11

Biosynthesis of fatty acidsBiosynthesis of fatty acids

22

Biosynthesis of fatty acids (Palmitic acid)Biosynthesis of fatty acids (Palmitic acid)Saturated FAsSaturated FAs

Fatty acid synthesis does not occurnot occur by a reversalreversal of the degradative pathway, it consists of a new set of reactions.

The steps of synthesisThe steps of synthesis of fatty acid (Palmitic (Palmitic acid )acid ) start from acetyl-CoA & malonyl-CoA.

1)- Acetyl-CoAAcetyl-CoA is formed from glucose via pyruvic acidpyruvic acid & malonyl-CoAmalonyl-CoA is produced by carboxylation of acetyl-CoAacetyl-CoA by the enzyme acetyl-CoA carboxylaseacetyl-CoA carboxylase.

33

COOH CH3CO-S – CoA + CO2 CH2CO-S – CoA +AMP+PPi Acetyl-CoA Malonyl-CoA

2)- Conversion of acetyl-CoA to acetyl-ACPacetyl-ACP (acyl carrier protein). It is catalyzed by acetyl acetyl transacylasetransacylase.

CH3-CO-SCoA +ACP-SH CH3-CO-S-ACP + CoASH Acety-CoA Acety-ACP

ATP Mn++

Acetyl-CoA carboxylase

44

3)- Conversion of malonyl-CoAmalonyl-CoA to malonyl-ACPmalonyl-ACP in the presence of malonyl transacylasemalonyl transacylase.

HOOC-CH2-CO-SCoA + ACP-SH malonyl-CoA HOOC-CH2-CO-S-ACP + CoASH malonyl-ACP

4)- Formation of acetoacetyl-ACPacetoacetyl-ACP by condensation of acetyl-ACPacetyl-ACP with malonyl-ACPmalonyl-ACP. Catalyzed by ββ-ketoacyl-ACP synthase-ketoacyl-ACP synthase.

CH3-CO-S-ACP + HOOC-CH2-CO-S-ACP Acetyl-ACP Malonyl-ACP CH3-CO-CH2-CO-S-ACP+ CO2 Acetoacetyl-ACP

55

5)- Reduction of acetyoacetyl-ACPacetyoacetyl-ACP to ββ--hydroxybutyryl-ACPhydroxybutyryl-ACP. By the enzyme ββ-ketoacyl--ketoacyl-ACPACP reductase reductase & NADPH+H as conenzyme.

CH3-CO-CH2-CO-S-ACP CH3-CHOH-CH2-CO-S-ACP β-hydroxy-butyryl-ACP

6)- Dehydration of ββ-hydroxybutyryl-ACP-hydroxybutyryl-ACP to crotonyl-ACPcrotonyl-ACP. Catalyzed by ββ-hydroxyacyl--hydroxyacyl-ACP dehydrogenaseACP dehydrogenase.

CH3-CHOH-CH2-CO-S-ACP CH3-CH=CH-CO-S-ACP Crotonyl-ACP

NADPH+H NADP+

+

H2O

66

7)- Reduction of crotonyl-ACPcrotonyl-ACP to butyryl-butyryl-ACPACP, by enoyl-ACP reductase & NADPH+H is coenzyme.

CH3-CH=CH-CO-S-ACP CH3-CH2-CH2-CO-S-ACP

All the steps are repeatedrepeated to elongateelongate the carbon chaincarbon chain by 2 C2 C atoms in each operationeach operation.

The The acyl groupsacyl groups are finally are finally set freeset free from the from the acyl carrier protein either by acyl carrier protein either by hydrolysishydrolysis or by or by reactionreaction with with CoASHCoASH. .

NADPH+H NADP++

+

77

O O CHCH33 – (CH – (CH22)n – C – S – ACP + CoASH)n – C – S – ACP + CoASH

OOCHCH33 – (CH – (CH22)n – C – SCoA + ACPSH )n – C – SCoA + ACPSH

CoASH

88

Biosynthesis of fatty acid stepwise reactions

99

A A general overall reactiongeneral overall reaction for the for the synthesissynthesis of of palmitic acidpalmitic acid as the principal product of as the principal product of synthetase system is:synthetase system is:

CHCH33-CO-S-ACP + 7HOOC-CH-CO-S-ACP + 7HOOC-CH22-CO-S-ACP + 14NADPH+14H-CO-S-ACP + 14NADPH+14H CHCH33-(CH-(CH22)14-COOH + 7CO)14-COOH + 7CO22 + 8ACP-SH + + 8ACP-SH + Palmitic acidPalmitic acid 14NADP + 6H 14NADP + 6H22OO

+

+

1010

Biosynthesis of Unsaturated fatty acids

Synthesis of monounsaturated fatty acids: The palmitoleic acidpalmitoleic acid (16-carbon) & oleic acidoleic acid

(18-carbon) are the two major monounsaturated FAsmonounsaturated FAs in the body.

Both having double bonddouble bond at C 9C 9 position. These are first produced as their coenzyme coenzyme

derivativesderivatives -- by the oxidationoxidation (loss of H) - palmitoyleoly-CoApalmitoyleoly-CoA & stearoyl-CoAstearoyl-CoA respectively in the presence of molecular O2.

1111

CH3(CH2)14CO – SCoA Palmitoyl-CoA

CH3(CH2)16COSCoA Stearyl-CoA CH3(CH2)5-CH=CH-(CH2)7-CO-SCoA Pamitoleoyl-CoA

CH3(CH2)7CH=CH(CH2)7CO-SCoA Oleoyl-CoA

NADP + ½ O2- H2

Acetyl-CoA+NADPH+NADH

(mitochondrial system

- H2 NADP + ½ O2+

+

1212

Palmitoleoyl-CoAPalmitoleoyl-CoA & oleoyl-CoAoleoyl-CoA then give rise to their respective fatty acidsfatty acids by the splitting up splitting up of coenzyme A coenzyme A portion of their molecules:e.g:

Palmitoleoyl-CoA + H2O Palmitoleic acid + CoA

1313

Biosynthesis of Cholesterol Cholesterol is synthesizedsynthesized by the body starting starting

fromfrom CH3CO-SCoA (acetyl-CoAacetyl-CoA). Synthesis has also been carried out in vitro

preparation of tissues. The main sitesmain sites of cholesterol biosynthesis are:

liverliver, small intestinesmall intestine, aortaaorta, skinskin, adrenal adrenal glandsglands & gonadsgonads etc.

Many reaction mechanismsreaction mechanisms have been proposed but the most acceptablemost acceptable by POPJACKPOPJACK.

1414

Synthesis of cholesterol involves 3 stages3 stages. (i) –(i) – ThreeThree units of acetateacetate condense to form 6- 6-

C C intermediate – mevalonatemevalonate. (ii) –(ii) – Mevalonate is convertedconverted to activated

isopreneisoprene units, which polymerizepolymerize to form 30-C30-C linearlinear structure of squalenesqualene.

(iii) –(iii) – Squalene is cyclizedcyclized to form lanosterollanosterol – converted to cholesterolcholesterol in few steps that removeremove 3 methyl3 methyl groups & rearrangerearrange double bonds.

1515

Structure

A B

C D

3 56

10

1317

18

19

2021

273 cyclohexane ringes

(phenanthrene arrangement

Cyclopentane ring

1616

Various reactionsVarious reactions involved in Biosynthesis of Biosynthesis of cholesterolcholesterol.

CH3CO-SCoA + CH3CO-SCoA Acetyl-CoA

CH3COCH2CO-SCoA Acetoacetyl-CoA

CH3

HOOC-CH2-C-CH2-CO-SCoA β-Hydroxy- β-methyl-glutaryl-CoA

OH

CondensationThiolase

- CoASH

+ (CH3CO-SCoA+H2O)- CoASH

HMG-CoA synthase

+ 2(NADPH + H- 2(NADP + CoASH)

(1)

(2)

(3)

1717

CH3 HOOC-CH2-C-CH2-CH2OH Mevalonic acid OH

CH3 O OHOOC-CH2-C-CH2-CH2O-P-O-P-OH Diphospho-mevalonic acid OH OH OH

CH3 O O C=CH-CH2-O-P-O-P-OH 3,3-Dimethyl allyl pyrophosphate CH3 OH OH

+ 2ATP- 2ADP

- (CO2 + H2O) ATP

Isomerization

(4)

(5)

(6)

1818

CH3 O O C-CH2-CH2-O-P-O-P-OH Isopentenyl-pyrophosphate (C5) CH2 OH OH

Geranyl-PP (C10) Farnesyl-PP (C15)

Squalene (C30) Lanosterol (C30)

Polymerizaion + IPP

Squalene synthetase

+Isopentenyl-PP

+ Farnesyl-PP

(7)

(8)

(9)

(10)

(11)

1919

Zymosterol

Desmosterol

CHOLESTEROL (C27)

The production of 1 molecule1 molecule of cholesterolcholesterol requires 18 molecule18 molecule of acetyl CoAacetyl CoA, 36 ATP36 ATP, 16 NADPH16 NADPH.

-

(12)

(13)

2020

Biomedical significance of cholesterol

The levellevel of cholesterolcholesterol in blood is directly directly relatedrelated to the development of atherosclerosisatherosclerosis.

Cholesterol is a componentcomponent of membranesmembranes & has a modulating effectmodulating effect on the fluid statefluid state of the membrane.

7-dehydrocholesterol7-dehydrocholesterol in skin is the precursorprecursor of Vitamin DVitamin D.

2121

Saponification number: HydrolysisHydrolysis of triacylglycerols by alkalialkali is

known as saponificationsaponification because one of the products is soap.

The number of mgs of KOHmgs of KOH used to saponify saponify fatty acids per gramper gram of a given fatfat is called saponification number.

2222

Iodine number: HalogensHalogens can be addedadded to the double bondsdouble bonds of

unsaturatedunsaturated acyl group of triacylglycerols. The amount of halognesamount of halognes absorbed is

proportionalproportional to the total number of double number of double bondsbonds present.

OneOne double bond absorbs 2 halogenabsorbs 2 halogen atoms. The # of gms# of gms of iodineiodine absorbed by

unsaturated fatty acidsunsaturated fatty acids present in 100 gms100 gms of fatfat or oiloil called iodine number.

2323

Biosynthesis of Triglycerides – Triacylglycereols (neutral lipids)(neutral lipids)

Recommended BooksRecommended Books

“HARPER’S Illustrated Biochemistry”, 28th Edition

&

“Essentials of Medical Biochemistry” Vol.1 by Mushtaq Ahmed

2424

Biosynthesis of Triacylglycerols (Neutral Fats)

These are triesterstriesters of different fatty acidsfatty acids with glycerolsglycerols.

The fatty cid partfatty cid part (RCOO, R'COO, RʺCOO) in triacylglycerols is known as acyl groupacyl group.

Acylglycerols differdiffer in number of OHnumber of OH groups of glycerol esterified with different fatty acids.

Simple triacylglycerolsSimple triacylglycerols, in which all OH groups esterified with similar ff,similar ff, are rarerare in humans but with different ff are called mixed mixed triacylglycerolstriacylglycerols & are very commonvery common.

2525

Structures O O O H2C – O – C – R H2C – O – C – R H2C – O – C – R

O O HC – O – C – R' HC – O – C – R' HC – OH

O H2C – O – C – Rʺ H2C – OH H2C – OH

Triacylglycerol Triacylglycerol 1,2-diacylglycerol1,2-diacylglycerol 1-monoacylglycerol1-monoacylglycerol

2626

Biosynthesis of TAGBiosynthesis of TAG Glycerol phosphateGlycerol phosphate is the initial acceptoracceptor of

fatty acids during TAG synthesisTAG synthesis. Two pathwaysTwo pathways for glycerol-P production. FirstFirst; In liverliver & adiposeadipose tissue, glycerol phosphate

can be produced from glucosefrom glucose, by glycolysisglycolysis to produce dihydroxyacetone-Pdihydroxyacetone-P.

Next, DHAP is reducedreduced to glycerol-P by glycerol phosphate dehydrogenaseglycerol phosphate dehydrogenase.

2727

Second pathwaySecond pathway found in liverliver, where by glycerol kinaseglycerol kinase convert free glycerolglycerol to glycerol phosphateglycerol phosphate.

Then there are four reactionsfour reactions, such as; Sequential additionaddition of two fatty acidstwo fatty acids from

fatty acyl CoA, the removal of phosphateremoval of phosphate, and the additionaddition of the third fatty acidthird fatty acid.

2828

H2C – OH H2C – OH H2C – OH HO – C – H HO – C – H C = O

GlycolysisGlycolysis H2C – OH H2C – O – P H2C – O – P

O H2C – O – C – R1 HO – CH H2C – O – P

Glycerol Glycerol 3-P Hydroxy acetone P

1-Acylglycerol 3-P(Lysophosphatidic acid)

Glycerol kinase Glycerol-3-Phosphate

dehydrogenas

ATP ADPNADH+HNAD

Acyltransferase Acyl-CoA (mainly saturated)

CoA

Acyl-CoA (mainly saturated)

Acyltransferase CoA

(1)

(2)

2929

O O H2C – O – C – R1 H2C – O – C – R1

R2 – C – O – C – H R2 – C – O – C – H O H2C – O – P O H2C – OH

O H2C – O – C – R1

R2 – C – O – C – H O O H2C – O – C – R3

1,2-DAG-P (Phosphatidic acid)

1,2-Diacylglycerol

Triacylglycerol

H2O Pi

Phosphatidate phosphohydrolase

Acyl-CoA

CoA

Acyltransferase

(3)

(4)

3030

Triacylglycerol producedproduced in the liverliver transportedtransported to;

Adipose tissueAdipose tissue & stores Blood VLDLVLDL During fastingfasting, insulin decreasesinsulin decreases while

glucagon increasesglucagon increases causing riserise in cAMPcAMP in adipose cells.

This stimulatesstimulates lipolysislipolysis, the process of breakdown of TAGs.

3131

Peroxisome = oxidizer of toxic substances: a tiny part within a cell containing enzymes that oxidize toxic substances such as alcohol and prevent them from doing any harm.

Porphyrin = compound in plants and animals: a metal-containing pigment in animal and plant tissue, consisting of four pyrrole rings linked by methylene groups, e.g. hemoglobin.

ACP = Acyl Carrier Protein HMG –CoA = Hydroxy Methyl Glutaryl Co A. Mushtaq Ahmad volume I. Lippincott’s