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LIPIDS AND MEMBRANES
Fatty acids
•Hydrocarbon chain (saturated or un-)•Carboxylic acid group
CC
H H
H H
Fatty acids
Nomenclature: C1 (COOH), C2, C3, etc. C = C2, ßC = C3, etc.18 C’s with 2 double bonds: C18:2(9,12)(“9” 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 (9)• Linoleic acid—C18:2 (9,12)
Fatty acids
• Palmitic acid—16:0• Stearic acid—18:0• Oleic acid—18:1 (9)• Linoleic acid—18:2 9,12)
Long, straight chains are less soluble (in aqueous medium)Short chains, and double bonds reduce melting temperature and increase solubility
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
P-lipid breakdown by hydrolysis: catalyzed by phospholipases e.g.: snake venom P-lipase (PLA2) hydrolyzes C2 fatty acid, which bursts erythrocytes
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: phosphtidylethnolmine,
phosphtidylserine predom inte • outer: phosphtidyl choline, sphingom yelin
predom inte
Sterols: note tetra-ring base, hydrophobic addition, hydrophillic -OH
Archeal membrane lipids have structures analogous to phospholipids
Lipid solubility
ß on water surface: heads in water, tails in air ß submerged single tail lipids (e.g., sodium
laurylsulfate) at “critical micelle concentration”: spontaneous formation of micelles
ß submerged phospholipids form liposomes, bilayer leaflets
Phase transitions
liquid crystal Lipid mobility heat phase transition absorption gel
ToC ToC
• 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
liquid crystal Lipid mobility heat phase transition absorption gel
ToC ToC
Resistance to cold is associated with higher concentrations of linolenic acid.
Lipid compositionUnsaturatedPalmitic 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%PolyunsaturatedLinoleic acid: 3.5–21.0 %α-Linolenic acid: <1.5%
ProblemsFree fatty acidsPeroxidesUV absorption (conjugated double bonds)1,2 and 1,3 diacylglycerols
Olive oil “Tests indicate that imported “extra virgin”olive oil often fails internationaland USDA standards - UC Davis Olive Center, July 2010”
Summary
Fatty acids are distinguished by length and presence of double bonds:palmitic, steric, oleic, and linoleic acids are common.
Storage lipids are generally triglyceridesMembrane lipids include phospholipids, sphingolipids, and sterolsTemperature-induced phase transitions represent a change from
close-packed to more open conformations