Medical chemistry (2)Part II (Lipids)

week 4 lectures1435-36

“Important reactions of lipids”

Taif UniversityCollege of Medicine

Preparatory Year Students

Lectures outlines

Definition and importance of lipids

Classification of lipids

Fatty acids structure and properties

Important reactions of FA

Glycerides

Non-glyceride lipids

Membranes Lipids

List the Important reactions of FA?

Esterification

Hydrolysis of esters

Hydrogenation

Oxidation and Rancidity

Effect of radiation

Esterification

H +/ H

eat

Fatty acids + Alcohol

Ester + Water

Esterification

Hydrolysis of esters

Acid hydrolysis

Base hydrolyses

(Saponification)

H +/ H

eatAcid + Alcohol

Ester

Acid Hydrolysis

Hydrolysis of triglycerides TGIn hydrolysis, • Triacylglycerols (TAG) split into glycerol and three fatty

acids• Acid or enzyme catalyst is required

OH

-Salt of the acid or soap + Alcohol

EsterBase Hydrolysis

Saponification

Soap

Saponification of TG and SoapSaponification• is the reaction of a fat with a strong base• splits triacylglycerols into glycerol and the salts of fatty acids • is the process of forming “soaps” (salts of fatty acids)• with KOH gives softer soaps

Hydrolysis of Triglycerides

Enzymatic or Acid

Hydrolysis

Alkaline Hydrolysis

Glycerol

Saponification

Lipase

Glycerol

Learning CheckWhat products are obtained from the complete hydrolysis of glyceryl trioleate?

A. glycerol and three oleic acids

B. glyceryl tristearate

C. glycerol and three stearic acids

Hydrogenation

Hydrogenation

When hydrogen adds to all the double bonds of glyceryl trioleate (triolein) using a nickel catalyst, the product is the saturated fat glyceryl tristearate (tristearin).

Learning Check

What products are obtained from the complete hydrogenationof glyceryl trioleate?

A. glycerol and three oleic acids

B. glyceryltristearate

C. glycerol and three stearic acids

Hydrogenation of Unsaturated Fats

X ? Carcinogenic

Learning Check

Which of the following statements are true and which are false?

A. There are more unsaturated fats in vegetable oils.

B. Vegetable oils have higher melting points than fats.

C. Hydrogenation of oils converts some cis double bonds to trans double bonds.

D. Animal fats have more saturated fats.

Summary of Organic and Lipid Reactions

Oxidation

Oxidation

• The carbon–carbon double bonds present in the fatty

acid residues of a triacylglycerol are subject to

oxidation with molecular oxygen (from air) as the

oxidizing agent.

• Such oxidation breaks the carbon–carbon bonds,

producing both aldehyde and carboxylic acid products

Oxidation The short-chain aldehydes and carboxylic acids often

have unpleasant odors and flavors. Fats and oils containing them are said to have become rancid.

• Rancidity is due to a combination of two reactions:Bacterial hydrolysis of ester bonds.Air oxidation of alkene double bonds.

AutoxidationPolyunsaturated Fatty Acids

Free Radical Initiation H-abstraction

Diene Conjugation

O2 uptake

Lipid PeroxidesCatalysts (Fe, Fe-O2)

Decomposition

Rancid off-flavor compounds such as ketones ,alcohols,hydrocarbons, acids, epoxides

Polymerization

(dark color, possibly toxic)

Rancidity

• In fats containing triacylglycerols with some low-molecular-

mass carboxylic acids, hydrolysis by airborne bacteria

under moist, warm conditions is directly responsible for

rancid odors and flavors.

To avoid this unwanted oxidation process, commercially

prepared foods always contain antioxidants—substances

that are more easily oxidized than the food:

vitamins C vitamin E (naturally occurring

antioxidants).

BHA (butylated hydroxyanisole) and BHT (butylated

hydroxytoluene) (synthetic oxidation inhibitors).

Consequences of Lipid Peroxidation

• Structural changes in membranes– Alter fluidity and ion channels– Alter membrane-bound signaling proteins– Increase membrane permeability

• Form lipid oxidation products adducts/crosslinks with non lipids– e.g., proteins and DNA

• Cause direct toxicity– e.g., 4-hydroxynonenal, Malondialdehyde “MDA”

• DNA damage and mutagenesis

Antioxidant Defenses in Biological Systems

• Fat-soluble cellular membrane consists – Vitamin E– beta-carotene– Coenzyme Q (10)

• Water soluble antioxidant scavengers – Vitamin C– Glutathione peroxidase, – Superoxide dismutase– Catalase

Effect of radiation

Effect of radiation

Changing the configuration of the double bond

cis biradical cis

biradical trans

Lipids Classification

Neutral-Glycerides

Phospho-Glycerides

Glycerides

Neutral Glycerides

Esters of Fatty acids with Glycerol

Neutral glycerides

Glycerides

HO

HO

HO

3 H2O

Ester Bond

Esterification

Triglyceride

The simple triacylglycerol produced from the triple esterification reaction between glycerol and three molecules of stearic acid (18:0 acid). Three molecules of water are a by-product of this reaction.

Structure of the simple triacylglycerol

Three different fatty acid residues are present

Structure of a mixed triacylglycerol

Triglycerides

Simple Mixed

Triglycerides (TG)

Tristearin

Stearo, Stearo, Palmitin Palmito, Oleo, Stearin

Di-acid Tri-acid

Question

Draw the structural formula of the triacylglycerol (TAG) produced from the

reaction between glycerol and three molecules of oleic acid?