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Lipids

• Lipids in biological systems can be hydrophobic or amphipathic.• Hydrophobic lipids, not surprisingly, are hydrophobic.• Amphipathic lipids have a polar group (head) and a hydrophobic group (tail) and areutilized in membranes. If the polar group has a carboxylic acid moiety, this is a fatty acid.

• Fatty acids can be saturated or unsaturated. • Saturated fatty acids have only single carbon-carbon bonds.• Unsaturated fatty acids have at least one double carbon-carbon bond.

• Monoumsaturated fatty acids have one double carbon-carbon bond• Polyunsaturated fatty acids have more than one double carbon-carbon bond.

Chapter 8 questions: 5,6,7,10,15,16,17,18,20,21,22.

Also, how does the fact that many glycerophospholipids are made of unsaturated fatty acids affect the physical properties of the fatty acids and the membrane lipids to which they are esterified?

Lipid nomenclature

•  Lipids have common names , e.g., myristic acid.

•  The systematic name takes into account the number of carbons in the hydrophobic chain, e.g., tetradodecanoic acid for the 14 carbons of myristic acid.

•  The symbol nomenclature takes into account the number of carbons in the chain and the number of unsaturated bonds. Myristic acids is described by 14:0, meaning there are 14 carbons and no unsaturated bonds.

•  Linoleic acid has 18 carbons and two double bonds, one between C9 and C10, and the other between C12 and C13. The systematic name is 9,12-octadecadienoic acid. The symbol nomenclature is 18:2 (9,12).

•  Carbon counting starts at the carboxylic acid carbon in the delta counting system. Carbon counting starts at the carbon end in the omega counting system. Thus linoleic acid is a delta-9,12 or an omega-6,9 fatty acid.

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Lipid structure

On the right are C18 fatty acids. All double bonds have a cis configuration.In membranes, most of the fatty acids are saturated becausethe double bond causes a severe“kink” in the molecules thatprevents close packing. Nevertheless, some flexibilitymust be maintained in biologicalmembranes.

Lipid structure

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Triacylglycerols• Function as energy storage molecules, not as membrane components. • Triacylglycerols are a main component of fats and oils. • Different oils contain different types of triacylglycerols.• Fats are used for energy storage because they are are less oxidized than carbohydrates.When completely oxidized, they yield more energy than carbohydrates.

Glycerophospholipids (1)Also called phosphoglycerides. These are the major components of biological membranes.The molecules are glycerol-3-phosphate, with the C1 and C2 positions esterified with fattyacids. Also, the phosphate group on C3 sometimes is coupled to another group.If the C3 phosphate is not coupled to another group, the molecule is phosphatidic acid, whichis not found in large amounts in biological membranes.

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Glycerophospholipids (2)

A common constituent of membranes is 1-stearoyl-2-oleoyl-phosphatidylcholine.One of the three chains is unsaturated, which is typical for biological membranes.

Glycerophospholipids (3)

• Lung surfactant is the major fat component in the lungs. Surfactant is the glycerophospholipiddipalmitoyl phosphatidylcholine (DPPC). This glycerophospholipid has only saturated palmitoyl chains, which allow the molecules to pack very closely in a single layer. The polar heads point towards the alveolar cells, whereas the non-polar tails point towards the inhaled air.

• The close packing of the non-polar tails prevents the alveoli from ‘collapsing” on each other after exhalation. This is an efficient design because if the lungs had collapsed after each breath, a lot of energy would have been required to inflate them after each exhalation.

• Premature infants, in general, do not have the appropriate amount of surfactant, so they getput on ventilators. Surfactant can be pumped into the lungs to prevent the alveoli from collapsing. Adults also can develop breathing disorders where surfactant is lacking. This also can be treated by introducing surfactant into the lungs.

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Glycerophospholipids (4)• Phospholypases can hydrolyze glycerophospholipids. • Snake and bee venoms contain phospholypase A2, an enzymes that cleaves glycerophospholipids and causes the release of lysophospholipids (aka lysolecithin). These products can insert into membranes and act as detergents. • Lysophospholipids in the blood stream can lyse red blood cell and cause death.• Other phospholypases cleave the glycerophospholipid at different positions.

Model of phospholipase A2 bound to a glycerophospholipid substrate

The cobra venom enzyme structure was determined using X-ray crystallography.The phosphate group of thelipid can fit precisely within the active site of the enzyme.The glycerophospholipidis modeled into the structure,A calcium ion, a cofactorfor the cleavage reaction, isshown in magenta.

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Plasmalogens - platelet activating factorPAF is a representative of ether glycerophospholipids, also known as plasmalogens. The attachment between the glycerol moiety C1 and the R1 hydrocarbon chain (C16 in PAF)is via an ether linkage. In addition, the acyl group attached to the C2 in PAF is an acetate. This makes the molecule more water-soluble than glycerophospholipids, thus it can function as a messenger in intercellular signal transduction.

Sphingolipids• A major component of many membrane types. They are derived from sphingosine (not glycerol) but their dimensions and charge distribution are very similar to the glycerophospholipids.• Sphingomyelin is found as an insulating sheath around nerve cell axons.• Cerebrosides contain a single sugar residue attached to the head group.• Gangliosides contain oligosaccharides attached to the head group.

phosphocholinehead group

sphingomyelin

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Sphingolipids

Electron micrograph of myelinated nerve fibers. There are 10-15 myelin layers surroundingeach axon, seen here in cross section. The myelin layers serve as electrical insulators. Defects in myelination result in neurological deficiencies such as multiple sclerosis.

Steroids (1)

The most abundant (and much maligned) steroid is cholesterol. It is made of 4 non-planar rings and a C3 hydroxyl group.

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Steroids (2)Cholesterol functions as a precursor to steroid hormones.Glucocorticoids affect many biological functions including inflammatory responses, mentalstress management, and carbohydrate, protein, and lipid metabolism. Example: cortisol.Aldosterones and mineracorticoids regulate salt balance, includingexcretion of salts bythe kidneys.Androgens and estrogens regulate sexual development and function.Examples: testosterone,β-estradiol (estrogen),progesterone.

Steroids (3)

• Many diseases are associated with steroid malfunction. Cholesterol-related heart disease isan obvious example.

• Vitamin D is a cholesterol derivative. The various forms of vitamin D, especially D2 and D3, regulate calcium absorption from food. Vitamins D2 and D3 are formed by exposure to sun light (although sun light does not contain vitamin D), which triggers a non-enzymatic conversion of a cholesterol derivative into vitamin D precursors. These precursors then become hydroxylated, thereby activated, in the kidneys and liver.

• In the absence of adequate amounts of active vitamin D, calcium cannot be absorbed efficiently from the diet, and this results in a disease called rickets. Rickets is characterized by bone malformation, bone softness, and tooth brittleness.

• Excessive vitamin D can cause high serum calcium levels, which can lead to kidney stones,kidney failure, and calcification of soft tissue. It has been suggested that the increase in skin pigmentation in populations living near the equator is a protective mechanism against excessive activation of vitamin D.

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Vitamin D synthesis - production of 1,25 dihydroxycholecalciferol

HO

CH3

A B

C D

CH3R

UV irradiation

HO

CH2

A B

C D

CH3R

In Vitamin D3 , R =

In Vitamin D2 , R =

C25 becaomes hydroxylated in the liver and C1 becomes hydroxylated in the kidney to yield 1,25-dihydroxycholecalciferol

HO OH

CH2

CH3

OH