Chemistry of Lipids (Part - III) Triacylglycerols chemistry. Chemistry, Structure, Properties of Triacylglycerols.

1. CH3

2. CH3
• Also called as
– Neutral fats
– Triacylglycerols (TAG) or
– Triglycerides (TG)
• Chemically these are esters of fatt
y acids with glycerol.
• Widely distributed in both plants an
d animals.
• They are insoluble in water and no
n-polar in character.
R2-C-O-CH
CH2-O-C-R3
CH2-O-C-R1
O
=
O
=
O
=
Triacylglycerols Ashok Katta

3. Fats as stored fuel
• TG are the most abundant group of lipids that primarily functi
on as fuel reserves of animals.
• The fat reserve of normal humans…
– men 20% by weight,
– women 25% by weight
• This is sufficient to meet
the body’s caloric requirements
for 2-3 months
Triacylglycerols Ashok Katta

4. Fats primarily occur in adipose tissue :
• Adipocytes of adipose tissue are specialized for stora
ge of triacylglycerols.
• The fat is stored in the form of globules dispersed in t
he entire cytoplasm.
• And surprisingly, TGL are not the structural
components of biological membranes.
Triacylglycerols Ashok Katta

5. CH3
Structures of Acylglycerols :
HO-CH
CH2-OH
CH2-O-C-R
O
=
R2-C-O-CH
CH2-O-C-R3
CH2-O-C-R1
O
=
O=
O
=
R-C-O-CH
CH2-OH
CH2-OH
O
=
R2-C-O-CH
CH2-OH
CH2-O-C-R1
O
=
O
=
1-Monoacylglycerol
2-Monoacylglycerol
1,2-Diacylglycerol Triacylglycerol
Triacylglycerols Ashok Katta

6. CH3
Simple Triacylglycerols vs Mixed Triacylglycerols:
TGs of plants, have higher content of unsaturated fatty acids compared to that of animals.
Triacylglycerols Ashok Katta

7. CH3
Important properties of TG:
Triacylglycerols Ashok Katta

8. Hydrolysis
• TGs undergo stepwise enzymatic
hydrolysis to liberate free fatty acids &
glycerol.
• And it is catalysed by lipases.
• Triacylglycerol Glycerol + 3 R-COOH
Saponification
• The hydrolysis of TGs by alkali
• To produce glycerol and soaps.
• Triacylglycerol + 3 NaOH Glycerol + 3 R-COONa
(soaps)Triacylglycerols Ashok Katta

9. CH3
Triacylglycerol
R2-C-O-CH
CH2-O-C-R3
CH2-O-C-R1
O
=
O
=
O
=
Diacylglycerol
2-monoacylglycerol
Glycerol
fatty acid
R3-COOH
R1-COOH
R2-COOH
Lipase
Lipase
Lipase
fatty acid
R2-C-O-CH
CH2-OH
CH2-O-C-R1
O
=
O
=
R2-C-O-CH
CH2-OH
CH2-OHO
=
HO-CH
CH2-OH
CH2-OH
fatty acid
Triacylglycerols Ashok Katta

10. Rancidity
• It represent the deterioration of fats & oils resulting in an
unpleasant taste
• Fats containing unsaturated fatty acids are more susc
eptible to rancidity.
• Rancidity occurs when fats and oils are exposed to air,
moisture, light, bacteria etc.
Triacylglycerols Ashok Katta

11. CH3
Rancidity
– Hydrolytic rancidity occurs due to partial hy
drolysis of TGs by bacterial enzymes.
– Oxidative rancidity is due to oxidation of u
nsaturated fatty acids.
• This results in the formation of unpleasant products such
as dicarboxylic acids, aldehydes, ketones etc.
• Rancid fats and oils are unsuitable for human co
nsumption.
• The substances which can prevent the occurrence of oxi
dative rancidity are known as antioxidants.
Triacylglycerols Ashok Katta

12. CH3
• In the living cells, lipids undergo
oxidation to produce peroxides and
free radicals which can damage the
tissue.
• The free radicals are believed to
cause inflammatory diseases,
ageing, cancer, atherosclerosis etc.
• Antioxidants prevent in vivo lipid
peroxidation
Lipid peroxidation
Triacylglycerols Ashok Katta

13. CH3
Triacylglycerols Ashok Katta

14. CH3
Lipid index Details Significance
Iodine number
Number of grams of iodine
absorbed by 100 gm of fat
It is a measure of degree of u
nsaturation of a fat
Saponification nu
mber
Number of mg of KOH req
uired to saponify 1gm of a fat or
oil.
Indicates molecular wt. & is
inversely proportional to it.
Acid number
Number of mg of KOH req
uired to neutralize the fatty ac
ids in a gram of a fat
Indicates the degree of ranc
idity of a fat
Reichert-Meissl (R
M) Number
Number of ml of 0.1 N alkali req
uired to neutralize the soluble
volatile fatty acids distilled fro
m 5 G of fat
Measures the amount of vola
tile soluble fatty acids (RM no
. is useful in testing the purity
of butter)
Lipid index Details Significance
Iodine number
Number of grams of io
dine absorbed by 100
gm of fat
It is a measure of degree o
f unsaturation of a fat
Saponification
number
Number of mg of KOH
required to saponify 1gm of
a fat or oil.
Indicates molecular wt. & i
s inversely proportional to i
t.
Acid number
Number of mg of KOH
required to neutralize the
fatty acids in a gram of a f
at
Indicates the degree of r
ancidity of a fat
Reichert-Meissl
(RM) Number
Number of ml of 0.1N
alkali required to neutralize
the soluble volatile fatty
acids distilled from 5 gm of
fat
Measures the amount of v
olatile soluble fatty acids (
RM no. is useful in testing t
he purity of butter)
Triacylglycerols Ashok Katta

15. 15
Triacylglycerols Ashok Katta