Lipid Catabolism

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Lipid Catabolism
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Lesson Learning Outcome

• Upon completion of this lecture, students should
  be able to
• understand lipid catabolism

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Lipid catabolism

• Fatty acids are the major source of energy in
  catabolism of lipids
• Sterols (eg steroids) are not catabolized but are
  excreted out
• Thus lipid molecules that contain fatty acids are
  the ones that can be catabolized for energy
  production
• The release of fatty acids from the glycerol
  backbone is done by enzyme lipases.
• The release of fatty acids from triacyglycerides
  (TAG) in adipose tissue is hormone-controlled

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Liberation of fatty acids from TAG in adipose
tissue is hormone-dependant.
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Fatty Acids and Energy

• Fatty acids in triacylglycerols are the principal
  storage form of energy for most organisms
• their carbon chains are in a highly reduced form
• the energy yield per gram of fatty acid oxidized
  is greater than that per gram of carbohydrate
  oxidized

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Fig. 21-1, p. 592
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1. Activation of fatty acid

• A thioester bond is formed between the carboxyl
  group of the fatty acid and the thiol group of
  the CoA ? acyl-CoA
• Enzyme responsible is acyl-CoA synthase, reaction
  require ATP
• Acyl-CoA can cross the outer mitochondrial
  membrane but not the inner membrane

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1. Activation of fatty acid
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Transport of Acyl-CoA

• In the inter membrane space, acyl group is
  transferred to carnithine by transesterification
  by carnitine acyltransferase (located in the
  inner membrane)
• Acyl carnitine can then cross the inner membrane
  via specific transporter called carnitine
  translocase

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?-oxidation

• In the matrix, repeated sequence of reactions
  cleaves two-carbon units from the fatty acids
  starting from the carboxyl end ? process called
  ß-oxidation
• The whole cycle requires four reactions.
• Each cycle produces single molecules of FADH2,
  NADH, and acetyl-CoA
• and yields a fatty acid shortened by two carbons.
• This series of reactions is then repeated on the
  shortened fatty acyl chain and continues until
  the entire fatty acid chain is degraded to
  acetyl-CoA

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• Reaction 1 oxidation of the ?,? carbon-carbon
  single bond to a carbon-carbon double bond

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• Reaction 2 hydration of the carbon-carbon double
  bond

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• Reaction 3 oxidation of the ?-hydroxyl group to
  a carbonyl group

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• Reaction 4 cleavage of the carbon chain by a
  reverse Claisen reaction

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• the overall equation for oxidation of stearic
  acid can be obtained by adding the equations for
  b-oxidation, the citric acid cycle, and oxidative
  phosphorylation

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Odd-Numbered Fatty Acids

• The last ?-oxidation cycle of a fatty acid with
  an odd number of carbons gives propionyl-CoA

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Ketone Bodies

• Ketone bodies acetone, ?-hydroxybutyrate, and
  acetoacetate
• formed principally in liver mitochondria
• can be used as a fuel in most tissues and organs
• Formation occurs when the amount of acetyl-CoA
  produced is excessive compared to the amount of
  oxaloacetate available to react with it
• intake high in lipids and low in carbohydrates
• diabetes not suitably controlled
• starvation

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Ketone Bodies
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End of lecture