BIOC 3560 FA metabolisn.docx

All reactions are catalyzed by a single multi enzyme

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All reactions are catalyzed by a single multi-enzyme complex: fatty acid synthase (FAS) 6 enzyme functions plus ACP in one polypeptide chain Domains: o KS – β-ketoacyl-ACP synthase o DH – β-hydroxyacyl-ACP dehydratase o ER – enoyl-ACP reductase o KR – β-ketoacyl-ACP reductase o MAT – malonyl/acetyl-CoA-ACP transferase Initiating FA synthesis Malonyl/acetyl-CoA ACP transferase (MAT) transfers acetyl from acetyl-CoA to acyl carrier protein (ACP) β-ketoacyl synthase (KS) transfers acetyl from ACP to itself MAT then transfers malonyl group to ACP KS is now ready for the first condensation step One round of FA synthesis Condensation: FAS complex charged with an acetyl and malonyl group undergoes condensation by KS β–keto reduction: reduction of β-keyobutyryl-ACP by KR Dehydration: β-hydroxybutyryl-ACp is formed then undergoes dehydration by DH Enoyl reduction: trans-Δ2-butenoyl-ACP is formed then undergoes reduction of its double bond by ER to form butyryl-ACP Translocation: butyryl group undergoes translocation to Cys on KS Beginning of second round of FA synthesis KS transfers butyryl group from ACP to itself MAT transfers new malonyl group to ACP KS condenses malonyl group with butyryl group Palmitate (C16) synthesis First acetyl group ends up furthest from the carboxylate Substrate is physically linked to FAS through the whole process Palmitic acid (C16 FA) is released from FAS by cleavage by the last enzyme, thioesterase Synthesis of longer chain FAs Palmitate is the precursor for longer chain FAs Elongation occurs 2 C at a time (still using malonyl) CoA instead of ACP Different enzymes for different steps (generic steps are elongases and desaturases) Palmitate (16:0) can either undergo desaturation to form palmitoleate (16:1 Δ9) or elongation to form stearate (18:0)
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Stearate can undergo elongation to form longer saturated FAs, or desaturation to form oleate (18:1 Δ9) In plants only, oleate will undergo desaturation to form linoleate (18:2 Δ9,12) Production of Δ9 unsaturated FAs In mammals, fatty acyl-CoA desaturases make cis double bonds They desaturate only at the 9 position, so palmitate (16:0) is oxidized to palmitoleate (16:1 Δ9), and stearate (18:0) to oleate (18:1 Δ9) Fatty acyl-CoA desaturases require O2 and cytochrome b5, which must be re-reduced using NADPH Other unsaturated FAs Mammals cannot further desaturate Δ9 FAs However, we absolutely require linoleate (18:2 Δ9,12) as a precursor for other products such as prostaglandins Plants and bacteria make linoleate to promote membrane fluidity Linoleate is therefore an essential dietary FA Triacylglycerol assembly FAs are stored as triacylglycerols Two sources of glycerol-3-PO4 Major source is dihydroxyacetone PO4 Acyl-CoA synthetases activate FA groups with CoA Acyl transferases then transfer the FA to glycerol-3-PO4 The triacylglycerol cycle
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