Structure of lipids
Pavla Balínová
Lipids
Lipids are a large and heterogenous group of substances
of biological origin.
They are easily dissolved in organic solvents but they
are insoluble or poorly soluble in water.
Biological roles of lipids:
● lipids are important source of energy – they serve as
metabolic fuel (ATP production)
amphipathic lipids are building blocks of cellular
membranes
● lipids are excellent insulators (subcutaneous tissue
and around various organs)
●
• special tasks – signaling functions, hormones,....
Classification of lipids
I. Simple lipids
● Triacylglycerols (fats)
● Waxes
II. Complex lipids
● Glycerophospholipids
● Sphingophospholipids
● Glycolipids
III. Isoprenoids and steroids
● Isoprenoids: vitamins A, D, E, K
● Steroids: sterols, bile acids,
steroid hormones
Figure is found on http://en.wikipedia.org/wiki/Triacylglycerol
Structural components of lipids
• alcohols
a)
a)a)
 glycerol
 sfingosine
(unsaturated C18 aminoalcohol)
 cholesterol
 inositol
• long chain carboxylic acids
(= fatty acids)
The figures were adopted from http://en.wikipedia.org
b)
Fatty acids (FA)
• are carboxylic acids with unbranched hydrocarbon
chains of 4 – 24 C atoms
• in fats or membrane lipids they are esterified with
alcohols (glycerol, sphingosine)
• free fatty acids (FFAs) = unesterified forms of FA
• in higher plants and animals, unbranched, longchain FA
with either 16 or 18 C atoms are the most common
(palmitic and stearic acid)
• the number of C atoms in the longer, natural FA is
always even (synthesis from acetyl residues)
The figure was adopted from: J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition
Saturated FA
• Palmitic acid CH3-(CH2)14-COOH
• Stearic acid CH3-(CH2)16-COOH
Unsaturated FA
• contain 1 or more double bond(s)
• Oleic acid 18:1, position 9
• Linoleic acid 18:2, positions 9, 12
• Linolenic acid 18:3, positions 9, 12, 15
• Arachidonic acid 20:4, positions 5, 8, 11, 14
• Greek letters (ω) are also commonly used:
• ω = the last C, ω-3 = the third last C
Essential FA
• are FA that have to be supplied in the diet
Polyunsaturated FA:
• Linoleic acid ω-6
• Linolenic acid ω-3
• Arachidonic acid ω-6
Human organism is not able to introduce double bonds
into the end sections of FA (after C-9).
Triacylglycerols (neutral fats)
• are esters of glycerol and 3 fatty acids
ester
bond
ester
bond
Diacylglycerol = glycerol + 2 FA
Monoacylglycerol = glycerol + 1 FA
Lipases = enzymes that hydrolyze ester bonds
Complex lipids
The figure was adopted from: J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme
2005
Complex lipids
A) PHOSPHOLIPIDS
● glycerophospholipids
● sphingophospholipids
B) GLYCOLIPIDS
●
sphingolipids
Glycerophospholipids
= glycerol + 2 FA + phosphate group + hydrophilic compound
- phosphatidic acid + polar compound (ethanolamine, inositol,
choline, serine)
- are the main constituents of biological membranes
Phosphatidylethanolamine (cephalin)
Phophatidylinositol
Phosphatidylcholine (lecithin)
choline
Sphingophospholipids
= sphingosine + FA + phosphate residue + amino alcohol or sugar
alcohol
- are found in large quantities in the membranes of nerve cells
Ceramide = sphingosine + fatty acid (amide bond)
Sphingomyelin (myelin sheaths)
sphingosine
Glycolipids
= sphingosine + FA + sugar or oligosaccharide residue
The phosphate group is absent.
Gangliosides = ceramide + oligosaccharide unit
Cerebrosides = ceramide + Gal (Glc)
Galactocerebroside
galactose
Isoprenoids and steroids
• isoprenoids (terpenes) are derived from an isoprene
(2-methyl-1,3-butadiene)
• isoprene metabolism in plants is very complex –
synthesis of many types of aromatic substances
(menthol, camphor, citronellal)
• activated isoprene can be used in synthesis of fatsoluble vitamins (A, D, E, K)
• isoprene is a precursor in synthesis of cholesterol (→
steroid hormones, bile acids)
Examples of terpenes
menthol (C10)
phytol (C 20)
squalene (C 30)
-carotene (C40)
The figures were adopted from http://en.wikipedia.org
Steroids - overview
Sterols are steroid alcohols
• -OH group in C-3 position and one or more double bonds
• cholesterol, ergosterol (plants)
Bile acids are synthesized from cholesterol in the liver
• cholic acid, chenodeoxycholic acid, lithocholic acid, deoxycholic acid
Steroid hormones regulate metabolism, growth and reproduction
• progesterone, estrogene, testosterone, aldosterone, cortisol,
calcitriol
Sterane
12
13
11
1
10
2
A
3
B
8
14
16
15
7
5
4
D
C
9
17
6
The figure is found at http://courses.cm.utexas.edu/archive/Spring2002/CH339K/
Robertus/overheads-2/ch11_cholesterol.jpg
Cholesterol
„free“ cholesterol
Cholesterol is a constituent of cellular membranes and it is present in all animal
tissues.
Cholesterol esters with FA are insoluble in the water.
Lipoproteins (LDL, HDL) are transporters of cholesterol in blood.
Figures were assumed from a book T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed., Wiley-Liss,
Inc., New York, 1997.
Steroid hormones
• cholesterol is a metabolic precursor of all steroid
hormones in human body
• number of C atoms is changing during synthesis of
hormones: from 27 to 21, 19 or 18
• glucocorticoids (21 C)
● mineralocorticoids (21 C)
● sex hormones (progesterone 21 C, testosterone 19 C,
estrogene 18 C)
Steroid hormones
Figure was adopted from: J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition, Thieme 2005
Bile acids
• are formed from cholesterol in
the liver and they are excreted
into a bile
• primary bile acids:
cholic and chenodeoxycholic acid
secondary bile acids:
deoxycholic and lithocholic acid
●
Function: emulsification of lipids in
intestine → digestion and
resorption
Figure was assumed from book T. M. Devlin et al.: Textbook of Biochemistry With Clinical
Correlations, 4th ed., Wiley-Liss, Inc., New York, 1997.