Lipids: Lecture 3

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Much of the plasma membrane is made up of lipids, they are small(ish) molecules that usually associate through non covalent forces. In aqueous solution the non polar tails associate via the hydrophobic effect, van der waal are formed between the hydrocarbon tails and surface monolayers, bilayers, micelles and vesicles are formed.

Fatty Acids

The simplest form of lipids often incorporated into triacylglycerols they have a hydrophobic (hydrocarbon) tail and hydrophillic (carboxylate group) head that has two H-bond acceptors. They are weak acids (pKa 4.5) and ionized at physiological pH. If their are any double bonds present then they are usually cis (same) causing a bend in the chain. Naturally occurring ones often have an even number of carbon atoms and are linear, however bacterial fatty acids often contain branched or cyclic structures. A FULLY SATURATED FATTY ACID has a HIGHER MELTING POINT than unsaturated fatty acids as they pack together better.

Organisms control their cell membrane fluidity by how many saturated or unsaturated fatty acids they have, this varys inversely with temperature

Bacteria in Arctic Ocean: high levels of unsaturation so their membranes remain fluid in cold conditions

Bacteria in hot springs: high levels of saturation so their membranes remain fluid in hot conditions

Reindeer have a concentration gradient along their leg so the percentage of unsaturation in cells varies, their is more unsaturation in the hoof,

Nomenclature

X:Y(c or t) Z X: total number of carbons Y: number of C=C c or t: cis or trans Z: starting atom of C=C

Carbon number starts at the end with the carboxylate group

Triacylglycerols

Consist of 3 fatty acids and glycerol the hydrocarbon chains store alot of enegy in organisms. Esterification of the fatty acids mean that triacylglycerols have a polar head group that is much less polar than in fatty acids and they aren't soluble in water and don't form miscelles very effectively.

Glycerophospholipids

These are naturally occuring lipids have a phosphate head groups that is VERY POLAR. The acyl side chains are derived from fatty acids so one is saturated and the other unsaturated.

Shape/Structure

Double tails usually for a cylindrical molecule that easily form bilayers. Fatty acids are usually wedge shape so form spherical micelles where as phospholipids are more cylindrical and pack together better to make the bilayer structure

Sphingolipids

These are the second class of major membrane lipids, they are built around sphingosine. The head group can contain saccharides if this is the case then they are called glycolipids. If a fatty acid is attached to the NH2 via an amide bond then the subclass is called ceramides, these can be modified by adding to the -OH of sphingosine. Glycosphingolipids are the 3rd major class of membrane lipids and include gangliosides (anionic glycosphingolipids containing at least one sialic acid.

Ganglioside GM2 accumulates in neural tissue of infant that have tay-sachs disease as they lack an enzyme that cleaves the terminal GalNAc.

Cholesterol

A member of the steriod group that is a precursor for bile acids. It is weakly amphipathic and found in many animal membranes as it influences fluidity. It does this by aligning itself with the -OH groups close to the hydrophilic heads of phospholipids this stiffens the membrane reducing it fluidity. The sterol ring structure interacts with regions of the hydrocarbon chain closest to the phospholipid head so that the rest of the tail is flexible.

Antidioxidants

Provide protection from oxidative damage from reactive oxygen species like superoxides, hydroxyl radicals and hydrogen peroxide by scavenging reactive oxygen molecules or chemically reducing oxidized components.

Uric acid

Glutathione: the free thiol group provides protection from oxidative stress.

Vitamins:

A: a fat soluble vitamin derived from B-carotene.

C: a water soluble potent antidioxidant.

E: a fat soluble antidioxidant, effective at preventing the attack of peroxides on unsaturated fatty acids in membrane lipids.

Bile Salts

These are derived from cholesterol and help to digest and absorb fatty products. The top is non polar and the bottom polar. Cholic acid and chenodeoxycholic acid the main bile acids in humans. Cholic acid with glycine is called glycocholate and cholic acid with taurine is called taurocholate.