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1. Details of Module and its Structure
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Module Id <4d>
Pre-requisites Basic knowledge about organic chemistry of fatty acids .
Objectives To make the students aware of the categories , physical and chemical properties and biosynthesis of fatty acids.
Keywords Fatty acids , Saturated , Unsaturated , Biosynthesis , Activation , Elongation , Desaturation , Regulation
Structure of Module / Syllabus of a module (Define Topic / Sub-topic of module )
1.Fatty acids 1.Introduction
2.Functions
3.Nomenclature 3.1. IUPAC System
3.2. Trivial names
3.3.Two abbreviation systems
3.3.1. The carboxyl-reference system
3.3.2. The omega-reference system
Biochemistry Botany Fatty acids
3.4.Delta-x system
3.5. Lipid numbers
4. Classification 4.1. Saturated fatty acids
4.2. Unsaturated fatty acids
4.2.1. Monoethenoid acids
4.2.2. Polyunsaturated fatty acids
4.2.2.1. Diethenoids
4.2.2.2. Triethenoids
4.2.2.3. Tetraethenoids
4.3. Branched chain fatty acids
4.4. Cyclic fatty acids
4.5. Essential fatty acids
4.6. Eicosanoids
5. Properties of fatty acids 5.1. Solubility in water
5.2. Melting (and boiling) point
5.3. Isomerism
5.4. Amphipathic nature
5.5. Oxidation
5.6. Halogenation
5.7. Formation of esters with alcohols
5.8. Formation of soaps with alkalies
Biochemistry Botany Fatty acids
6. Characterisation of fats 6.1. Saponification number
6.2. Iodine number
6.3. Acid number
6.4. Reichert-meissl number
7. Fatty acid biosynthesis Phase 1- Activation Phase 2-Elongation Phase 3- Termination
8. The stoichiometry of fatty acid synthesis
9. Elongation of saturated fatty acids 9.1. In mitochondria
9.2. In microsomes
10. Introduction of double bonds 10.1. Formation of monoenoic acid
10.1.1. Aerobic reactions
10.1.2. Anaerobic reactions
10.2. Formation of polyenoic acid
11. Regulation of fatty acid biosynthesis
Biochemistry Botany Fatty acids
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TABLE OF CONTENTS (for textual content)
1. INTRODUCTION
2. FUNCTIONS
3. NOMENCLATURE
3.1. IUPAC SYSTEM
3.2. TRIVIAL NAMES
3.3. TWO ABBREVIATION SYSTEMS
3.3.1. THE CARBOXYL-REFERENCE SYSTEM
Biochemistry Botany Fatty acids
3.3.2. THE OMEGA-REFERENCE SYSTEM
3.4. DELTA-X SYSTEM
3.5. LIPID NUMBERS
4. CLASSIFICATION
4.1. SATURATED FATTY ACIDS
4.2. UNSATURATED FATTY ACIDS
4.2.1. MONOETHENOID ACIDS
4.2.2. POLYUNSATURATED FATTY ACIDS
4.2.2.1. DIETHENOIDS
4.2.2.2. TRIETHENOIDS
4.2.2.3. TETRAETHENOIDS
4.3. BRANCHED CHAIN FATTY ACIDS
4.4. CYCLIC FATTY ACIDS
4.5. ESSENTIAL FATTY ACIDS
4.6. EICOSANOIDS
5. PROPERTIES OF FATTY ACIDS
5.1. SOLUBILITY IN WATER
5.2. MELTING (AND BOILING) POINT
5.3. ISOMERISM
5.4. AMPHIPATHIC NATURE
5.5. OXIDATION
5.6. HALOGENATION
Biochemistry Botany Fatty acids
5.7. FORMATION OF ESTERS WITH ALCOHOLS
5.8. FORMATION OF SOAPS WITH ALKALIES
6. CHARACTERISATION OF FATS
6.1. SAPONIFICATION NUMBER
6.2. IODINE NUMBER
6.3. ACID NUMBER
6.4. REICHERT-MEISSL NUMBER
7. FATTY ACID BIOSYNTHESIS
PHASE 1- ACTIVATION , PHASE 2-ELONGATION , PHASE 3- TERMINATION
8. THE STOICHIOMETRY OF FATTY ACID SYNTHESIS
9. ELONGATION OF SATURATED FATTY ACIDS
9.1. IN MITOCHONDRIA
9.2. IN MICROSOMES
10. INTRODUCTION OF DOUBLE BONDS
10.1. FORMATION OF MONOENOIC ACID
10.1.1. AEROBIC REACTIONS
10.1.2. ANAEROBIC REACTIONS
10.2. FORMATION OF POLYENOIC ACID
11. REGULATION OF FATTY ACID BIOSYNTHESIS
Biochemistry Botany Fatty acids
e-Text
FATTY ACIDS 1. INTRODUCTION :- In natural fats fatty acids mostly occur as monocarboxylic acids containing an even number of carbon atoms . These contain four to twenty carbon atoms and are straight chain compounds. Out of these C12 to C24 are common in nature, while C16 and C18(generally aliphatic, unbranched ) are prevalent in our body. The chain is saturated if it lacks double bonds otherwise it remains unsaturated and contains one or more double bonds . Hydroxy fatty acids contain hydroxyl group(s) , while cyclic fatty acids have ring structure . Acetic , propionic and butyric acids are short chain fatty acids which occurs as intermediates of metabolism . Fatty acids differ from each other in a) length of hydrocarbon tail, b) degree of unsaturation, c) position of double bonds in the chain. Glycerides and phospholipids are esterified forms of fatty acids . A carboxyl group becomes ionized when fatty acids exist in free form.
2. FUNCTIONS :- 1) They serve as stored forms of energy . 2) Vitamins like A, D , E , and K are soluble in fats. 3) Fatty acids are the building units of majority of lipids. 4) Fatty acids are constituents of phospholipids. 3. NOMENCLATURE :- Nomenclature of fatty acids follows many systems and these are described as follows:-
3.1. IUPAC SYSTEM( SYSTEMATIC NAMES) Conventions of the International Union Of Pure and Applied Chemistry are followed by IUPAC names. Names describe the structure in detail. • The carboxyl carbon of the fatty acids is denoted by the number one , and in reference to it other carbon positions are numbered. E.g. a double bond between ninth and tenth carbon in the chain is said to be at -9 carbon. • Cis or trans carbons are denoted by either cis/trans notion or E-/Z- notion according to need. This notion is technically clear , descriptive and generally more verbose than common nomenclature.
3.2. TRIVIAL NAMES Trivial names give no clue for the structure of the compound . Here name may be derived from either the common source of the compound or the source from which it was first isolated. Few examples are as follows :- - The name palmitic acid is associated with palm oil because it is present in it.
Biochemistry Botany Fatty acids
- In olive oil (oleum) oleic acid is a major constituent. - Stearic acid remains solid at room temperature ( originated from a Greek word meaning solid) -Arachidonic acids are present in spiders (arachnids) 3.3 & 3.4) TWO ABBREVIATION SYSTEMS Fatty acids are also named by abbreviation ; they follow two such systems details of which are given below. 3.3. THE CARBOXYL-REFERENCE SYSTEM indicates the number of double bonds , the number of carbons , and the positions of the double bonds , counting from the carboxyl carbon ( which is numbered 1, like in the IUPAC system ) .But carboxyl reference system uses a number ( e.g.14) to denote chain length and IUPAC system uses a name derived from Greek word ( e.g. tetradecanoic acid). 3.4.THE OMEGA-REFERENCE SYSTEM( N-X NOMENCLATURE )) indicates the number of double bonds , the number of carbons and the position of the double bond closest to the omega carbon , counting from the omega carbon ( which is numbered 1 for this purpose ) . Omega -3 and omega -6 fatty acids are physiologically different and can not be interconverted in human body and so this system is useful for physiological purposes . The omega-x , ω-x , or “ omega” notion is commonly used in popular nutritional literature , but IUPAC has deprecated it in favour of n-x notation in technical documents. n-3 and n-6 notions are used in literature for citing researches for fatty acid biosynthetic pathways. (Rigaudy , J. ; Klesney , S.P. (1979 ) . Nomenclature of Organic Chemistry . Pergamon . ISBN o-o8-o22369-9 OCLC 5oo8199 )
The diagram given below shows the way in which Greek letters denote positions of carbons relative to either end of a fatty acid chain.
Biochemistry Botany Fatty acids
Summary of the four common systems of nomenclature of fatty acid is given below is given below :-
3.5. X ( OR DELTA-X ) NOMENCLATURE – In this system each double bond is shown by Δx, where the double bond is located on the xth C-C bond , counting from carboxyl end of fatty acid chain . Each double bond preceds by cis / trans prefix , indicating the conformation of the molecule around the bond , for e.g. linoleic acid is designated as cis-Δ9, cis-Δ12 octadecadienoic acid.This nomenclature is less verbose and technically less clear or descriptive than IUPAC system .
3.6. LIPID NUMBERS - IN THE FORM OF C:D , Here C is the number of carbon atoms in the fatty acid chain and D is the number of double bonds in the fatty acid ( if more than one , the double bonds are assumed to be interrupted by CH2 units , i.e. ,at intervals of 3 carbon atoms along the chain ) . This can be ambiguous because some different fatty acids can have same numbers . And so , when ambiguity exists this notation is usually paired with either a delta-x or n-x term.
Most of theNomenclature has been taken(with modifications) from following resources:- http://library.med.utah.edu/NetBiochem/FattyAcids/4_1a.html & en.wikipedia.org/wiki/Fatty_acid
4. CLASSIFICATION OF FATTY ACIDS:-
4.1. SATURATED FATTY ACIDS-: Have no double bonds and thus chain is saturated . They have following molecular formula CnH2n+1COOH e.g. palmitic acid is represented by C15H31COOH.
Biochemistry Botany Fatty acids
e.g.Butyric acid (4) , Caproic acid (6) , Caprylic acid (8) , Capric acid (10) , Lauric acid (12) Myristic acid (14) , Palmitic acid (16) , Stearic acid (18) , Arachidic acid (20), Behenic acid (22) , Lignoceric acid (24) , Cerotic acid (26) , Montanic acid (28) [Figures in the bracket is number of carbons].
Common name Systematic name Structural formula Lipid numbers
Propionic acid Propanoic acid CH3CH2COOH C3:0
Butyric acid Butanoic acid CH3(CH2)2COOH C4:0
Valeric acid Pentanoic acid CH3(CH2)3COOH C5:0
Caproic acid Hexanoic acid CH3(CH2)4COOH C6:0
Caprylic acid Octanoic acid CH3(CH2)6 COOH C8:0
Capric acid Decanoic acid CH3(CH2)8COOH C10:0
Lauric acid Dodecanoic acid CH3(CH2)10COOH C12:0
Myristic acid Tetradecanoic acid CH3(CH2)12COOH C14:0
Palmitic acid Hexadecanoic acid CH3(CH2)14COOH C16:0
Stearic acid Octadecanoic acid CH3(CH2)16COOH C18:0
Biochemistry Botany Fatty acids
Arachidic acid Eicosanoic acid CH3(CH2)18COOH C20:0
This list has been adapted from:- http://en.wikipedia.org/wiki/List_of_saturated_fatty _acids
4.2. UNSATURATED FATTY ACIDS: May have one to six double bonds and thus degree of unsaturation varies with the number of double bonds present in the chain. Depending upon the degree of unsaturation these are further classified as follows:-
4.2.1. MONOETHENOID ACIDS [ monounsaturated fatty acids (MUFAs ) ]:These contain one double bond e.g. oleic acid and palmitoleic acid 4.2.2. POLYUNSATURATED FATTY ACIDS (PUFAs)- Have nonconjugated double bond system i.e. the double bonds alternate with-CH2- groups . Configuration around double bonds remain usually of cis type . These are divided as follows- 4.2.2.1. DIETHENOID ACIDS: Contain two double bonds.e.g.Linoleic acid. 4.2.2.2. TRIETHENOID ACIDS: Contain three double bonds e.g.Linolenic acid ,Eleostearic acid. 4.2.2.3. TETRAETHENOID ACIDS: Contain four double bonds e.g. Arachidonic acid.
Trans or ω-n Common Name Lipid Numbers Structural Formula Cis
ω-3 α-Linolenic acid C18:3 CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH Cis
ω-6 Linoleic acid C18:2 CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH Cis
ω-6 y-Linolenic acid C18:3 CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4COOH Cis
ω-7 Palmitoleic acid C16:1 CH3(CH2)5CH=CH(CH2)7COOH Cis
ω-9 Oleic acid C18:1 CH3(CH2)7CH=CH(CH2)7COOH Cis
Biochemistry Botany Fatty acids
ω-9 Elaidic acid C18:1 CH3(CH2)7CH=CH(CH2)7COOH trans
ω-9 Mead acid C20:3 CH3(CH2)7CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH Cis
ω-6 Arachidonic acid C20:4 CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH Cis
This list has been taken from:-http://en.wikipedia.org/wiki/List_of_unsaturated _fatty acids
4.3. BRANCHED CHAIN FATTY ACIDS: Fatty acids containing hydroxyl group in their chain. e.g.ricinoleic acid, cerebronic acid.
Ricinoleic acid
This structure has been taken from:- http://en.wikipedia.org/wiki/List_of_unsaturated _fatty acids
The structure given below is of Cerebronic acid
Biochemistry Botany Fatty acids
This structure has been taken from:-
http://www.ebi.ac.uk/chebi/chebiOntology.do?chebiId=CHEBI:61302
4.4. CYCLIC FATTY ACID: Contain ring structure e.g.chaulmoorgric acid and hydnocarpic acid.
Chaulmoogric acid Hydnocarpic acid.
These Figs. have been taken from:- http://www.chemicalbook.com/ChemicalProductProperty_EN_CB7338591.htm
Biochemistry Botany Fatty acids
4.5. ESSENTIAL FATTY ACIDS: Some fatty acids are not synthesised by the mammalian cells because enzymes required to introduce double bonds beyond C9 in the fatty acid chains are not present , and so such fatty acids must be obtained from dietary sources. These are called essential fatty acids (EFA).Names of essential fatty acids are – Linoleate (18:2∆9,12) and Linolenate (18:3∆9,12,15) ; these are the two essential fatty acids in mammals . Other fatty acids are derived from these two and one such example is- arachidonic acid (20:4∆5,8,11,14) .
4.6. EICOSANOIDS are derivatives of arachidonic acid and have both harmonal and signalling properties. These can be classified into thromboxanes , leukotrienes and prostaglandins..
5.PROPERTIES OF FATTY ACIDS :-
The degree of unsaturation and length of the hydrocarbon chain determines many properties of the fatty acids.
5.1. SOLUBILITY IN WATER
Solubility in water decreases with increase in degree of unsaturation and chain length of the fatty acids . Polar nature of the –COOH group makes short chains soluble in water and the carboxyl group ionizes at neutral pH.
5.2. MELTING ( AND BOILING ) POINT
Melting and boiling points increase with increase in the chain length but decrease with the increase in the number of double bonds .Fatty acids with the chain length <8C are liquid at room temperature (R.T.), saturated fatty acid of 12-24 chain lengths have a waxy consistency at R.T., and unsaturated fatty acids of the same chain length are oily liquids .Fats containing fatty acids 18:2 are liquid below 00 C. Configuration of saturated chains remains extended and zig zag at low temperatures , but short at high temperatures (due to rotation of the C-C bonds).This explains why biomembranes get thinner with increasing temperatures.
5.3. ISOMERISM
Fatty acids show isomerisation due to the presence of double bonds. Thus, Oleic acid can have 15 possible positional isomers depending upon the location of the double bond in the chain . Trans isomers with higher stability are less common while cis isomers of lesser stability are found more in nature and in our body . Trans fatty acids are commonly found in the triglycerides of dairy and meat products , and in partially hydrogenated oils . Trans fatty acids cause increase of blood LDL and decrease of HDL and thus should be consumed less in amount.Saturated fatty acids like stearic acid, and unsaturated fatty acids with trans bonds, have linear shapes but the presence of a cis bond in other fatty acids bends the shape of the molecule .
Biochemistry Botany Fatty acids
The constituent fatty acids effect the spatial configuration of lipids in which they are present . The bends are of special significance in the fluid nature of the membranes . Saturated fatty acids in a membrane are tightly packed in a stable , orderly arrays while one or more “ kinks” in unsaturated fatty acid molecules prevent orderly packing and allow lateral movement.
5.4. AMPHIPATHIC NATURE
The hydrophilic carboxylic ends of the fatty acid molecules react with the cellular environment , while the ‘R’ chains are hydrophobic and interact with each other. This amphipathic nature of fatty acids is important for formation and functioning of micelles and membranes . The addition of cation (Na++) makes the carboxylic head more polar as in soaps and bile salts .
5.5. OXIDATION
Oxidation of unsaturated fatty acids ( double bonds ) yield hydroxy , aldehydic and ketonic derivatives which on polymerisation turn into resins .Oxidation is a cause of rancidity in fats , oxidation of some oils like linseed oil produces waterproof films . Linseed is also known as drying oil and is used in manufacturing of paints and varnishes .
5.6. HALOGENATION
Halogenation of the double bonds is analytically useful .e.g. absorption of UV radiation by the double bonds is a basis for characterisation of fats .
5.7. FORMATION OF ESTERS WITH ALCOHOLS
The esters of fatty acids with glycerol ( trihydric alcohol ) are called neutral fats or triglycerides . Waxes are esters with some higher monohydroxy alcohols .
5.8 . FORMATION OF SOAPS WITH ALKALIES
Fatty acids make soap with alkali.The soaps of Na and K are useful in daily life.
Sodium soaps are hard.Potassium soaps are soft but costly . To make sodium soaps usable as toilet soaps, sodium carbonate or silicate is added in small amounts .This makes soaps lather with even hard water.
Soaps used for shaving are usually potassium soaps using coconut oil or palm oil as the source of fatty acids . To make soap less alkaline and more smooth to the skin, excess of fatty acids are added.
Zinc stearate is a soft powder which is non- irritant to the skin and water repellent.It is commonly used in dusting powders . Calcium and magnesium soaps are insoluble in water and
Biochemistry Botany Fatty acids
do not lather . Hard water which contains salts of calcium and magnesium is hence unsuitable for washing purposes.
6. CHARACTERISATION OF FATS:-
Fats are characterised and their purity or otherwise assessed by determining certain chemical constants for individual fats-
6.1. SAPONIFICATION– Number of mgs of KOH required to neutralize the free and combined fatty acids in a gm of a given fat is its saponification number. A high saponification number indicates that the fat is made up of low molecular weight fatty acids and vice versa.
6.2. IODINE NUMBER - is the number of gms of Iodine required to saturate 100 gms of fat .Since Iodine is taken up by the double bonds , a high Iodine number indicates a high degree of unsaturation of the fatty acids of the fat.
6.3. ACID NUMBER – Number of mgs of KOH required to neutralize the free fatty acids in a gm of fat is known as acid number .The acid number indicates the degree of rancidity of the given fat .
6.4. REICHERT – MEISSL( R.M. )NUMBER – The number of mls of 0.1 N alkali required to neutralize the volatile fatty acids ( separated by saponification , acidification and steam distillation of the fat ) contained in 5 gms of the fat is the R.M. number .
7. FATTY ACID BIOSYNTHESIS: –
Fatty acid biosynthesis is stepwise assembly of acetyl CoA units (mostly as malonyl CoA) ending with palmitate(saturated). The fatty acid biosynthesis is catalysed by fatty acid synthase ( FAS ) multienzyme complex. Important points of the overall reaction are written bellow.
Palmitic acid ( 16:0) is the end product of FAS action . Modifications of this primary fatty acid leads to formation of other longer ( and shorter) saturated and unsaturated fatty acids . The fatty acid molecule is synthesized 2 carbons at a time. Acetyl- CoA derived from carbohydrate or amino acid sources is the ultimate precursor of all the carbon atoms of the fatty acid chain . Out of eight acetyl units required for biosynthesis of palmitic acid , only one is provided by acetyl-CoA ( called starter or primer ; the two carbons of this acetyl group become the two terminal carbon atoms ,i.e. 15 and 16 carbon atoms of the palmitic acid) . The other seven arrive in the form of malonyl-CoA formed from acetyl-CoA and HCO3- in the carboxylation reaction. Fatty acid synthesis begins from the methyl end and proceeds towards the carboxylic acid end . Thus , C16 and C15 are added first and C2 and C1 are added in the last.
Biochemistry Botany Fatty acids
There are 3 phases in the fatty acid biosynthesis.: Phase -1-Activation
Phase -2-Elongation
Phase-3- Termination
PHASE -1-ACTIVATION
Synthesis of malonyl-CoA via Acetyl-CoA Carboxylase
For step-wise 2- carbon extensions , acetylCoA is first activated to malonyl CoA ( a three carbon compound ) by the addition of a CO2 .The ATP-dependent carboxylation provides energy for the reaction . The CO2 is lost later during condensation with the growing fatty acid chain. The spontaneous decarboxylation drives the condensation . The enzyme Acetyl-CoA Carboxylase with biotin as prosthetic group catalyzes the above reaction in the cytosol .
Figure 1 :
.
ATP- dependent carboxylation of the biotin is carried out at one active site denoted by (1) .This is followed by transfer of the carboxyl group to acetyl –CoA at a the second active site denoted by(2).
This overall spontaneous reaction is summarized as follows
HCO3- +ATP + acetyl-CoA ADP +Pi + malonyl CoA
Biotin is linked to the enzyme by an amide bond between the terminal carboxyl group of the biotin side chain and the Є – amino group of a lysine residue . The combined biotin and lysine side chains act as a long flexible arm that allows the biotin ring to translocate between the
Biochemistry Botany Fatty acids
two active sites .
Figure 2 :
Acetyl-CoA Carboxylase , which converts acetyl-CoA to malonyl-CoA , is the committed step ( irreversible & rate limiting )of the faty acid synthesis pathway . The mammalian enzyme is regulated by phosphorylation , besides , there is allosteric control via local metabolites.
AMP- Activated kinase catalyzes phosphorylation of acetyl-CoA carboxylase which inturn inhibits the ATP-utilizing production of malonyl – CoA.
Regulation of Acetyl-CoA Carboxylase by local metabolites :
Palmitoyl-CoA (negative effector) , the product of Fatty Acid Synthase , promotes the inactive conformation of Acetyl-CoA Carboxylase and diminishes the production of malonyl-CoA (, the precursor of fatty acid synthesis) . This is an example of feedback inhibition .
Citrate ( positive effector ) allosterically activates Acetyl-CoA Carboxylase . Citrate concentration becomes high when adequate acetyl – CoA enters Krebs’ Cycle. Excess acetyl- CoA is then converted via malonyl-CoA to fatty acids for storage. - Figure 3:
Regulation of Acetyl-CoA Carboxylase
Biochemistry Botany Fatty acids
Conformational changes associated with regulation :
When acetyl –CoA Carboxylase is active it self- associates to form multimeric filamentous complexes. Transition to the inactive conformation results in dissociation of the enzyme to its monomeric (protomer) form.
Fatty acid synthesis , from acetyl –CoA and malonyl-CoA , occurs by a series of reactions that are :
in bacteria , catalyzed by six different enzymes and a separate acyl carrier protein .
in mammals , catalyzed by individual domains of a very large polypeptide that includes an acyl carrier domain.During evolution the mammalian Fatty Acid Synthase apparently has involved gene fusion .
In humans , fatty acids are formed predominantly in the liver and lactating mammary glands and , to a lesser extent , the adipose tissue . Most acetyl –CoA is formed from pyruvate by the action of enzyme pyruvate dehydrogenate in the mitochondria . Acetyl-CoA produced in
mitochondria is condensed with oxaloacetate by citrate synthase to form citrate , which is then transported into the cytosol via Citrate Shuttle and broken down to yield acetyl-CoA and
oxaloacetate by ATP citrate lyase. Oxaloacetate in the cytosol is reduced to malate by cytoplasmic malate dehydrogenate , and malate is then transported back into the
mitochondria to participate in the citric acid cycle . NADPH serves as electron donor in the two reactions involving substrate reduction and is produced mainly by the Pentose Phosphate Pathway . Prosthetic groups of Fatty Acid Synthase include :
The thiol of the side- chain of a cysteine residue in the Condensing Enzyme domain of the complex. The thiol of phosphopantetheine.
Phosphopantetheine is covalently linked via a phosphate ester to a serine hydroxyl of the acyl carrier protein domain of Fatty Acid Synthase . The long flexible arm of phosphopantetheine helps its thiol to move from one active site to another within the complex . Structure of Pant is given below :– Figure 4 :
Biochemistry Botany Fatty acids
Individual steps of the reaction pathway are catalyzed by the catalytic domains of the mammalian Fatty Acid Synthase , listed in the diagrams below.
Initiation- As each of the substrates acetyl –CoA and malonyl –CoA bind to the complex (designated steps 1 &2 ) , the initial attacking group is the oxygen of a serine hydroxyl group of the malonyl / acetyl-CoA transacylase enzyme domain . Each acetyl or malonyl moiety is transiently in ester linkage to this serine hydroxyl , before being transferred into thioester linkage with the phosphopantetheine thiol of acyl carrier protein (ACP ) domain . (Figure 5) Acetate is subsequently transferred to a cysteine thiol of the Condensing Enzyme domain .
The condensation reaction ( step 3) involves decarboxylation of the malonyl moiety , followed by attack of the resultant carbanion on the carbonyl carbon of the acetyl ( acyl )
moiety.
PHASE -2 ELONGATION
(Figure 6) :
Biochemistry Botany Fatty acids
In steps 4-6 : the β ketone is reduced to an alcohol , by electron transfer from NADPH . Dehydration yields a trans double bond . Reduction at the double bond by NADPH yields a saturated chain
Following transfer of the growing fatty acid from phosphopantetheine to the Condensing Enzyme’s cysteine sulfhydryl , the cycle begins again , with another malonyl-CoA. Figure 7
PHASE 3- TERMINATION-
Product release : When the fatty acid is 16 carbon atoms long , a thioesterase domain catalyzes hydrolysis of the thioester linking the fatty acid to phosphopantetheine . The 16 –C saturated fatty acid palmitate is the final product of the fatty Acid Synthase Complex :
The figures(1-7) have been taken from : https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/fasythesis.htm#malcoa
Biochemistry Botany Fatty acids
Saturated fatty acids having an odd number of carbon atoms , which are found in many marine organisms ,are also made by the Fatty Acid Synthase Complex . In this case the synthesis is primed by a starter molecule of propionyl-S-ACP , to which are added successive two-carbon units via condensation with malonyl-S-ACP .
8.THE STOICHIOMETRY OF FATTY ACID SYNTHESIS:-
The stoichiometry of the synthesis of palmitate is
The equation for the synthesis of the malonyl-CoA used in the preceding reaction is
Hence , the overall stoichiometry for the synthesis of palmitate is
9. ELONGATION OF SATURATED FATTY ACIDS: -
9.1. IN MITOCHONDRIA – palmitic and other saturated fatty acids are lengthen by successive additions of acetyl units in the form of acetyl –CoA(malonyl –ACP cannot replace acetyl - CoA) to the carboxyl terminal end. Condensation of palmityl-CoA with acetyl –CoA yields β- Ketostearyl-CoA, which is reduced by NADPH to b- Hydroxystearyl-CoA . The latter is dehydrated to the ∆2-unsaturated stearyl – CoA , which is then reduced to yield stearyl-CoA at the expense of NADPH.This system also elongates unsaturated fatty acids.
9.2 IN MICROSOMAL FRACTION - both saturated and unsaturated fatty acyl – CoA esters are elongated., but in this case malonyl-CoA rather than acetyl- CoA serves as source of the acetyl groups. The reaction sequence is identical to that in the Fatty Acid Synthase system except that the microsomal system employs CoA and not ACP as acyl carrier.A family of enzymes
Biochemistry Botany Fatty acids
designated Fatty Acid Elongases or ELOVL ( elongation of very long fatty acid )catalyze the initial condensation step .
10. INTRODUCTION OF DOUBLE BONDS (DESATURATION):-
10.1. FORMATION OF MONOENOIC ACIDS
Palmitic acid and stearic acids serve as precursors of the two monoenoic fatty acids of animal 9 tissues , namely , palmitoleic and oleic acids , both of which possess a cis double in the position . Although most organisms can form palmitoleic and oleic acids , the pathway and enzymes employed differ between aerobic and anaerobic organisms.
10.1.1. AEROBIC REACTIONS
Desaturases (enzymes) introduce double bonds at a specific positions in a fatty acid chain .There are 4 fatty acyl desaturase enzymes in mammals designated 9 , 6, 5, and 4 fatty acyl- CoA desaturase . Mammals can synthesize long chain unsaturated fatty acids using desaturation and elongation . Mammalian cells are unable to produce double bonds at certain 12 locations ,e.g. . Thus some polyunsaturated fatty acids are dietary essentials , e.g. linoleic 9,12 acid , 18:2 cis (18 carbon atoms long , with cis double bonds between carbons 9-10 & 12-13 ).
Formation of a double bond in a fatty acid involves the following endoplasmic reticulum membrane proteins (monooxygenase system ) in mammalian cells ( and other vertebrate aerobic cells ):
NADH-cyt b5 Reductase , a flavoprotein with FAD as prosthetic group . Cytochrome b5 , which may be a separate protein or a domain at one end of the desaturase.. Desaturase , with an active site that contains two iron atoms complexed by histidine residues.
The desaturase catalyses a mixed function oxidation reaction . There is a 4- electron reduction of O2 to form 2H2O as a fatty acid is oxidized to form a double bond.
Two electrons pass from NADH to the desaturase via the FAD-containing reductase and cytochrme b5 , the order of electron transfer being : NADH , FAD , Cyt b5 , desaturase . Two electrons are extracted from the fatty acid as the double bond is formed . A terminal cyanide -sensitive factor (CSF) , a protein , is required to activate the acyl-CoA and the oxygen . In some plants and some lower aerobic organisms cytochrome b5 does not participate but is replaced by an iron-sulfur protein .
Biochemistry Botany Fatty acids
10.1.2. ANAEROBIC REACTIONS
In many bacteria an entirely different mechanism , one not employing molecular oxygen , comes into play.It involves dehydration of a specific saturated intermediate- length β- hydroxyacyl-ACP, rather than oxidative desaturation of fatty acyl-CoA . In E.coli the biosynthesis of palmitoleic acid starts with β- hydroxydecanoyl- ACP , formed by the fatty acid synthase system as follows. Fab A is a β- hydroxydecanoyl-ACP dehydrase – it is specific for the 10 carbon saturated fatty acid synthesis intermediate ( β –hydroxydecanoyl-ACP ) . Fab A catalyzes the dehydration of β- hydroxydecanoyl-ACP ,causing the release of water and insertion of the double bond between C7 and C8 counting from the methyl end. This creates the trans-2-decenoyl intermediate. Either the trans-2 decenoyl intermediate can be shunted to the normal saturated fatty acid synthesis pathway by FabB , where the double bond will be hydrolysed and the final product will be a saturated fatty acid , or FabA will catalyze the isomerization into the cis- 3- decenoyl intermediate. FabB is a β-ketoacyl- ACP synthase that elongates and channels intermediates into the mainstream fatty acid synthesis pathway . When FabB reacts with the cis-decenoyl intermediate , the final end product after elongation will be an unsaturated fatty acid . The two main unsaturated fatty acids made are Palmitoleoyl-ACP ( 16:1ω7 ) and cis – vaccenoyl-ACP ( 18:1ω7 ) .
This figure has been taken from : http://en.wikipedia.org/wiki/Fatty_acid_synthesis
Biochemistry Botany Fatty acids
10.2. FORMATION OF POLYENOIC ACIDS:-
Bacteria do not contain polyenoic acids ; however , these are found abundantly in higher plants and animals. Mammals contain four distinct families of polyenoic acids, which differ in the number of carbon atoms between the terminal methyl group and the nearest double bond .. These are named from their precursor fatty acids , namely , palmitoleic , oleic , linoleic , linolenic acids , and are formed either by the process of elongation and / or desaturation .The elongation of chain occurs at the carboxyl end by the mitochondrial or microsomal systems described above . The desaturation steps occur by the action of the cytochrome b5- oxygenase system with NADPH as coreductant of oxygen like the steps in the formation of palmitoleic and oleic acids .
11. REGULATION OF FATTY ACID BIOSYNTHESIS:-
There are three metabolic control mechanisms that regulate flux through the fatty acid synthesis pathway .
1. Excess acetyl- CoA in the mitochondria results in citrate export to the cytosol which activates acetyl-CoA carboxylase activity (stimulates enzyme polymerization ) , thereby producing malonyl- CoA . 2. Malonyl-CoA inhibits carnitine acyltransferase I activity to prevent mitochondrial import and degradation of newly synthesized fatty acyl –CoA molecules . 3. When palmitoyl –CoA levels exceed the metabolic needs of the cell , feedback inhibition of acetyl-CoA carboxylase activity by palmitoyl-CoA ( stimulates enzyme depolymerization ) decreases flux through the fatty acid synthesis pathway .
This figure has been taken from:- www.learningace.com/doc/476
Biochemistry Botany Fatty acids