LipidsIntroduction to Lipids
• Lipids are biomolecules that are soluble in organic solvents and insoluble in water.
• They are defined on the basis of a physical property not by a particular functional group, thus they
have a variety of structures and functions.
• They contain many nonpolar C—C and C—H bonds and few polar bonds resulting in their water insolubility.
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LipidsIntroduction to Lipids
Lipids can be categorized as:
1. Hydrolyzable lipids can be converted into smaller molecules by hydrolysis with water.
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LipidsIntroduction to Lipids
Lipids can be categorized as:
2. Nonhydrolyzable lipids cannot be cleaved into smaller units by aqueous hydrolysis.
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Fatty Acids
• Hydrolyzable lipids are derived from fatty acids.
• Fatty acids are carboxylic acids (RCOOH) with long C chains of 12-20 C atoms.
• An example is CH3(CH2)14COOH (palmitic acid):
nonpolar portion = hydrophobic
polar portion = hydrophillic
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Fatty Acids
• Naturally occurring fatty acids have an even number of C atoms.
• Saturated fatty acids have no double bonds in their long hydrocarbon chains.
• Unsaturated fatty acids have 1 or more double bonds (generally cis) in their long hydrocarbon chains.
• As the number of double bonds in the fatty acid increases, the melting point decreases.
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Fatty Acids
• Stearic acid (mp 71 oC) is a saturated fatty acid.
• Oleic acid (mp 16 oC) is an unsaturated fatty acid.
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Fatty Acids
• Linoleic and linolenic acids are essential fatty acids; they cannot be produced by the body and must be consumed.
• Linoleic acid is called an omega-6 acid, because of the position of the first C=C in the nonpolar chain.
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Fatty Acids
• Linolenic acid is called an omega-3 acid, because of the position of the first C=C in the nonpolar chain.
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Waxes
• Waxes are esters (RCOOR’) formed from a fatty acid and a high molecular weight alcohol.
• General structure of waxes:
• General formation of waxes:
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Waxes
• For example, shown below is the formation of spermaceti wax, isolated from the heads of sperm whales.
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Waxes
• Waxes form a protective coating on the feathers of birds to make them water repellent, and on leaves to prevent water evaporation.
Beeswax (myricyl palmitate):
CH3(CH2)14 C
O
O(CH2)29CH3
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• Beeswax contains the wax myricyl palmitate as a major component.
Waxes
• Like other esters, waxes are hydrolyzed with water in the presence of acid or base to re-form the carboxylic acid and alcohol they came from.
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Triacylglycerols—Fats and Oils
• Triacylglycerols (triglycerides) are triesters formed from glycerol and three molecules of fatty acids.
• Below is a generalized block diagram of a triacylglycerol:
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Triacylglycerols—Fats and Oils
• The general reaction for the formation of a triacylglycerol is:
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Triacylglycerols—Fats and OilsGeneral Features
• Triacylglycerols may be composed of three identical fatty acid side chains, or from two or three different fatty acids.
• Animal fats and vegetable oils, the most abundant lipids, are triacylglycerols with different physical properties.
• The fatty acids may be saturated or unsaturated.
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Triacylglycerols—Fats and OilsGeneral Features
• Fats have higher melting points; they are solids at room temperature.
• Oils have lower melting points; they are liquids at room temperature.
• Fats are derived from fatty acids with few double bonds.
• Oils are derived from fatty acids having a larger number of double bonds.
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Triacylglycerols—Fats and OilsGeneral Features
• Solid fats have a relatively high percentage of saturated fatty acids and are generally animal in origin.
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Triacylglycerols—Fats and OilsGeneral Features
• Liquid oils have a higher percentage of unsaturated fatty acids and are generally vegetable in origin.
• In the unsaturated lipid, a cis double bond places a kink in the side chain, making it more difficult to pack efficiently in the solid state, thus leading to a lower melting point.
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Triacylglycerols—Fats and OilsGeneral Features
Anunsaturated
triacylglycerol:
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Focus on Health & MedicineFats and Oils in the Diet
• Unsaturated triacylglycerols lower the risk of heart disease by decreasing the level of cholesterol in the blood.
• Triglycerols formed from omega-3 fatty acids are very helpful in lowering the risk of a heart attack.
• However, if the double bond of the unsaturated triacylglycerol is trans, the beneficial effect is lost.
• Trans fats, which are primarily synthesized instead of naturally occurring, act like saturated fats and increase the cholesterol levels in the blood.
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Focus on Health & MedicineFats and Oils in the Diet
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Hydrolysis of Triacylglycerols
• Triacylglycerols are hydrolyzed with water in the presence of acid, base, or enzymes (in the body).
C
O
C
O
C
O
CH2—O
CH— O
CH2—O
The 3 bonds that break
are drawn in red.
H2SO4
CH2—OH
CH— OH
CH2—OH
glycerol
+ C
O
(CH2)16CH3HO
3 stearic acids
3
(CH2)16CH3
(CH2)16CH3
(CH2)16CH3
+ 3 H2O
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Hydrolysis of TriacylglycerolsSoap Synthesis
• Soaps are metal salts of fatty acids prepared by basic hydrolysis (saponification) of a triacyl-glycerol.
polar headionic end
nonpolar tailnonpolar end 23
Hydrolysis of TriacylglycerolsSoap Synthesis
C
O
C
O
C
O
CH2—O
CH —O
CH2—O
H2O
CH2—OH
CH —OH
CH2—OH
glycerol
+ C
O
RNa+ −O
3 soapmolecules
3
R
R
R
+ 3 NaOH
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• The nonpolar tails dissolve grease and oil and the polar head makes it soluble in water.
Phospholipids
• Phospholipids are lipids that contain a P atom.
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• Phosphoacylglycerols are the most common phospholipid, they are the principal component of most cell membranes.
• Structurally, they resemble a triacylglycerol, except the third fatty acid has been replaced with a phosphodiester bonded to an alcohol.
PhospholipidsPhosphoacylglycerols
• There are two main types of phosphoacylglycerols that differ in the identity of the R“ group in the phosphodiester.
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PhospholipidsPhosphoacylglycerols
• One of the main types of phosphoacylglycerols is cephalin:
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PhospholipidsPhosphoacylglycerols
• The second of the main types of phosphoacyl- glycerols is lecithin:
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Focus on Health & MedicineCholesterol, the Most Prominent Steroid
• Steroids are a group of lipids whose carbon skeletons contain several fused rings:
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Focus on Health & MedicineCholesterol, the Most Prominent Steroid
• Cholesterol, the most prominent steroid, is synthesized in the liver and found in almost all body tissues.
• It is obtained in the diet from many sources, including meat, cheese, butter, and eggs.
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Focus on Health & MedicineCholesterol, the Most Prominent Steroid
• Elevated levels of cholesterol in the bloodstream lead to coronary artery disease, heart attack, etc.
• Cholesterol is insoluble in the aqueous medium of blood.
• It is transported through the bloodstream by lipoproteins, aggregates of phospholipids and proteins.
• Low-density lipoproteins (LDLs) transport cholesterol from the liver to the tissues.
• High-density lipoproteins (HDLs) transport cholesterol from tissues back to the liver.
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Focus on Health & MedicineCholesterol, the Most Prominent Steroid
• LDLs deposit cholesterol on the walls of arteries when they carry more than is needed to form cell membranes.
• This forms plaque, which restricts blood flow; thus, LDL cholesterol is called “bad” cholesterol.
• HDLs reduce the level of cholesterol in the blood- stream by bringing excess back to the liver; HDL cholesterol is called “good” cholesterol.
• Recommended levels are: HDL > 40 mg/dL, LDL < 100 mg/dL, total serum cholesterol < 200 mg/dL.
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Steroid Hormones
• A hormone is a molecule that is synthesized in one part of an organism, which then elicits a response at a different site.
• Two important classes of steroid hormones include sex hormones and adrenal cortical steroids.
• The female sex hormones are estrogens and progestins.
• The male sex hormones are called androgens.
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• The estrogens estradiol and estrone control development of secondary sex characteristics, regulate the menstrual cycle, and are made in the ovaries.
Steroid HormonesFemale Sex Hormones: Estrogens
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Steroid HormonesFemale Sex Hormones: Progestin
• The progestin progesterone is called the “pregnancy hormone”; it is responsible for the preparation of the uterus for implantation of a fertilized egg.
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Steroid HormonesMale Sex Hormones: Androgens
• Testosterone and androsterone are androgens made in the testes.
• They control the development of secondary sex characteristics in males.
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Steroid HormonesAnabolic Steroids
• Synthetic androgen analogues, called anabolic steroids, promote muscle growth.
• They have the same effect as testosterone, but are more stable, so they are not metabolized as quickly.
• They have come to be used by athletes and body builders, but are not permitted in competitive sports.
• Prolonged use of anabolic steroids can cause physical and psychological problems.
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Steroid HormonesAnabolic Steroids
• Some examples of anabolic steroids:
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Steroid HormonesAdrenal Cortical Steroids
• Three examples of adrenal cortical steroids are:
aldosterone cortisone cortisol
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Steroid HormonesAdrenal Cortical Steroids
• Aldosterone regulates blood pressure and volume by controlling the concentration of Na+ and K+
in body fluids.
• Cortisone and cortisol serve as anti-inflammatory agents, which also regulate carbohydrate metabolism.
• Prolonged use of these steroids can have undesired side effects, including bone loss and high blood pressure.
• Prednisone, a synthetic alternative, has similar anti-inflammatory properties.
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Focus on Health & MedicineFat-Soluble Vitamins
• Vitamins are organic compounds required in small quantities for normal metabolism and must be obtained from the diet.
• Vitamins are either water soluble or fat soluble.
• The four fat-soluble vitamins (A, D, E, and K) are found in fruits, vegetables, fish, liver, and dairy products.
• They are stored in adipose cells to be used when needed.
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Focus on Health & MedicineFat-Soluble Vitamins
• Vitamin A is found in liver, fish, and dairy products, and is made from β-carotene.
• It is needed for vision and for healthy mucous membranes.
• Vitamin A deficiency causes night blindness and dry eyes and skin.
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Focus on Health & MedicineFat-Soluble Vitamins
• Vitamin D can be synthesized from cholesterol.
• It can be obtained in the diet from many foods, especially milk, and helps regulate Ca+ and K+
metabolism.
• A deficiency of vitamin D causes rickets (bone malformation).
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Focus on Health & MedicineFat-Soluble Vitamins
• Vitamin E is an antioxidant, protecting unsaturated side chains in fatty acids from unwanted oxidation.
• Deficiency of vitamin E causes numerous neurological problems, although it is rare.
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Focus on Health & MedicineFat-Soluble Vitamins
• Vitamin K regulates the synthesis of clotting proteins (prothrombin), and deficiency of this leads to excessive or fatal bleeding.
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