Medical chemistry (2) Part II (Lipids) week 4 lectures 1435-36 “Important reactions of lipids” Taif University College of Medicine Preparatory Year Students
Dec 26, 2015
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
Hydrolysis of triglycerides TGIn hydrolysis, • Triacylglycerols (TAG) split into glycerol and three fatty
acids• Acid or enzyme catalyst is required
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
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
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.
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
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
Triglycerides
Simple Mixed
Triglycerides (TG)
Tristearin
Stearo, Stearo, Palmitin Palmito, Oleo, Stearin
Di-acid Tri-acid