CHAPTER 10 LIPIDS

CHAPTER 10 LIPIDS

10.1 Storage Lipids

10.2 Structural Lipids in Membranes 10.3 Lipids as Signals, Cofactors, and

Pigments

10.4 Working with Lipids

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• Based on the structure and function

• Lipids that do not contain fatty acids: cholesterol, terpenes, …

• Lipids that contain fatty acids (complex lipids) – Storage lipids and membrane lipids

Classification of Lipids

‡The fats and oils used almost universally as stored forms of energy in living organisms are derivatives of fatty acids.

Fatty Acids Are Hydrocarbon Derivatives

‡Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C₄to C₃₆).

‡A simplified nomenclature for unbranched fatty acids specifies the chain length and number of double bonds, separated by a colon (Fig. 10–1a).

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10.1 Storage Lipids

FIGURE 10-1

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FIGURE 10–1 Two conventions for naming fatty acids.

TABLE 10-1

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‡The most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons.

‡The family of polyunsaturated fatty acids (PUFAs) with a double bond between the third and fourth carbon from the methyl end of the chain are of special importance in human nutrition.

‡PUFAs with a double bond between C-3 and C-4 are called omega-3 (ω-3) fatty acids, and those with a double bond between C-6 and C-7 are omega-6 (ω-6) fatty acids.

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FIGURE 10–2 The packing of fatty acids into stable aggregates.

FIGURE 10-2

Triacylglycerols Are Fatty Acid Esters of Glycerol

‡The simplest lipids constructed from fatty acids are the triacylglycerols, also referred to as triglycerides, fats, or neutral fats. Triacylglycerols are composed of three fatty acids each in ester linkage with a single glycerol (Fig. 10–3).

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FIGURE 10-7

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FIGURE 10–7 Some common types of storage and membrane lipids.

FIGURE 10-3

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FIGURE 10–3 Glycerol and a triacylglycerol.

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Triacylglycerols Provide Stored Energy and Insulation

‡In vertebrates, specialized cells called adipocytes, or fat cells, store large amounts of triacylglycerols as fat droplets that nearly fill the cell (Fig. 10–4a).

‡ Triacylglycerols are also stored as oils in the seeds of many types of plants, providing energy and

biosynthetic precursors during seed germination (Fig.

10–4b). Adipocytes and germinating seeds contain lipases, enzymes that catalyze the hydrolysis of stored triacylglycerols, releasing fatty acids for export to sites where they are required as fuel.

FIGURE 10-4(a) 4 guinea pig adipocytes

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Fat stores in cells

FIGURE 10-4(b)

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Figure 10–4(b) Cotyledon cell from a seed of Arabidopsis Protein

body oil body

‡Two significant advantages to using triacylglycerols as stored fuels:

(1) the carbon atoms of fatty acids are more reduced than those of sugars, and oxidation of triacylglycerols yields more than twice as much energy.

(2) Triacylglycerols are hydrophobic and therefore unhydrated, the organism that carries fat as fuel does not have to carry the extra weight of water of

hydration.

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Partial Hydrogenation of Cooking Oils Produces Trans Fatty Acids

‡Most natural fats, such as those in vegetable oils, dairy products, and animal fat, are complex mixtures of simple and mixed triacylglycerols. These contain a variety of fatty acids differing in chain length and degree of saturation (Fig. 10–5).

‡Many fast foods are deep-fried in partially

hydrogenated vegetable oils and therefore contain high levels of trans fatty acids (Table 10–2).

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FIGURE 10-5

p.348 FIGURE 10–5 Fatty acid composition of three food fats.

Trans Fatty Acids in Foods

• partial dehydrogenation of unsaturated fatty acids

• A trans double bond allows a given fatty acid to adopt an extended conformation.

• Trans fatty acids can pack more regularly, and show higher melting points than cis forms.

• Consuming trans fats increases risk of cardiovascular disease

„Avoid deep-frying partially hydrogenated vegetable oils

„Current trend: reduce trans fats in foods (Wendy’s, KFC)

TABLE 10-2

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Waxes

‡Waxes are esters of long-chain saturated and unsaturated fatty acids with long-chain alcohols

‡Insoluble and have high melting points

‡Variety of functions:

„Storage of metabolic fuel in plankton (浮游生物)

„Protection and pliability for hair and skin in vertebrates

„Waterproofing of feathers in birds

„Protection from evaporation in tropical plants and ivy

„Used by people in lotions, ointments, and polishes

Wax: the material of the honeycomb

‡Beeswax is a mixture of a large number of lipids, including esters of triacontanol, and a long-chain palmitic acid.

10.2 Structural Lipids in Membranes

‡In glycerophospholipids and some sphingolipids, a polar head group is joined to the hydrophobic moiety by a phosphodiester linkage; these are the phospholipids.

‡Other sphingolipids lack phosphate but have a simple sugar or complex oligosaccharide at their polar ends; these are the glycolipids(Fig. 10–7).

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FIGURE 10-7

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FIGURE 10–7 Some common types of storage and membrane lipids.

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Glycerophospholipids Are Derivatives of Phosphatidic Acid

‡Glycerophospholipids, also called phosphoglycerides, are membrane lipids in which two fatty acids are attached in ester linkage to the first and second carbons of glycerol.

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FIGURE 10–8 the backbone of phospholipids.

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FIGURE 10–9 Glycerophospholipids.

backbone of phospholipids

O

FIGURE 10-9 Part 2 _p.351

-5 (磷脂醯膽鹼)

‡The properties of head groups determine the surface properties of membranes

‡Different organisms have different membrane lipid head group compositions

‡Different tissues have different membrane lipid head group compositions

Examples of Glycerophospholipids:

Phosphatidylcholine

(磷脂醯膽鹼,卵磷脂)

‡Phosphatidylcholine is the major component of most eukaryotic cell membranes

‡Many prokaryotes, including E. coli cannot synthesize this lipid; their membranes do not contain

phosphatidylcholine

Some Glycerophospholipids Have Ether-Linked Fatty Acids

‡Some animal tissues and some unicellular organisms are rich in ether lipids, in which one of the two acyl chains is attached to glycerol in ether, rather than ester, linkage.

FIGURE 10-10

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Ether lipids:

Plasmalogen

‡Vinyl ether analog of phosphatidylethanolamine

‡Common in vertebrate heart tissue

‡Also found in some protozoa and anaerobic bacteria

‡Function is not well understood

„Resistant to cleavage by common lipases but cleaved by few specific lipases

„Increase membrane rigidity?

„Sources of signaling lipids?

„May be antioxidants?

FIGURE 10-10

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Ether lipids:

Platelet- activating factor

‡Aliphatic ether analog of phosphatidylcholine

‡Acetic acid has esterified position C2

‡First signaling lipid to be identified

‡Stimulates aggregation of blood platelets

‡Plays role in mediation of inflammation

Chloroplasts Contain Galactolipids and Sulfolipids

‡The second group of membrane lipids are those that predominate in plant cells: the

galactolipids

connected by

a glycosidic linkage to C-3 of a 1,2-diacylglycerol.

FIGURE 10-11

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Fig. 10–11 Three glycolipids of chloroplast thylakoid membranes

Archaea contain unique membrane lipids

‡Most of the archaea live in ecological extreme conditions (e.g. high temperatures, low pH, high ionic strength).

‡The have membrane lipids containing long-chain branched hydrocarbons linked at each end to glycerol through ether bonds.

‡Ether bonds are more stable to hydrolysis at low pH and high temperatures.

‡These archaea lipids are twice the length of phospholipids and sphingolipids, and can span the full width of the surface membrane.

FIGURE 10-12

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FIGURE 10–12 A typical membrane lipid of archaea.

Linked at each end to glycerol through ether bonds

General name:

glycerol dialkyl glycerol tetraethers

Sphingolipids Are Derivatives of Sphingosine

‡Sphingolipids (神經鞘脂質), the fourth large class of membrane lipids, also have a polar head group and two nonpolar tails

‡The backbone of sphingolipids is NOT glycerol.

The backbone of sphingolipids is a long-chain amino alcohol, sphingosine

‡A fatty acid is joined to sphingosine via an amide linkagerather than an ester linkage as usually seen in lipids

‡A polar head group is connected to sphingosine by a glycosidic or phosphodiester linkage

‡The sugar-containing glycosphingolipids are found largely in the outer face of plasma membranes

Glycerol backbone Sphingosine backbone

‡There are three subclasses of sphingolipids.

(1) Sphingomyelins (神經鞘磷脂質)contain

phosphocholine or phosphoethanolamine as their polar head group and are therefore classified along with glycerophospholipids as phospholipids (Fig. 10–7).

Sphingomyelin is abundant in myelin sheath(髓鞘)that surrounds some nerve cellsin animals

(2) Glycosphingolipids, which occur largely in the outer face of plasma membranes, have head groups with one or more sugars connected directly to the ─OH at C-1 of the ceramide moiety

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Sphingolipids

(3) Gangliosides, the most complex sphingolipids, have oligosaccharides as their polar head groups and one or more residues of N-acetylneuraminic acid (Neu5Ac), a sialic acid, at the termini.

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FIGURE 10-7

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FIGURE 10–7 Some common types of storage and membrane lipids.

Glycosphingolipids and Blood Groups

‡The blood groups are determined in part by the type of sugars located on the head groups in glycosphingolipids.

‡The structure of sugar is determined by a expression of specificglycosyltransferases

„Individuals with no activeglycosyltransferase will have the O antigen

„Individuals with a glycosyltransferase that transfers an N- acetylgalactosaminegroup have A blood group

„Individuals with a glycosyltransferase that transfers a galactosegroup to phosphate will have B blood group

N-acetylgalactosamine

galactose

Phospholipids and sphingolipids are degraded in lysosome

‡For each hydrolyzable bond in

glycerophospholipids, there is a specific hydrolytic enzyme in lysosome.

‡Phospholipasesof the A type remove one of the two fatty acids (These esterasesdo not attach the ether bond of plasmalogens), producing a

lysophospholipid. Lysophospholipases remove the remaining fatty acid.

‡Gangliosides are degraded by a set of lysosomal enzymes the remove sugar units. A genetic defect leads to accumulation of gangliosides in the cell with severe medical consequences. (see box 10-2)

FIGURE 10-16

p.355 FIGURE 10–16 The specificities of phospholipases.

Brain cell of Tay-Sachs disease

Sterols(固醇) Have Four Fused Carbon Rings

‡Sterolsare structural lipids present in the membranes of most eukaryotic cells. The characteristic structure of this fifth group of membrane lipids is the steroid (類固醇) nucleus, consisting of four fused rings, three with six carbons and one with five (Fig. 10–17).

‡Cholesterol (膽固醇), the major sterol in animal tissues, is amphipathic, with a polar head group (the hydroxyl group at C-3) and a nonpolar hydrocarbon body, about as long as a 16-carbon fatty acid in its extended form.

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FIGURE 10-17

p.355 FIGURE 10–17 Cholesterol.

‡Bile acids (膽酸)are polar derivatives of cholesterol that act as detergents in the intestine, emulsifying dietary fats to make them more readily accessible to digestive lipases.

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Physiological Role of Sterols

‡Cholesteroland related sterols are present in the membranes of most eukaryotic cells.

„Modulate fluidity and permeability

„Thicken the plasma membrane

„Most bacteria lack sterols

‡Mammals obtain cholesterol from foodand synthesizeit de novo in the liver

‡Cholesterol, bound to proteins, is transported to tissues via blood vessels

„Cholesterol in low-density lipoproteinstends to deposit and clog arteries

‡Many hormonesare derivatives of sterols

Steroid Hormones

‡Steroids are oxidized derivatives of sterols

‡Steroids have the sterol nucleus, but lack the alkyl chain found in cholesterol. This makes them more polarthan cholesterol.

‡Steroid hormones are synthesized in gonads and adrenal glands from cholesterol

‡They are carried through the body in the blood stream, usually attached to carrier proteins

‡Many of the steroid hormones are male and female sex hormones

FIGURE 10-19

p.359 FIGURE 10–19 Steroids derived from cholesterol.

10.3 Lipids as Signals, Cofactors, and Pigments

Eicosanoids Carry Messages to Nearby Cells

Ecosanoids are paracrine hormones, are are present in small amounts but play vital roles as signaling molecules

between nearby cells

‡All eicosanoids are derived from arachidonic acid (花

生酸)

(1) Prostaglandins(PG) contain a five-carbon ring originating from the chain of arachidonic acid.

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(2)Thromboxanes have a six-membered ring containing an ether. They are produced by platelets (also called thrombocytes) and act in the formation of blood clots and the reduction of blood flow to the site of a clot.

(3)Leukotrienescontain three conjugated double bonds.

• Variety of functions:

Functions:

• Inflammation and fever (prostaglandins)

• Formation of blood clots (thromboxanes)

• Smooth muscle contraction in lungs (leukotrienes)

• Smooth muscle contraction in uterus (prostaglandins)

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Vitamins A and D Are Hormone Precursors

‡Vitamin D₃, also called cholecalciferol, is normally formed in the skin from 7-dehydrocholesterol in a photochemical reaction driven by the UV component of sunlight (Fig. 10–20a).

‡Vitamin A (retinol), in its various forms, functions as a hormone and as the visual pigment of the vertebrate eye (Fig. 10–21).

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FIGURE 10-20

FIGURE 10–20 Vitamin D₃production and metabolism.

FIGURE 10-21

p.361 FIGURE 10–21 Vitamin A₁and its precursor and derivatives.

Vitamins E and K and the Lipid Quinones Are Oxidation- Reduction Cofactors

‡Vitamin Eis the collective name for a group of closely related lipids called tocopherols, all of which contain a substituted aromatic ring and a long isoprenoid side chain (Fig. 10–22a).

‡The aromatic ring of vitamin K(Fig. 10–22b) undergoes a cycle of oxidation and reduction during the formation of active prothrombin, a blood plasma protein essential in blood clotting.

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FIGURE 10-22(a-c)

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FIGURE 10-22(d-f)

p.362 FIGURE 10–22 Some other biologically active isoprenoid

compounds or derivatives.

FIGURE 10-23

p.363 FIGURE 10–23 Lipids as pigments in plants and bird feathers.

10.4 Working with Lipids

‡Some methods commonly used in lipid analysis are shown in Figure 10–24.

(1) Lipid Extraction Requires Organic Solvents (2) Adsorption Chromatography Separates Lipids of Different Polarity

(3) Gas-Liquid Chromatography Resolves Mixtures of Volatile Lipid Derivatives

(4) Specific Hydrolysis Aids in Determination of Lipid Structure

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FIGURE 10–24 Part 1

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FIGURE 10–24 Part 2

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FIGURE 10–24 Part 3

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Common

procedures in the extraction, separation, and identification of cellular lipids.

(5) Mass Spectrometry Reveals Complete Lipid Structure (6) Lipidomics Seeks to Catalog All Lipids and Their Functions

„The structural lipids of membranes include both glycerophospholipids and sphingolipids, separate

categories in Table 10–3. Each method of categorization has its advantages.

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TABLE 10-3

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