Lipids

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Chapter 8

• Lipids
• Biochemistry
• by
• Reginald Garrett and Charles Grisham

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Essential Question

• What is the structure, chemistry, and biological
  function of lipids?
• Lipids low solubility in water and high
  solubility in nonpolar solvents.
• The hydrophobic nature of lipids allows membranes
  to act as effective barriers to more polar
  molecules.

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Outline

• What Is the Structure and Chemistry of Fatty
  Acids?
• What Is the Structure and Chemistry of
  Triacylglycerols?
• What is the Structure and Chemistry of
  Glycerophospholipids?
• What Are Sphingolipids, and How Are They
  Important for Higher Animals?
• What Are Waxes, and How Are They Used?
• What Are Terpenes, and What Is Their Relevance to
  Biological Systems?
• What Are Steriods, and What Are Their Cellular
  Functions?

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Classes of Lipids

• All biological lipids are amphipathic (containing
  both polar and nonpolar groups).
• Fatty acids
• Triacylglycerols
• Glycerophospholipids
• Sphingolipids
• Waxes
• Isoprene-based lipids (including steroids)

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8.1 - Fatty acids

• Know the common names and structures for fatty
  acids up to 20 carbons long
• Saturated
• Lauric acid (12 C)
• Myristic acid (14 C)
• Palmitic acid (16 C)
• Stearic acid (18 C)
• Arachidic acid (20 C)

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Fatty acids - II

• Unsaturated fatty acids
• Palmitoleic acid (161)
• Oleic acid (181)
• Linoleic acid (182)
• ?-Linolenic acid (183)
• ?-Linolenic acid (183)
• Arachidonic acid (204)

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Fatty acids - III

• Structural consequences of unsaturation
• Saturated chains pack tightly and form more
  rigid, organized aggregates (i.e., membranes)
• Unsaturated chains bend and pack in a less
  ordered way, with greater potential for motion

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Fatty acids in Food saturated vs.
unsaturated Fatty acid compositions of some
dietary lipids
Source Lauric Myristic
Palmitic Stearic Oleic Linoleic Beef
5 24-32 20-25 37-43 2-3 Milk 25
12 33 3 Coconut 74 10 2
7 Corn 8-12 3-4 19-49
34-62 Olive 9 2 84
4 Palm 39 4 40
8 Safflower 6 3 13
78 Soybean 9 6 20
52 Sunflower 6 1 21 66
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8.2 - Triacylglycerols

• Also called triglycerides
• A major energy source for many organisms
• Why?
• Most reduced form of carbon in nature
• No solvation needed
• Efficient packing

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Triacylglycerols - II

• Other advantages accrue to users of
  triacylglycerols
• Insulation
• Energy without nitrogen
• Metabolic water

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8.3 - Glycerophospholipids

• Glycerophospholipids are phospholipids but not
  necessarily vice versa.
• Know the names and structures in Figure 8.6
• Understand the prochirality of glycerol
• Remember that, if phospholipid contains
  unsaturation, it is at the 2-position.

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Ether Glycerophospholipids

• An ether instead of an acyl group at C-1
• Figure 8.8

1-alkyl 2-acyl-phosphatidylethanolamine (an
ether glycerophospholipid)
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Ether Glycerophospholipids

• Platelet activating factor (PAF) is an ether
  glycerophospholipid.
• PAF is a potent biochemical signal molecule
• Note the short (acetate) fatty acyl chain at the
  C-2 position in PAF.
• PAF is much more water soluble, functions as a
  soluble messenger in signal transduction.

Figure 8.9 1-alkyl 2-acyl-phosphatidylcholine
(PAF)
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8.4 Sphingolipids ?????

• Base structure is sphingosine.
• Sphingosine is an 18-carbon amino alcohol.
• Ceramides are amide linkages of fatty acids to
  the nitrogen of sphingosine.
• Glycosphingolipids are ceramides with one or more
  sugars in beta-glycosidic linkage at the
  1-hydroxyl group.

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Sphingolipids

• Base structure is sphingosine.
• Sphingosine is an 18-carbon amino alcohol.
• Ceramides (????) are amide linkages of fatty
  acids to the nitrogen of sphingosine.
• Glycosphingolipids are ceramides with one or more
  sugars in beta-glycosidic linkage at the
  1-hydroxyl group.

Figure 8.11  Formation of an amide linkage
between a fatty acid and sphingosine produces a
ceramide.
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• Sphingolipids
• Backbone of sphingosine 18-carbon amino alcohol
  with C-C trans double bond
• ? Ceramide sphingosine with fatty acid in amide
  linkage
• ? Sphingomyelins alcohol esterified to
  phosphoceramide
• Choline
• Ethanolamine
• Glycosphingolipids ceramide with sugars in
  ?-glycosidic linkage
• Cerebroside Sugar is glucose or galactose.
• Sulfatide Sugar is galactose with sulfate
  esterified at carbon 3 of galactose.
• Gangliosides ceramide with three or more
  sugars
• including sialic acid

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8.5 - Waxes

• Esters of long-chain alcohols with long-chain
  fatty acids
• Highly insoluble
• Animal skin and fur are wax-coated
• Leaves of many plants
• Bird feathers

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8.5 Waxes

• Esters of long-chain alcohols with long-chain
  fatty acids
• highly insoluble
• Animal skin and fur are
• wax-coated .
• leaves of many plants
• bird feathers

Figure 8.15 An example of a wax. Oleoyl alcohol
is esterified to stearic acid in this case.
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8.6 - Terpenes

• Based on the isoprene structure
• Know nomenclature
• Understand linkage modes (Figure 8.16)
• See structures in Figure 8.17
• All sterols (including cholesterol) are
  terpene-based molecules.
• Steroid hormones are terpene-based.

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8.6 Terpenes

• Based on the isoprene structure
• Know nomenclature
• All sterols (including cholesterol) are
  terpene-based molecules.
• Steroid hormones are terpene-based.

Figure 8.16
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8.7 - Steroids

• Based on a core structure consisting of three
  6-membered rings and one 5-membered ring, all
  fused together.
• Cholesterol is the most common steroid in animals
  and precursor for all other steroids in animals.
• Steroid hormones serve many functions in animals
  - including salt balance, metabolic function and
  sexual function.

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An A-B cis steroid
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Figure 8.19 The structure of cholesterol, shown
with steroid ring designations and carbon
numbering.