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. 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

http://www.uptodate.com/contents/fish-oil-and-marine-omega-3-fatty-acids

Long chain n-3polyunsaturatedfatty acids (PUFA),found in fish oil butprobably synthesizedby algae in fish diets,have important effects on humanhealth.

Long chain saturated fattyacids are pheromones formany insects, advertisingfecundity in queens and/orsuppressing reproduction inworkers.

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

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: phosphatidylethanolamine,

phosphatidylserine predominate • outer: phosphatidyl choline, sphingomyelin

predominate

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.

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