Fatty acids are a class of compounds containing a long hydrophobic hydrocarbon chain and a terminal carboxylate group They exist free in the body as well as fatty acyl esters in more complex molecules such as triglycerides or phospholipids. Fatty acids can be oxidized in all tissues, particularly liver and muscle to provide energy They are also structural components of membrane lipids such as phospholipids and glycolipids. Esterified fatty acids, in the form of triglycerides are stored in adipose cells Fatty acids are also precursors of Eicosanoids
Diet Adipolysis De novo synthesis(from precursors)- Carbohydrates, protein, and other molecules obtained from diet in excess of the body’s need can be converted to fatty acids, which are stored as triglycerides
Fatty acids are synthesized by an extramitochondrial systemThis system is present in many tissues,including liver, kidney, brain, lung, mammarygland, and adipose tissue.Acetyl-CoA is the immediate substrate, andfree palmitate is the end product.Its cofactor requirements include NADPH,ATP, Mn2+, biotin, and HCO3– (as a source ofCO2).
FA synthasecomplex isfoundexclusively inthe cytosol.The locationsegregatessyntheticprocesses fromdegradativereactions.
NADPH is involved as donor of reducingequivalentsThe oxidative reactions of the pentose phosphatepathway are the chief source of the hydrogenrequired for the reductive synthesis of fatty acids.Tissues specializing in active lipogenesis—ie, liver, adipose tissue, and the lactatingmammary gland—possess an active pentosephosphate pathway.Other sources of NADPH include the reaction thatconverts malate to pyruvate catalyzed by the "Malicenzyme" (NADP malate dehydrogenase) and theextra mitochondrial isocitrate dehydrogenasereaction (probably not a substantial source, except
In hepatocytes, adipose tissue and the lactatingmammary glands, the NADPH is supplied primarily bythe pentose phosphate pathway.
Reversible reaction, pyruvate produced in thereaction reenters the mitochondrion for furtherutilization
There are three isoenzymes of isocitratedehydrogenase. One, which uses NAD+, is foundonly in mitochondria. The other two use NADP+and are found in mitochondria and the cytosol.Respiratory chain-linked oxidation of isocitrateproceeds almost completely through the NAD+-dependent enzyme.
Acetyl co A, theprecursor for fattyacid synthesis isproduced frompyruvate,ketogenic aminoacids, fatty acidoxidation and byalcoholmetabolismIt is a substratefor TCA cycle anda precursor forfatty acids, ketonebodies andsterols.
Fattyacid synthesis requires considerableamounts of acetyl-CoANearly all acetyl-CoA used in fatty acidsynthesis is formed in mitochondria Acetyl co A has to move out from themitochondria to the cytosol Cytosol – site of acetate utilization Mitochondria – site of acetate synthesis
Acetate is shuttled out of mitochondria ascitrateThe mitochondrial inner membrane isimpermeable to acetyl-CoAIntra-mitochondrial acetyl-CoA first reactswith oxaloacetate to form citrate, in theTCA cycle catalyzed by citrate synthaseCitrate then passes into the cytosolthrough the mitochondrial inner membraneon the citrate transporter.In the cytosol, citrate is cleaved by citratelyase regenerating acetyl-CoA.
The other product of Citrate cleavage,oxaloacetate can be-Channeled towards glucose productionConverted to malate by malatedehydrogenaseConverted to Pyruvate by Malic enzyme,producing more NADPH, that can be used forfatty acid synthesisPyruvate and Malate pass through specialtransporters present in the innermitochondrial membrane
Two main enzymes-Acetyl co A carboxylaseFatty acid SynthaseBoth the enzymes are multienzymecomplexesCoenzymes and cofactors are-BiotinNADPHMn++Mg++
Acetyl co A carboxylase -Is the Initial & ControllingStep in Fatty Acid SynthesisMultienzyme complex containing-BiotinBiotin CarboxylaseBiotin carboxyl carrier proteinTranscarboxylaseA regulatory allosteric site
Fatty acid Synthase complex-The Fatty Acid Synthase Complex is apolypeptide containing seven enzymeactivitiesIn bacteria and plants, the individualenzymes of the fatty acid synthase systemare separate, and the acyl radicals are foundin combination with a protein called the acylcarrier protein (ACP).In yeast, mammals, and birds, the synthasesystem is a multienzyme polypeptidecomplex that incorporates ACP, which takes
In mammals, the fatty acid synthase complex isa dimer comprising two identical monomers,each containing all seven enzyme activities offatty acid synthase on one polypeptide chainThe use of one multienzyme functional unit hasthe advantages of achieving the effect ofcompartmentalization of the process within thecell without the erection of permeability barriers,Synthesis of all enzymes in the complex iscoordinated since it is encoded by a single gene.
Step-1The input to fatty acid synthesis is acetyl-CoA, which iscarboxylated to malonyl-CoA. The reaction is catalyzed by Acetylco A carboxylase
ATP-dependent carboxylation provides energyinput.The CO2 is lost later during condensation with thegrowing fatty acid.The spontaneous decarboxylation drives thecondensation reaction.As with other carboxylation reactions, theenzyme prosthetic group is biotin.The reaction takes place in two steps:carboxylation of biotin (involving ATP) and transferof the carboxyl to acetyl-CoA to form malonyl-CoA.
Biotin is linked to the enzyme by an amide bondbetween the terminal carboxyl of the biotin sidechain and the -amino group of a lysine residue. O O C C N NH O CH CH O O C H2C CH S (CH2)4 C NH (CH2)4 CH Carboxybiotin lysine NH residue
Enzyme-biotin - HCO3 + ATP 1 ADP + Pi - Enzyme-biotin-CO2 O ll 2 CH3-C-SCoA Enzyme-biotin acetyl-CoA O - ll O2C-CH2-C-SCoA malonyl-CoAThe overall reaction, which is spontaneous, may besummarized as:HCO3 + ATP + acetyl-CoA ADP + Pi + malonyl-CoA
Once malonyl-CoA is synthesized, longcarbon FA chains may be assembled in arepeating four-step sequence.With each passage through the cycle thefatty acyl chain is extended by two carbons.When the chain reaches 16 carbons, theproduct palmitate (16:0) leaves the cycle.
All the remaining steps are catalyzed byFatty acid synthase complexFatty Acid Synthase prosthetic groups:The thiol (-SH)of the side-chain of acysteine residue of keto acyl synthaseenzyme(also called condensing enzyme)The thiol (-SH)ofphosphopantetheine, equivalent instructure to part of coenzyme A. It is acomponent of Acyl carrier protein
Each segment of the disk represents one of the six enzymatic activities of the complex (Thioesterase not shown) At the center is the ACP – acyl carrier protein - with its phosphopantethein -e arm ending in – SH.
SH phosphopantetheinePhosphopantetheine CH2 of acyl carrier protein(Pant) is covalently CH2inked via a phosphate -mercaptoethylamine NHester to a serine OH ofthe acyl carrier protein C Odomain of Fatty Acid CH2Synthase. CH2 pantothenate NHThe long flexible arm C Oof phosphopantetheine HO C Hhelps its thiol to move H3C C CH3 O NHfrom one active site to serineanother within the H2C O P O CH2 CH residuecomplex. O C O phosphate
Serve as a flexiblearm, tethering thegrowing fatty acyl chain tothe surface of thesynthase complexCarrying the reactionintermediates from oneenzyme active site to thenext.
To initiate FA biosynthesis, malonyl and acetylgroups are activated on to the enzyme fatty acidsynthase.Initially, a priming molecule of acetyl-CoAcombines with a cysteine —SH group catalyzedby acetyl transacylaseMalonyl-CoA combines with the adjacent —SHon the 4-phosphopantetheine of ACP of theother monomer, catalyzed by malonyltransacylase (to form acetyl (acyl)-malonylenzyme.
The acetyl groupfrom acetyl-CoA istransferred to theCys-SH group of the -ketoacyl ACPsynthaseThis reaction iscatalyzed by acetyl-CoA transacetylase.
Transfer of themalonyl group to the –SH group of the ACP iscatalyzed by malonyl-CoA ACP transferase.The charged acetyland malonyl groupsare now in closeproximity to eachother
After activation, the processes involved are-1. Condensation2. Reduction3. Dehydration4. ReductionThese steps are repeated till a fatty acid with 16carbon atoms is synthesized
The acetyl group attacks the methylenegroup of the malonyl residue, catalyzed by3-ketoacyl synthase, and liberates CO2,forming 3-ketoacyl enzyme (Acetoacetylenzyme),freeing the cysteine —SH group.Decarboxylation allows the reaction to go tocompletion, pulling the whole sequence ofreactions in the forward direction.
Condensation –Condensation of theactivated acetyl andmalonyl groups takesplace to formAcetoacetyl-ACPThe reaction iscatalyzed by β-ketoacyl-ACPsynthase.
Reduction-The Acetoacetyl-ACP is reduced to b-hydroxybutyryl-ACP, catalyzed by b-ketoacyl-ACPreductaseNADPH + H+ arerequired forreduction
Dehydration –Dehydration yieldsa double bond inthe product, trans-∆2-butenoyl-ACP,Reaction iscatalyzed by β-hydroxybutyryl-ACP dehydratase.
ReductionReduction of thedouble bond takesplace to form butyryl-ACP, Reaction is catalyzedby enoyl-reductase.Another NADPHdependent reaction.
This reactionmakes way for thenext incomingmalonyl group.The enzymeinvolved is acetyl-CoAtransacetylase
The butyryl group is on the Cys-SH group The incoming malonyl group is first attached to ACP. In the condensation step, the entire butyryl group is exchanged for the carboxyl group on the malonyl residue
The 3-ketoacyl group isreduced, dehydrated, and reduced again(reactions 2, 3, 4) to form the correspondingsaturated acyl-S-enzyme.A new malonyl-CoA molecule combines withthe —SH of 4-phosphopantetheine, displacingthe saturated acyl residue onto the free cysteine—SH group.The sequence of reactions are repeated until asaturated 16-carbon acyl radical (Palmityl) hasbeen assembled. It is liberated from the enzyme complex by theactivity of a seventh enzyme in thecomplex, Thioesterase (deacylase).
Seven cycles of condensation and reduction producethe 16-carbon saturated palmitoyl group, still bound toACP.Chain elongation usually stops at this point, and freepalmitate is released from the ACP molecule byhydrolytic activity in the synthase complex.Smaller amounts of longer fatty acids such as stearate(18:0) are also formedIn mammary gland, there is a separate Thioesterasespecific for acyl residues of C8, C10, or C12, which aresubsequently found in milk lipids.
First, the formation of seven malonyl-CoA molecules: 7Acetyl-CoA + 7CO2 + 7ATP 7malonyl CoA + 7ADP + 7Pi
Then the seven cycles of condensation and reductionAcetyl-CoA + 7malonyl-CoA + 14NADPH + 14H+ palmitate + 7CO2 + 8CoA + 14NADP+ + 6H2OThe biosynthesis of FAs requires acetyl-CoA and theinput of energy in the form of ATP and reducing powerof NADPH.
Βeta Oxidation Fatty acid Synthesis pathwayLocation Mitochondrial CytoplasmicAcyl Carriers(Thiols) Coenzyme A 4’ Phosphopantetheine and CysteineElectron acceptors FAD/NAD NADPHand donorsOH Intermediates L D2 Carbon Acetyl co A Acetyl co A/ Malonylproduct/donor co A
When a cell has Glycerol-Pmore energy, the Glucoseexcess is generally Triacylglycerolconverted to FattyAcids and stored aslipids such as Fatty acyl CoAtriacylglycerol. Malonyl CoA Pyruvate Acetyl CoA TCA cycle
O Acetyl-CoAThe reaction =catalyzed by acetyl- CH3-C-S-CoACoA carboxylase is HCO3-the rate limitingstep in thebiosynthesis of Ofatty acids. = -OOC-CH -C-S-CoA 2 Malonyl-CoA
The mammalian enzyme is regulated, by Allosteric control by local metabolites Phosphorylation Conformational changes associated with regulation: In the active conformation, Acetyl-CoA Carboxylase associates to form multimeric filamentous complexes. Transition to the inactive conformation is associated with dissociation to yield the monomeric form of the enzyme (protomer).
Allosteric controlPalmitoyl-CoA acts as afeedback inhibitor of theenzyme, and citrate is anactivator.When there is anincrease in mitochondrialacetyl-CoA andATP, citrate istransported out ofmitochondria,Citrate becomes boththe precursor of cytosolicacetyl-CoA and a signalfor the activation ofacetyl-CoA carboxylase.
PhosphorylationAcetyl-CoAcarboxylase isalso regulated byhormones such asglucagon,epinephrine, andinsulin viachanges in itsphosphorylationstate
Additionally, these pathways are regulated at thelevel of gene expressionLong-chain fatty acid synthesis is controlled inthe short term by allosteric and covalentmodification of enzymes and in the long term bychanges in gene expression governing rates ofsynthesis of enzymes.
Excess carbohydrates is stored as fat inmany animals in anticipation of periods ofcaloric deficiency such asstarvation, hibernation, etc, and to provideenergy for use between meals inanimals, including humans, that take theirfood at spaced intervals.The nutritional state of the organism is themain factor regulating the rate oflipogenesis.
The rate is higher in the well-fed state if thediet contains a high proportion ofcarbohydrateLipogenesis converts surplus glucose andintermediates such as pyruvate, lactate, andacetyl-CoA to fat, assisting the anabolicphase of this feeding cycleLipogenesis is increased when sucrose isfed instead of glucose because fructosebypasses the phosphofructokinase controlpoint in glycolysis and floods the lipogenicpathway
It is depressed by restricted caloricintake, high fat diet, or a deficiency ofinsulin, as in diabetes mellitusThese conditions are associated withincreased concentrations of plasma free fattyacidsAn inverse relationship has beendemonstrated between hepatic lipogenesisand the concentration of serum-free fattyacids.
Insulin stimulates lipogenesis by severalother mechanisms as well as by increasingacetyl-CoA carboxylase activity.It increases the transport of glucose into thecell (eg, in adipose tissue),Increases the availability of both pyruvatefor fatty acid synthesis and glycerol 3-phosphate for esterification of the newlyformed fatty acids,
Insulin converts the inactive form ofpyruvate dehydrogenase to the active form inadipose tissue but not in liver, thus providesmore of Acetyl co AInsulin also acts by inhibiting c AMPmediated lipolysis in adipose tissue andthereby reduces the concentration of plasmafree fatty acids (long-chain fatty acids areinhibitors of lipogenesis.
Palmitate in animal cells is the precursorof other long-chained FAs.By further additions of acetyl groups, fattyacid chain length is elongated through theaction of FA elongation systems present inthe smooth endoplasmic reticulum and themitochondria.
Palmitate and stearate serve as precursorsof the two most common monounsaturatedfatty acids of animal cells: palmitoleate(16:1 9), and Oleate (18:1 9).The double bond is introduced by fattyacyl-CoA desaturase in the smoothendoplasmic reticulum.
Mammalian hepatocytes readily introducedouble bonds at the D9 position of FAs butcannot between C-10 and the methyl-terminalend.Linoleate, 18:2D9,12 and linolenate 18:3D9,12,15cannot be synthesized by mammals, but plantscan synthesize both.Arachidonic acid is semi essential, since it canbe synthesized from Linoleic acid
Most of the FAs synthesized or ingested byan organism have one of two fates:Incorporated into triacylglycerols for thestorage of metabolic energyIncorporated into the phospholipidcomponents of membranes