Structure of lipids

Pavla Balínová

LipidsLipids 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 hormonesFigure is found on http://en.wikipedia.org/wiki/Triacylglycerol

Structural components of lipids

• alcohols glycerol sfingosine (unsaturated C18 aminoalcohol)

cholesterol inositol

• long chain carboxylic acids (= fatty acids)

The figures were adopted from http://en.wikipedia.org

a)a)

a) 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

Diacylglycerol = glycerol + 2 FAMonoacylglycerol = glycerol + 1 FALipases = enzymes that hydrolyze ester bonds

ester bondester bond

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 cellsCeramide = 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 fat-soluble vitamins (A, D, E, K)

• isoprene is a precursor in synthesis of cholesterol (→ steroid hormones, bile acids)

menthol (C10) phytol (C 20)

squalene (C 30)

-carotene (C40)

The figures were adopted from http://en.wikipedia.org

Examples of terpenes

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

The figure is found at http://courses.cm.utexas.edu/archive/Spring2002/CH339K/

Robertus/overheads-2/ch11_cholesterol.jpg

1

2

3

45

6

7

8

910

11

12

13

1415

16

17

A B

C D

Sterane

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.

„free“ cholesterol

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.