capric |
10:0 |
Docosanoic |
Behenic |
22:0 |
|
|
Dodecanoic |
Lauric |
12:0 |
Tetrcosanoic |
Lignoceric |
24:0 |
|
b.
unsaturated- the hydrocarbon tail contains on or more double bonds.
-
monounsaturated- tail contains 1 double bond
-
polyunsaturated- tail contains 2 or more double bonds
|
Table
2: Unsaturated fatty acids |
||
|
Systematic Name |
Common
Name |
Notation |
|
9-Hexadecenoic |
Palmitoleic |
16:1 |
|
9-Octadecenoic |
Oleic |
18:1 |
|
9,12-Octadecadienoic |
Linoleic |
18:2n-6 |
|
9,12,15-Octadecatrienoic |
a-Linolenic |
18:3n-3 |
|
6,9,12-Octadecatrienoic acid |
g-Linolenic |
18:3n-6 |
|
5,8,11,14-Eicosatetraenoic |
Arachidonic |
20:4n-6 |
|
15-Tetracosenoic |
Nervonic |
24:1 |
1.
More common in nature than saturated.
Oleic acid is the most common unsaturated fatty acid.
2.
Most double bonds are found in the cis conformation. Produces a
“kink” in the hydrocarbon tail, essential for the function of biological
membranes.
3.
Essential fatty acids- those that are not synthesized by mammals but are
critical for normal growth and life.
a.
Linoleic acid
b.
g-Linolenic
**
Arachidonic acid is synthesized by mammals but only from linoleic acid.
This is an important fatty acid, precursor for the eicosanoids.
4.
Melting points of unsaturated fatty acids are commonly lower than melting
points of the saturated fatty acid counterparts
5.
Shorter chain lengths have lower melting points than longer chain lengths
**Important
note: Short chain length and
unsaturation enhance the fluidity of fatty acids and their derivatives.
2. Triacylglycerols (Triglycerides
or Fats)
- major energy reserve for plants and animals (adipose tissue)
a.
composition
1.
glycerol (1,2,3-Trihydroxypropane)
2.
3 fatty acids linked to the glycerol via ester linkages
b.
classification
1.
simple- contain the same 3 fatty acids. Named
for the fatty acid
ex. Tristearoylglycerol (tristearin) & Trioleoylglycerol (triolein)
2.
mixed- composed of different fatty acids. Named according to alphabetic
priority
ex.
1-Myristoyl-2-Stearoyl-3-Palmitoloylglycerol
c.
characteristics: based on
fatty acid composition
1.
fats- contain saturated fatty acids, solids at room temperature
-saturated
fatty acids have linear conformation therefore lending to a greater degree of
Van der Waals interactions
2.
oils- contain unsaturated fatty acids, liquids at room temperature
3.
large energy reserves.
-complete
oxidation of 1 g of triacylglycerols yields around 38 kJ of energy whereas 1 g
of protein/carbohydrate yields around 17 kJ of energy.
4.
Form highly anhydrous aggregates.
5.
Provide good insulation in animals
6.
Fats (lard) are used to make soap, saponification.
-
hydrolysis of the ester linkage using a concentrated base (NaOH- lye)
-
produces neutralized fatty acids and glycerol
-
in the body, fats are broken down with enzymes called lipases
3.
Glycerophospholipids (phosphoglyceride or glycerol phosphatide)
-consist
of a 1,2-Diacylglycerol (DA) with a phosphate group esterfied to the C-3 of
the glycerol molecule.
-largest class of natural lipids and probably the most important (cell
membranes)
-belong to a larger class of lipids. Phospholipids.
The phosphate group becomes an attachment site for various molecules
Common phospholipids- based on phosphate-ester attachments
a.
Phosphatidylcholine
-choline
b.
Phosphatidylethanolamine
-ethanolamine
c.
other “head” groups- serine, glycerol, & inositol
-
inositol 
- Most glycerol
phosphatides have a saturated fatty acid at C-1 and an unsaturated at C-2.
d.
Ether Glycerophospholipids
-contains
an ether linkage (alkyl group) at C-1 rather than an ester linkage (acyl)
ex.
Platelet activating factor (PAF)- 1-alkyl-2-acetyl-phosphatidylcholine
-causes platelet aggregrations and dilation of blood vessel dilation.
-recently been found to be a potent mediator in inflammation, allergic
response
caused by enterotoxins of some
bacteria)
-also found to be released by fertilized egg which assists in binding to uterine wall
4. Sphingolipids-
-contain sphingosine (18-C amino alcohol) as the backbone for the lipid
(C=C; is in the trans conformation)
a. Ceramide- an acyl group
attached to the sphingosine via an amide linkage
b. Sphingomyelins- phosphorus-containing class of sphingolipids.
-usually an esterification of a phosphorylcholine (or ethanolamine) to a
ceramide
-common in nervous tissue of higher animals
c. Glycosphingolipids- contain a ceramide with a sugar group esterfied to the C-1 hydroxyl
