Taif University
College of Medicine
Preparatory Year Students
Medical chemistry (2)
Part II (Lipids)
week 4 lectures
1435-36
“Important reactions of lipids”
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
Esterification
Fatty acids
+
Alcohol
H +/ Heat
Ester
+
Water
Hydrolysis of esters
Acid hydrolysis
Base hydrolyses (Saponification)
Acid Hydrolysis
Ester
H +/ Heat
Acid
+
Alcohol
Hydrolysis of triglycerides TG
In hydrolysis,
• Triacylglycerols (TAG) split into glycerol and three fatty
acids
• Acid or enzyme catalyst is required
Base Hydrolysis
OH -
Saponification
Ester
Salt of the acid or soap +
Alcohol
Soap
Saponification of TG and Soap
Saponification
• 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
Saponification
Learning Check
What 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 hydrogenation
of 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.
Autoxidation
Polyunsaturated Fatty Acids
Free Radical Initiation
H-abstraction
Diene Conjugation
O2 uptake
Lipid Peroxides
Catalysts (Fe, Fe-O2)
Decomposition
Polymerization
(dark color,
possibly toxic)
Rancid off-flavor
compounds such as
ketones ,alcohols,
hydrocarbons, acids,
epoxides
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
cis
biradical
cis
biradical
trans
Changing the configuration of the
double bond
Lipids Classification
Glycerides
NeutralGlycerides
PhosphoGlycerides
Neutral Glycerides
Glycerides
Neutral glycerides
Esters of Fatty acids with Glycerol
Esterification
HO
HO
HO
3 H2O
Ester Bond
Triglyceride
Structure of the simple triacylglycerol
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 a mixed triacylglycerol
Three different fatty acid residues are present
Triglycerides
Triglycerides (TG)
Simple
Mixed
Di-acid
Tri-acid
Tristearin
Stearo, Stearo, Palmitin
Palmito, Oleo, Stearin
Question
Draw the structural formula of the
triacylglycerol (TAG) produced from the
reaction between glycerol and three
molecules of oleic acid?