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LIPIDS AND MEMBRANES
Fatty acids
H
H
C
C
H
H
•Hydrocarbon chain (saturated or un-)
•Carboxylic acid group
Fatty acids
Nomenclature: C1 (COOH), C2, C3, etc. αC = C2, ßC = C3, etc.
18 C’s with 2 double bonds: C18:2(Δ9,12)
(“Δ9” means between C9 and C10)
(double bonds are normally at 9, 12, 15 and are cis)
•
•
•
•
Palmitic acid—C16:0
Stearic acid—C18:0
Oleic acid—C18:1 (Δ9)
Linoleic acid—C18:2 (Δ9,12)
Fatty acids
•
•
•
•
Palmitic acid—16:0
Stearic acid—18:0
Oleic acid—18:1 (Δ9)
Linoleic acid—18:2 (Δ9,12)
Long, straight chains are less soluble (in aqueous medium)
Short chains, and double bonds reduce melting temperature and increase solubility
Long chain n-3
polyunsaturated
fatty acids (PUFA),
found in fish oil but
probably synthesized
by algae in fish diets,
have important
effects on human
health.
http://www.uptodate.com/contents/fish-oil-and-marine-omega-3-fatty-acids
Long chain saturated fatty
acids are pheromones for
many insects, advertising
fecundity in queens and/or
suppressing reproduction in
workers.
Van oystaeyen et al., Science 343, 287 (17 Jan 2014)
Triacyl glycerol:
energy storage (fats and oils): 38 kJ/mol (vs protein
17 kJ/mol)
Fats and oils --storage forms of C and energy-- accumulate in lipid bodies
An adipocyte
Membrane lipids (phospholipids)
glycerol C1-attached fatty acid normally saturated
glycerol C2-attached fatty acid normally unsaturated
glycerol C3: phosphate plus hydrophilic group...
Membrane lipids (phospholipids): note the different
head groups
Other lipids: e.g. sphingolipids on a sphingosine base
See below: sphingosine is outlined
Membranes
Lipid bilayer: heads in contact with aqueous solution;
tails isolated from it.
Note the different lipids in membranes: inner and
outer leaflets are distinct.
erythrocyte:
• inner: phosphatidylethanolamine,
phosphatidylserine predominate
• outer: phosphatidyl choline, sphingomyelin
predominate
P-lipid breakdown by hydrolysis:
catalyzed by phospholipases
e.g.: snake venom P-lipase (PLA2) hydrolyzes
C2 fatty acid, which bursts erythrocytes
Sterols: note tetra-ring base, hydrophobic addition,
hydrophillic -OH
Archeal membrane lipids have structures analogous to phospholipids
Lipid solubility
! on water surface: heads in water, tails in air
! submerged single tail lipids (e.g., sodium
laurylsulfate) at “critical micelle concentration”:
spontaneous formation of micelles
! submerged phospholipids form liposomes,
bilayer leaflets
Phase transitions
• longer chains raise the transition temperature,
decrease fluidity
• double bonds lower the transition temperature,
increase fluidity
• membranes leak during the transition
• cholesterol (et al.) makes the gel more fluid and
the liquid crystal less fluid (also Ca2+)
• enzymes in membranes generally work better in
liquid crystal phase, but complexes may stay
together better in a gel
Resistance to cold is associated with higher concentrations of linolenic acid.
Olive oil
“Tests indicate that imported “extra virgin”olive oil often fails international
and USDA standards - UC Davis Olive Center, July 2010”
Lipid composition
Unsaturated
Palmitic acid: 7.5–20.0%
Stearic acid: 0.5–5.0%
Mono-unsaturated
Oleic acid: 55.0–83.0%
Palmitoleic acid: 0.3–3.5%
Polyunsaturated
Linoleic acid: 3.5–21.0 %
α-Linolenic acid: <1.5%
Problems
Free fatty acids
Peroxides
UV absorption (conjugated
double bonds)
1,2 and 1,3 diacylglycerols
Summary
Fatty acids are distinguished by length and presence of double bonds:
palmitic, steric, oleic, and linoleic acids are common.
Storage lipids are generally triglycerides
Membrane lipids include phospholipids, sphingolipids, and sterols
Temperature-induced phase transitions represent a change from
close-packed to more open conformations