LIPID BIOSYNTHESISFatty acid biosynthesis-basic fundamentalsFatty acid biosynthesis-elongation and desaturationTriacylglycerolsPhospholipidsCholesterolCholesterol metabolism

Fatty Acid BiosynthesisCytosolRequires NADPHAcyl carrier proteinD-isomerCO2 activationKeto saturatedMitochondriaNADH, FADH2CoA L-isomerNo CO2 Saturated keto Beta OxidationSynthesis

Rule:Fatty acid biosynthesis is a stepwise assemblyof acetyl-CoA units (mostly as malonyl-CoA) ending with palmitate (C16 saturated)ActivationElongationTermination3 Phases

ACTIVATIONCarboxybiocytinBiotinAcetyl-CoA carboxylaseCofactor

Acetyl-CoA CarboxylaseThe rate-controlling enzyme of FA synthesis In Bacteria -3 proteins(1) Carrier protein with Biotin(2) Biotin carboxylase (3) TranscarboxylaseIn Eukaryotes - 1 protein(1) Single protein, 2 identical polypeptide chains (2) Each chain Mwt = 230,000 (230 kDa)(3) Dimer inactive (4) Activated by citrate which forms filamentous form of protein that can be seen in the electron microscope

Yeast Fatty Acid Synthase Complex2,500 kDa Multienzyme Complex6 molecules of 2 peptide chains called A and B(66)A: (185,000) Acyl Carrier protein -ketoacyl-ACP synthase (condensing enzyme) -ketoacyl-ACP reductaseB: (175,000) -hydroxy-ACP dehydrase enoyl-ACP reductase palmitoyl thioesteraseFatty AcidSynthaseComplex

Acyl carrier protein10 kDaCysteaminePhosphopantetheineCoenzyme AAcyl Carrier Protein

Overall ReactionHS-CoACO2NOTE:Malonyl-CoA carbons become new COOH endNascent chain remains tethered to ACPAcyl CarrierProteinCO2, HS-CoA are released at each condensationMalonyl-CoA + ACPInitiation

NADPH-H2ONADPHElongationReductionDehydrationReduction-Ketoacyl-ACP reductase -Hydroxyacyl-ACP dehydraseEnoyl-ACP reductase

-KSCO2-S-ACPTERMINATIONKetoacyl ACPSynthaseTransfer to KSSplit out CO2Transfer to Malonyl-CoAWhen C16 stage is reached, instead of transferring to KS,the transfer is to H2O and the fatty acid is released

CO2H2OInitiation or primingElongationFatty Acid Synthase-Ketoacyl-ACP reductase-Hydroxyacyl-ACP dehydraseEnoyl-ACP reductase-Keto-ACP synthase (condensing enzyme)Malonyl-CoA-ACP transacylaseAcetyl-CoA-ACP transacylase-Ketoacyl-ACP synthase

TETESubstrate EntryReductionThioesterasepalmitate releaseSubstrate EntryReductionThioesterasepalmitate releaseACPACPHSSHATATCH2CH2HSSHCECEMTMTERERKRKRDHDHTranslocationTranslocation

Overall Reactions7H+

PROBLEM:Fatty acid biosynthesis takes place in thecytosol. Acetyl-CoA is mainly in the MitochondriaHow is acetyl-CoA made available to the cytosolicfatty acyl synthase?SOLUTION:Acetyl-CoA is delivered to cytosol from the mitochondria as CITRATE

Citrate lyaseAcetyl-CoANADHOAAL-malateL-malatemitochondriaCytosolPyruvateMalic enzymeOAAPyrMalatedehydrogenase

Post-Synthesis ModificationsC16 satd fatty acid (Palmitate) is the productElongationUnsaturationIncorporation into triacylglycerolsIncorporation into acylglycerol phosphates

Elongation of Chain (two systems)Malonyl-CoA* (cytosol)Elongation systems arefound in smooth ER andmitochondriaAcetyl-CoA(mitochondria)

DesaturationRules:The fatty acid desaturation system is in the smooth membranes of the endoplasmicreticulumThere are 4 fatty acyl desaturase enzymes in mammals designated 9 , 6, 5, and 4 fattyacyl-CoA desaturaseMammals cannot incorporate a double bondbeyond 9; plants can.Mammals can synthesize long chain unsaturated fatty acids using desaturation and elongation

The Desaturase System requires O2 andresembles an electron transport systemRule:NADHCyt b5 reductaseCyt b5O2Saturated FA-CoA(FAD)NOTE:1. System is in ER membrane2. Both NADH and the fatty acid contribute electrons3. Fatty acyl desaturase is considered a mixed function oxidase

DesaturaseCyt b5reductaseCyt b5C18-stearoly-CoA + O2 + 2H+C18 9-oleyl-CoA + 2H2O2 cyt b5 Fe2+2 cyt b5 Fe2+2H+ + cyt b5 reductaseFADcyt b5 reductase FADH2NADH + H+NAD+Fatty acid desaturation systemDesaturase

PalmitateStearateOleateLinoleate-Linolenate-LinolenateEicosatrienoateArachidonate18:3(9,12,15)18:2(9,12)18:3(6,9,12)16:018:0Elongase18:1(9)Palmitoleate16:1(9)DesaturaseDesaturaseDesaturaseDesaturaseDesaturaseDesaturaseElongase20:3(8,11,14)20:4(5,8,11,14)Other lipidsPermittedtransitionsin mammalsEssentialfatty acid