BIOC 3560 FA metabolisn.docx

Nadh and fadh2 donate e to mitochondrial respiratory

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NADH and FADH2 donate e- to mitochondrial respiratory chain, ultimately yielding ATP β-oxidation converts FAs to acetyl-CoA One cycle of β-oxidation results in one acetyl-CoA being removed from the carboxyl end of the fatty acid chain In each pass two carbon atoms are removed Continues until only acetyl-CoA is left From complete oxidation of C15-CoA, 108 ATP is formed FA degradation: steps 1 and 2 Dehydrogenation o Fatty acyl-CoA (Cn) FADH2 + trans-Δ2-enoyl-CoA by acyl-CoA dehydrogenase Hydration o Trans-Δ2-enoyl-CoA + H2O L-β-hydroxyacyl-CoA by enoyl-CoA hydratase FA degradation: steps 3 and 4 Dehydrogenation o L-β-hydroxyacyl-CoA NADH + β-ketoacyl-CoA by β-hydroxyacyl-CoA dehydrogenase Cleavage o β-ketoacyl-CoA acetyl-CoA + fatty acyl-CoA (Cn-2) by acyl-CoA acetyltransferase (thiolase) Problem cases – oxidation of unsaturated FAs and odd chain FAs Problem 1: cis bond - β-oxidation works on trans double bonds
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β-oxidation enzymes ignore the kink and process cis-fatty acids normally β-oxidation proceeds as normal for three rounds Cis-Δ3 FAs are not substrates for acyl-CoA dehydrogenase because C3 is already in a double bond The enzyme Δ3,Δ2-enoyl-CoA isomerase can move the double bond It converts the cis-Δ3 FA to trans-Δ2 Trans-Δ2 FA is a substrate for enoyl-CoA hydratase, it enters in step 2 β-oxidation proceeds normally Net result: one fewer FADH2 (bc acyl-CoA dehydrogenase skipped this cycle) Problem 2: two double bonds – similar to cis bond problem While the first double bond is in the 9 position (odd), the second will be at the 12 position (even) β-oxidation will sequentially produce both cis-Δ3 and cis-Δ4 FA intermediates The reaction proceeds as with one double bond to produce a cis-Δ3,Δ6 FA Cis-Δ3,Δ6 FA is converted to trans-Δ cis-Δ6 by Δ3,Δ2 enoyl-CoA isomerase Trans-Δ cis-Δ6 is a substrate for the hydratase This β-oxidation cycle then completes normally, yielding a cis Δ4 FA With a cis-Δ4 FA substrate, enoyl-CoA reductase produces a trans-Δ2,cis-Δ4 FA The resulting conjugated double bond cannot be hydrated by enoyl-CoA hydratase Reduced 2,4dienoyl-CoA reductase, yielding a trans-Δ3 FA Trans-Δ3 FA is then converted to trans-Δ2 FA by Δ3,Δ2 enoyl-CoA isomerase β-oxidation then proceeds normally Problem 3: odd number carbons - β-oxidation works on even number of carbons Initially β-oxidation proceeds normally Propionyl-CoA (3C atoms) cannot be oxidized by acyl-CoA dehydrogenase Instead a separate three enzyme pathway carboxylates propionyl-CoA to succinyl-CoA (4C atoms) Succinyl-CoA is part of TCA Ketone bodies Acetyl-CoA can then enter the TCA Optionally, acetyl-CoA in the liver can be turned into ketone bodies Ketone bodies allow acetyl-CoA to enter the citric acid cycle in a different cell This frees up CoA, so further FA can be degraded Ketone bodies act as an alternative fuel eg. for brain (which cannot use FAs)
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  • Spring '10
  • dawson
  • FAS, Ketone bodies

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