Chapter 10

Lipids

Lipids

Characteristics

Water insolubility

Chemical diversity

Biological functions

Energy storage; fats & oils

Components of biological membrane; phospholipids & sterols

Enzyme cofactors, electron carriers, light-absorbing pigments, molecular anchors, emulsifying agent, hormones, intracellular messengers

1. Storage lipids2. Structural lipids3. Biologically active lipids

10.1 Storage Lipids

“Cellular oxidation of fatty acids is highly exergonic”

Fatty Acids

Fatty acids Carboxylic acids with hydrocarbon chains

C4 to C36

Saturated or unsaturated (one or more double bonds)

A few contain C3 ring, -OH, -CH3 branches

Nomenclature Chain length: # of double bonds ( positions of double bonds)

Common Fatty Acids

Even number of carbon atoms Common patterns of double bonds

Monounsaturated fatty acids: 9

Polyunsaturated fatty acids : 9, 12, 15

No conjugated patterns (separated by methylene group)

cis configuration of double bonds

Trans fatty acids Fermentation in the rumen of dairy animals

Partial hydrogenation of vegetable oils

Increase blood levels of LDL (bad cholesterol) and decrease HDL (good cholesterol)

Omega-3 & -6 PUFA

Omega-3 PUFA -linolenic acid ALA; 18:3(9,12,15)

Essential PUFA in human

; synthesis of eicosapentaenoic acid (EPA; 20:5(5,8,11,14,17)) & docosahexaenoic acid (DHA; 22:6(4,7,10,13,16,19))

Supply via the diet (rich in vegetable & fish oils) Maintenance of the optimal dietary ratio of -6 to -3 PUFA

critical for health

Omega-6 PUFA

Physical Properties of Fatty Acids

Length & unsaturation degrees of hydrocarbon chain Water solubility

Longer chain lower solubility Polar carboxylic group a little solubility

Melting point Depending on the degrees of packing Saturated fatty acid ; tight packing (waxy or cystalline arrays) high melting point Unsaturated fatty acid ; loosely packing by kinks of cis double bond (oily liquid) low melting point

Triacylglycerol (triglycerides, fats, neutral fats)

3 fatty acids each in ester linkage with a single glycerol Simple triacylglycerols

Same 3 fatty acids Tripalmitin (16:0), tristearin (18:0), triolein (18:1)

Mixed triacylglycerols (most natural triacylglycerols)

Nonpolar & hydrophobic

Triacylglycerol (triglycerides, fats, neutral fats)

Function Energy storage

Lipid droplets in cytosol Adipocytes in vertebrate & seeds in plants

Lipase hydrolysis of stored TAG & release of fatty acids

Advantages of TAG as stored fuels High energy yield No extra weight of water of hydration

Fat tissues; under the skin, in the abdominal cavity & mammary glands

Insulation against low temperature Seals, penguins, hibernating animals Dual functions of energy storage & insulation

Matching the buoyancy of the body Sperm whale Spermaceti oil in the head

Increase in density during cold deep sea diving

Triacylglycerols in Food

Triacylglycerols in food Vegetable oils

TAG with unsaturated fatty acids Liquids at room temperature

Animal fats High content of TAG with saturated fatty

acids Solids at room temperature

Oxidation of fats in air Oxidative cleavage of double bonds

Generation of aldehydes and carboxylic acids of shorter chain length volatile rancidity

Partial hydrogenation (cis double bonds single bond)

increase shelf-life; margarine

harmful effect; formation of trans fatty acids

Waxes

Structure Esters of long-chain (C14 to C36)

saturated and unsaturated fatty acids with long-chain (C16 to C30) alcohols

Properties High melting points (60 to 100 oC) Water repellent Firm consistency

Functions In plankton : energy storage Water repellent

Protection of hair in vertebrates, feather in birds, and leaves in plants

Industrial applications Lotions, ointments, polishes

10.2 Structural Lipids in Membranes

Types of Membrane Lipid

Amphipathic Hydrophobic chain

Hydrophilic head Phosphate: phospholipid Oligosaccharide : glycolipids

Glycerophospholipids (Phosphoglycerides)

Ester linkage to glycerol Two fatty acids

C-1 : C16 or C18 saturated fatty acid C-2: C18 to C20 unsaturated fatty acid

Highly polar or charged group via phosphodiester linkage Wide variety

Glycerophospholipids (Phosphoglycerides)

Ether-Linked Fatty Acids

Ether lipids One of the acyl chains has ether linkage Plasmalogens

~ 50% of phospholipids in heart tissue

Platelet-activating factor Signal molecule released from basophils (leukocytes)

Aggregation of platelet Release of serotonin (vasoconstrictor) from platelets Regulation of inflammation & allergic response

Galactolipids and Sulfolipids

Galactolipid 1 or 2 galactose to C-3 via glycosidic linkage Localized in the thylakoid membranes of chloroplasts 70% to 80% of the total membrane lipids in vascular plants Most abundant membrane lipids in the biosphere

Sulfolipid Slufonated glucose to C-3 via glycosidic linkage

Membrane Lipids of Archaebacteria

Glycerol dialkyl glycerol tetraethers (GDGT)

Ether linkages More resistant to hydrolysis at low pH and high

temperature than ester bonds

Polar head at each end

Sphingolipids

General structure Spingosine : long-chain amino alcohol One long-chain fatty acid (amide linkage to –NH2 on C-2) Polar head group ; glycosidic or phosphodiester linkage

Ceramide; structural parent of all sphingolipids

Sphingolipids

3 subclasses of sphingolipids (polar head group) Sphingomyelins

Phosphocholine or phosphoethanolamine Prominent in myelin sheath, the insulator of

axons

Glycosphingolipids Neutral glycolipids

Cerebrosides ; single sugar: Gal (neural tissue), Glc (non-neural tissue)

Globoside : Two or more sugars Gangliosides

Oligosaccharides with one or more N-acetylneuraminic acid (sialic acid) negatively charged ganglioside

» GM: 1 sialic acid, GD: 2 sialic acid etc.

Sphingolipids

Sphingolipids for Biological Recognition

Sphingolipids Named after Sphinx because of

their enigmatic functions

Functions of sphingolipids Largely undiscovered

Blood typing Glycosphingolipid

Signal recognition Gangliosides; concentrated

in the outer surface of cells Embryonic development Induction of tumor

Degradation of Phospholipids and Sphingolipids

Degradation in lysosome Phospholipids

Phospholipase Phospholipase A, C, D Lysophospholipase

Gangliosides

Stepwise removal of sugar units

Ceramide as a final product

Sterols

Structure Sterol nucleus; 4 fused rings Planar & rigid

3 with C6 and 1 with C5

Types Cholesterol; Major sterol in animal

Amphipathic polar head + nonpolar hydrocarbon body

Stigmasterol; In plant Ergosterol; In fungi

Functions Membrane structure Precursors

Steroid hormones Bile acids: detergents in the intestine