Lipids Introduction to Lipids Lipids are biomolecules that are soluble in organic solvents and...

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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Lipids can be categorized as:

1. Hydrolyzable lipids can be converted into smaller molecules by hydrolysis with water.

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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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• 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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• Solid fats have a relatively high percentage of saturated fatty acids and are generally animal in origin.

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• 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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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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• One of the main types of phosphoacylglycerols is cephalin:

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• The second of the main types of phosphoacylglycerols 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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• 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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• 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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• 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 bloodstream 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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• 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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• 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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• 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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• Vitamin K regulates the synthesis of clotting proteins (prothrombin), and deficiency of this leads to excessive or fatal bleeding.

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Lipids Introduction to Lipids Lipids are biomolecules that are soluble in organic solvents and...

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