MS1005N General Biochemistry Dr Sundus Tewfik s.tewfik@londonmet.ac.uk

Define “lipid” and classify lipids by their chemistry;Draw saturated & unsaturated fatty acids;Illustrate a steroid, a TG and a PL;

Learning outcomes:

1. More commonly known as fats or oils2. Includes many compounds which are poorly soluble in water3. Many are amphiphiles (meaning that they are soluble in both polar and non-polar solvents)

4. some of these are weak amphiphiles eg triglycerides and poorly soluble in water while others are strong amphiphiles eg phospholipids

benzene

diethyl etherchloroform methanol

5. solvents capable of dissolving lipids are called fat solvents and include benzene, diethyl ether, chloroform. A common solvent mixture is chloroform:methanol.

6. The following classification is one which is often used. There are 5 classes recognised;I fatty acids - these are long chain carboxylic acids

RCOOH

II acylglycerols, glycerides or neutral fats - these are neutral esters of glycerol and fatty acids

Esters; A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind.

III phosphoacylglycerols or phosphoglycerides - ionic esters of glycerol, fatty acids and phosphate

IV lipids (apart from fatty acids) not containing glycerol - includes waxes, polyprenyls, terpenes and steroids

V lipids combined with other biomolecules - such as proteins or carbohydrates

Lipoproteins lipopolysaccharides

7. FATTY ACIDSit is necessary to distinguish between free fatty acids (FFAs) which are not combined with other structures and those fatty acids (FAs) which are found as residues as part of other lipids such as glycerides and phosphoglycerides

8. There are only small amounts of FFAs found in nature the remaining FAs are combined into other structures.

9. Fatty acids have an ionisable carboxyl group and a nonpolar unbranched hydrocarbon chain

RCOOH RCOO- + H

•usually possess an even number of carbon atoms (acetyle CoA)

•can be saturated (no carbon-carbon double bonds) or unsaturated (possess some carbon-carbon double bonds )

-C=C-•nomenclature (systematic naming) is very important since it aids understanding of structure

CH3CH2CH2CH2CH2COOH

6 5 4 3 2 1 number of C atom

•caproic acid

•the systematic name of the above compound is based on the number of

carbon atoms and is related to alkane structure and the fact that it is a carboxylic acid

•(Can you suggest its name?)

•it is n-hexanoic acid and it has a common name,

alkane

Palmitic acid - saturated

stearic acid-Saturated

10. In animal fats the most abundant saturated fatty acids are palmitic acid (C16 ie hexadecanoic acid) and stearic acid (C18 ie octadecanoic acid)

11. C12, C14 and C20-C28 FAs do occur in animal fats but in much smaller quantities. FAs below C10 occur very rarely in animal fats

12. Unsaturated FAs contain carbon:carbon double bondsand their names end in enoic as follows

number of double bonds suffix (name ending)

1 -enoic

2 -dienoic

3 -trienoic

4 -tetraenoic

5 -pentaenoic

13. How would you name the following unsaturated FA?(Hint - the numbering of the double bond position is taken from the lowest numbered carbon atom involved in the double bond)

16 15-11 10 9 8-2 1 number of C atom14. CH3[CH2]5CH=CH[CH2]7COOH

16. even this apparently complex systematic name does not define it completely since unsaturated FAs can exist in cis or trans forms ( hint - revise geometric isomerism)

15. 9- hexadecenoic acid (common name - palmitoleic acid)

Cis Configuration Trans Configuration

unlike the cis acids, which occur naturally, the trans acids are artificial and so cannot be metabolised in the human body as efficiently as their isomers. Over the past 3 decades TFAs have been linked to cancer, diabetes, heart disease, low birth rate and obesity.

Since the trans acids are straighter than their bent cis isomers, they can pack together easier and so have a higher melting point. By selecting a particular hydrogenation catalyst, temperature, stirring speed and pressure, manufacturers can control the precise composition of the margarine to create a particular 'mouth feel', melting range and stability.

9 octadecenoic acid

17. In polyunsaturated FAs (PUFAs) the double bonds are usually separated as follows:-[CH=CH-CH2-CH=CH]- divinyl methane structure but also the following may be found:-[CH=CH-CH=CH]- conjugated diene structure.

18. In animal tissues the most commonly found PUFAs are

linoleic acid 18:2linolenic acid 18:3arachidonic acid 20:4

19. Note also that the position of the double bonds can be given in shorthand form, thus oleic acid is 18:1 Δ9

20. The last carbon atom is denoted as ω thus the position of the last (and only) double bond in oleic acid is ω -921. What is the ω position of linolenic acid 18:3 Δ9,12,15 ?

A. ω-3

•commonly occur as storage fats in, for example, adipose tissue; they provide a reservoir of energy•they are neutral fats•they comprise 3 fatty acids combined with glycerol•note that mono and diglycerides also exist as storage fats

22. TRIACYLGLYCEROLS (OR TRIGLYCERIDES)

CH2OH CHOH + 3 HOOC[CH2]16CH3

stearic acidCH2OHglyserol

esterification

CH2O.OC[CH2]16CH3

CHO.OC[CH2]16CH3 + 3H2O CH2O.OC[CH2]16CH3

tristearin

the reverse of this process is a form of hydrolysis called lipolysis. This may be carried by enzymes called lipases yielding FAs and glycerol. In the case of FAs a process called β oxidation followed by the TCA cycle and oxidative phosphorylation provides a large yield of ATP

Condensation Reactions (Building of larger molecules) add energy to organic molecules.Hydrolysis Reactions (those that tear apart molecules) release energy from the molecules.

Formation of a Triglyceride via Dehydration Synthesis

http://www.lipidsonline.org/

23. PHOSPHOACYLGLYCEROLS (OR PHOSPHOGLYCERIDES, GLYCEROPHOSPHOLIPIDS)these are strong amphiphiles they are found in biological membranes

CH2OH HOCH O-

CH2 O - P - O-

O-

glycerol 3 phosphate

+ 2H2O

+ 2 FAs

phosphatidic acid

usually saturated

may be unsaturated

What has this got to do with reindeer?

phosphatidic acid is both an example of a phosphoacylglycerol and a precursor of phosphoacylglycerols.

CH3[CH2]nCO.OCH O-

CH2 O - P - O-

O-

CH2O.OC[CH2]nCH3

examples include fat soluble vitamins such as vitamin K and vitamin E (α-tocopherol)

24. LIPIDS (APART FROM FAs) NOT CONTAINING GLYCEROL- these include structures based on isoprene units

- lipids which contain the isoprene arrangement are called polyprenyls (terpenes if they are of plant origin)

polyprenyls

isoprene

head

tail

Vitamin K

Vitamin E (α- tocopherol)

other fat-soluble vitamins are A (trans-retinol) and D (cholecalciferol)

26. STEROIDSthese have the characteristic steroid ring structure :-

they occur widely in all types of living organisms except prokaryotes

versatile group of compounds often found as hormones

27. The most abundant steroid in the human body is cholesterol (~ 240g per body) which is essential for proper membrane function

28. Cholesterol is often viewed as a ‘bad’ molecule despite having a varied essential and normal roles in the human body.29. This image stems from a possible correlation between blood cholesterol levels and risk of heart disease.30. Some cholesterol is converted to other compounds eg bile salts, sex hormones, adrenal hormones and vitamin D (cholecalciferol)

31. The corticosteroids are products of the adrenal cortex which help regulate the metabolism of electrolytes, proteins and carbohydrates

32. Human sex hormones (and their derivatives) are also steroids - these include testosterone (male sex hormones), and progesterone and œstradiol (female sex hormones)

Commercially, derivatives of these female hormones are used in birth control pills and therapeutic agents such as those designed to treat post-menopausal symptoms. The male hormone and derivatives can be abused as anabolic steroids where they increase muscle production (but also increase aggressiveness, liver damage and heart disease)

33. Bile includes steroids such as taurocholate. This is a bile salt which is an amphipathic steroid (how might this occur?)

34. These bile salts aid digestion in the small intestine by helping nonpolar lipids mix with the more polar digestive enzymes

A. it has a polar and and a non-polar part to its structure

hydrophilic

3. Which of the following lipids form a bilayer between two watery regions, such as the plasma membrane of a cell? a) steroids    b) neutral fats    c) waxes    d) phospholipids

1. A lipid is a polymer made up of which kind of monomers? a) amino acids                                                     c) nucleotides b) alternating sugar and phosphate groups            d) fatty acids and glycerol

2. A dehydration synthesis reaction is also known as a) a condensation reaction b) a hydrolysis reaction c) an isomeric reaction d) an energy releasing reaction e) monomer formation

Study questions

4. Which of the following is Not a lipid? a) steroids    b) fats    c) phospholipids    d) glycogen    e) cholesterol

5. Which of the following lipids is saturated?