Chapter 21: Lipids

Chapter 21: Lipids

Triglyceride: A triester of glycerol with three fatty acids.• Hydrophobic (most of the molecule is non-polar)

• Two or three different fatty acid components are present

The Fatty acids:• Are unbranched carboxylic acids• 10 to 20 carbons long, even number of carbons• Cis-isomer

Chapter 21: Lipids

Hardening: The reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H2/transition metal catalyst, which converts a liquid triglyceride to a semisolid.• In practice, the degree of hardening is carefully controlled to

produce fats of a desired consistency.• The resulting fats are sold for kitchen use (Crisco, Spry, Dexo,

and others).• Margarine and other butter substitutes are produced by partial

hydrogenation of polyunsaturated oils derived from corn, cottonseed, peanut, and soybean oils.

• The hardening process is the source of trans fatty acids

Chapter 21: Lipids

Saponification: the base-promoted hydrolysis of fats and oils in aqueous NaOH and produces glycerol and a mixture of fatty acid salts called soaps.

O

CH2OCR

CH2OCR

RCOCH

O

O

CH2OH

CH2OH

HOCH RCO-Na+

O

A triglyceride 1,2,3-Propanetriol(Glycerol, glycerin)

+

Sodium soaps

3saponification

Chapter 21: Lipids

Phospholipids and Glycolipids

Chapter 21: Lipids

Complex lipids form the membranes around cells and around small structures within cells.In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers.• Polar (hydrophilic) head groups are in contact with the aqueous

environment.• Nonpolar (hydrophobic) tails are buried within the bilayer and

shielded from the aqueous environment.• The major force driving the formation of lipid bilayers is

hydrophobic interaction.• The arrangement of hydrocarbon tails in the interior can be rigid

(if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids).

Chapter 21: Lipids

Chapter 21: Lipids

Glycerolphospholipids

X = AlcoholR1 = Fatty acid tail (saturated or unsaturated)R2 = Fatty acid tail (this one is always unsaturated)

Chapter 21: Lipids

X = Choline

If the other alcohol is choline, the glycerophospholipid is called a phosphatidylcholine (common name lecithin).

Chapter 21: Lipids

X = ethanolamine or serine

If the other alcohol is ethanolamine or serine, the glycerophospholipid is called a cephalin.

Chapter 21: Lipids

Sphingolipids: Contain sphingosine instead of glycerol, from which this class of compounds in named. The sphingolipid myelin is found in the coatings of nerve axons.

OH

(CH2)12CH3

HO

NH2

SphingosineOH

(CH2)12CH3

HO

NHCR

A ceramide(an N-acylsphingosine)

OPOCH2CH2N(CH3)3

(CH2)12CH3

HO

NHCR

A sphingomyelinO

O-+

O O

Chapter 21: Lipids

GlycolipidGlycolipid:: a complex lipid that contains carbohydrates and ceramides.• The carbohydrate is either glucose or galactose.• The cerebrosides are ceramide mono- or oligosaccharides.• The following is a glucocerebroside.

O

(CH2)12CH3

HO

NHCR

O

H

HO

H

HO

H

HOH

H

OHO

a-glycosidic bond

a unit of -D-glucopyranose

a ceramide

Chapter 21: Lipids

Steroids: A group of plant and animal lipids that contain this tetracyclic ring structure

A B

C D

Cholesterol is the most abundant steroid in the human body, and also the most important.• It is a component in plasma membranes in all animal cells.• It is the precursor of all steroid hormones and bile acids.

HO

Chapter 21: Lipids

21-15

Steroid Hormones• Male sex hormonesMale sex hormones• Synthesized in the testes from cholesterol.• Responsible for the development of male secondary

sex characteristics.

AndrosteroneTestosteroneO

OH

H3C

H3C H3C

H3C

O

HO

21-16

Steroid HormonesAmong the synthetic anabolic steroids are:

O

H3C

H3C

MethandienoneO

CH3

H3CH3C

OHOH

CH3

MethenoloneO

H3C

H3CO

4-Androstene-3,17-dione

21-17

Steroid HormonesFemale sex hormones• Synthesized in the ovaries from progesterone.• Responsible for the development of female secondary

sex characteristics and control of the menstrual cycle.

Progesterone Estradiol

H3C

H3C

C=O

O

CH3OH

HO

H3Cseveralsteps

21-18

Steroid Hormones• Progesterone-like analogs are used in oral

contraceptives:

H3C

O

OHC CH

H3C

O

OHC CCH3

NH3C

CH3

"Nor" refers tothe absence of a methyl group here

Norethindrone

Mifepristone(RU486)

21-19

Glucocorticoid Hormones• Glucocorticoid hormonesGlucocorticoid hormones• Synthesized in the adrenal cortex.• Regulate metabolism of carbohydrates.• Decrease inflammation.• Involved in the reaction to stress.

Aldosterone

C=O

CH2OH

O

H

H3C H

H

CHHO

O

21-20

Bile SaltsBile saltsBile salts are oxidation products of cholesterol.• Synthesized in the liver, stored in the gallbladder, and

secreted into the intestine where they emulsify dietary fats and aid in their absorption and digestion.

HO

H3C

CH3

H3C

OH

HO NH

O

HO

H3C

CH3

H3C

OH

HO NH

O

SO32-

Glycocholate(from glycine)

Taurocholate(from taurine)

COO-

Chapter 21: Lipids

Lipoproteins: Lipoproteins: Carriers of cholesterol.

Chapter 21: Lipids

LipoproteinsLipoproteins

Chapter 21: Lipids

•Chylomicrons carry triglycerides (fat) from the intestines to the liver, skeletal muscle, and to adipose tissue.•Very low density lipoproteins (VLDL) carry (newly synthesised) triacylglycerol from the liver to adipose tissue.•Intermediate density lipoproteins (IDL) are intermediate between VLDL and LDL. They are not usually detectable in the blood.•Low density lipoproteins (LDL) carry cholesterol from the liver to cells of the body. LDLs are sometimes referred to as the "bad cholesterol" lipoprotein.•High density lipoproteins (HDL) collect cholesterol from the body's tissues, and bring it back to the liver. HDLs are sometimes referred to as the "good cholesterol" lipoprotein.

Chapter 21: Lipids

• LDL Normal plasma levels are 175 mg/100 mL.• If there are sufficient LDL receptors on the surface of cells, LDL is removed from circulation and its concentration in blood plasma drops.• In the disease called famelial hypercholesterolemia, there are not enough LDL receptors and plasma levels of cholesterol may be as high as 680 mg/100 mL.• The commonly used statin drugs inhibit the synthesis of cholesterol by blocking HMG-CoA reductase

LDL:LDL: