18_Lipids.pdf

CHAPTER 18

Structure and Classification

•

A lipid is an organic compound found in living organisms that

is insoluble (or only sparingly soluble) in water but soluble in

non-polar organic solvents.

•

Classification: They are classified on the basis of solubility not

on any functional groups

• Insoluble or sparingly soluble in water

5 Categories of Lipids

Energy –

Storage lipids

• Triacylglycerols

Membrane

Lipids

• Phospholipids

• Sphingoglycolipids

• Cholesterol

Emulsification

Lipids

• Bile Acids

Messenger

Lipids

• Steroid hormones

• Eicosanoids

Protective

-Coating

Lipids

Fatty Acid: Basic Lipid Building Block

•

Carboxylic acids with linear (unbranched) carbon chain

-Fatty acids are naturally occuring monocarboxylic acids

• Even # of Carbon atoms:

• Long chain fatty acids: C₁₂ - C₂₆

• Medium chain fatty acids: C₆ - C₁₁

• Short-chain fatty acids: C₄ - C₅

• Two Types:

• Saturated - all C-C bonds are single bonds (SFA’s)

• Unsaturated

• Monounsaturated: one C=C bond (MUFA’s)

Saturated Fatty Acids (SFAs)

•

Numbering starts from the end of -COOH group

•

See structural notation: it indicates number of C atoms

•

Example - Lauric acid has 12 C atoms and no double bonds

so it is (12:0)

O OH

Monounsaturated Fatty Acids (MUFAs)

Polyunsaturated Fatty Acids (PUFAs)

•

Two or more carbon–carbon double bonds are present.

•

Up to six double bonds are found in biochemically important

PUFAs.

•

Two types of unsaturated fatty acids.

• Omega (ω)-3 fatty acids - An unsaturated fatty acid with its

endmost double bond three carbon atoms away from its methyl end.

• Omega(ω)-6 fatty acid is an unsaturated fatty acid with its endmost

double bond six carbon atoms away from its methyl end.

• Numbering starts from the other end of COOH

• See structural notation: it indicates number of C atoms

Physical Properties of Fatty Acids

•

Water solubility: Short chain fatty acids have some solubility

whereas long chain fatty acids are insoluble

• Short chain fatty acids are sparingly soluble because of carboxylic acid polar group

•

Physical properties such as melting point depends on the

number of C atoms and degree unsaturation

•

The number of bends in a fatty acid chain increase as the

number of double bonds increase

• Less packing occurs

• Melting point is lower

Energy-Storage Lipids:

Triacylglycerols

•

With the notable exception of nerve cells, human cells store

small amounts of energy providing materials:

• The most widespread energy storage material -carbohydrate glycogen

• Present in small amounts

•

Most abundant storage material is the triacylglycerols:

• Triacylglycerols are concentrated primarily in adipose tissue (adipocytes)

• Nearly filled with the material

Triacylglycerol Structures

•

Simple Triacylglycerols: Three identical fatty acids are

esterified

•

Mixed Triacylglycerols: A triester formed from the

Triacylglycerols: Fats vs. Oils

•

Physical State:

• Fats

•

Predominantly Saturated

•

Solids or semisolids at

room temperature

• Oils:

•

Predominantly

unsaturated

•

Liquids at room

temperature

•

Source:

• Fats: Animal source and tasteless

• Oils: Plants and fish oil

“Good Fats” vs. “Bad Fats”

• Studies indicate that type of dietary fat and amount of dietary fat are important for balanced diet:

• Current recommended amounts are: total fat intake in calories:

• 15% - Monounsaturated fat

• 10% - Polyunsaturated • <10% - Saturated fats

• Studies also indicate that:

• Saturated fats are considered “bad fats”

• Monounsaturated fats are considered “good fats”

• Trans-monounsaturated fats are considered “bad fats”

• Polyunsaturated fats can be both “good fats” and “bad fats”

Essential Fatty Acids

• Fatty acids that must be obtained from dietary sources – are not

synthesized within the body

• Two most important essential fatty acids are:

• Linoleic acid (18:2) - omega 6

• Linolenic acid (18:3) - omega 3

• Both are needed for:

• Proper membrane structure

• Serve as starting materials for the production of several nutritionally important longer-chain omega-6 and omega-3 fatty acids

Practice: Fatty Acid Structure

Chemical Reactions of Triacylglycerols

•

Hydrolysis

: Breaking of 1-2 ester bonds to give rise to

mono-or diacylglycerol and fatty acid(s)

Chemical Reactions of Triacylglycerols

•

Saponification

:Hydrolysis in basic solution: Produce salt

of fatty acid and glycerol

RCOOR’ + NaOH



RCOONa (soap) + R’OH

H₂C

CH

H₂C O O O C C C O O O R R

R + 3NaOH

H₂C

HC

H₂C OH

OH

OH

+ 3RCOONa

Micelle Formation

Oil

droplet

COO

-COO

-COO

-COO

-Chemical Reactions of Triacylglycerols

•

Hydrogenation

: Addition of hydrogen across double bond

-increases degree of saturation

• Many food products are produced by partial hydrogenation of oils and fats

• Peanut oil + H₂  Peanut Butter

• Vegetable oil + H₂  Margarine

Chemical Reactions of Triacylglycerols

•

Oxidation

:

• Oxidation of alkenes may result into two short chain molecules – an

aldehyde or a carboxylic acid:

• The aldehydes and/or carboxylic acids so produced often have objectionable odors - fats and oils are said to be rancid

• To avoid this unwanted oxidation process antioxidants are added as preservatives, e.g., Vitamin C and vitamin E are good

5 Categories of Lipids

Energy –

Storage lipids

• Triacylglycerols

Membrane

Lipids

• Phospholipids

• Sphingoglycolipids

• Cholesterol

Emulsification

Lipids

• Bile Acids

Messenger

Lipids

• Steroid hormones

• Eicosanoids

Protective

-Coating

Lipids

Membrane Lipids: Phospholipids

•

Up to 80% of the mass of a cell membrane can be lipid

materials

•

Phospholipids are the most abundant

•

Contains one or more fatty acids, a phosphate group, a

platform molecule (glycerol or sphingosine) to which the

fatty acid(s) and the phosphate group are attached, and

an alcohol that is attached to the phosphate group

Alcohol = Choline Ethanolamine

Membrane Lipids: Phospholipids

•

Glycerophospholipid

•

4 ester linkages

Polar head group

Membrane Lipids: Sphingoglycolipids

• Sphingoglycolipids: Contains both a fatty acid and carbohydrate

• Simple sphingoglycolipids are called cerebrosides: contains a single monosaccharide unit - either glucose or galactose

•

Lipids: Fused Rings

•

Cholesterol: C27 steroid molecule

•

A steroid is a lipid whose structure is based

on a fused ring system of three 6 carbon

rings and one 5 carbon ring.

•

Important in human cell membranes, nerve

tissue and brain tissue

• Important in chemical synthesis: Hormones, vitamins essential for life

Transport Across Cell Membranes

•

To maintain cellular processes various molecules

transported across the cell membranes.

5 Categories of Lipids

Energy –

Storage lipids

• Triacylglycerols

Membrane

Lipids

• Phospholipids

• Sphingoglycolipids

• Cholesterol

Emulsification

Lipids

• Bile Acids

Messenger

Lipids

• Steroid hormones

• Eicosanoids

Protective

-Coating

Lipids

Emulsification Lipids: Bile Lipids

• Cholesterol derivatives that functions as emulsifying agents that make dietary lipids soluble in aqueous environment of the digestive tract:

• Approximately one third of cholesterol produced by liver is converted to bile acids.

• Action similar to soap in washing

Messenger Lipids

•

The lipids that play the role of “chemical messengers” include:

• Steroid hormones – derivatives of cholesterol

• Eicosanoids- derivatives of arachidonic acid

•

A hormone is a biochemical substance produced by a ductless

gland that has a messenger function.

•

Hormones serve as a means of communication between

various tissues.

• There are two major classes of steroid hormones:

• Sex hormones - control reproduction and secondary sex characteristics

• Adrenocorticoid hormones – control numerous biochemical

Messenger Lipids: Steroid Hormones

•

Classified into three major groups:

• Estrogens - the female sex hormones

• Androgens - the male sex hormones

Messenger Lipids:

Adrenocorticoid Hormones

• Produced by the adrenal glands -small organs located on top of each kidney

• 28 Different hormones have been isolated from the adrenal cortex

• Two types of adrenocorticoid hormones:

• Mineralocorticoids - control the balance of Na and K ions in cells

• Glucocorticoids - control glucose metabolism and counteract

Messenger Lipids: Eicosanoids

• Arachidonic acid (20:4) derivatives:

• Severe physiological effects at extremely low concentrations.

• Hormone-like molecules

• Exert their effects in the tissues where they are synthesized.

• Usually have a very short “life.” • Physiological effects:

• Inflammatory response

• Production of pain and fever

• Regulation of blood pressure

• Induction of blood clotting

• Control of reproductive functions, such as induction of labor

•

Biological Waxes

• The fatty acids:

• Generally are saturated fatty acids

• Contain 14 to 36 carbon atoms.

• The alcohols:

• May be saturated or unsaturated

• May contain 16 to 30 carbon atoms.

Biological Waxes

•

Properties: Water-insoluble and water-repellent because of long

nonpolar hydrocarbon chains.

•

Humans and animals secrete biological waxes from skin

glands

•

Functions:

•

Protect hair and skin; keeps it pliable and lubricated.

•

Impart water repellency to animal fur.

• Birds keep their feathers water repellent and help minimize loss

of body heat

•

Plants:

• prevent excessive evaporation of water

Practice: Types of Lipids