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Lipid Catabolism

Lehninger 5th ed. Chapter 17

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Fatty acid oxidation

•  Central energy producing pathway in animals, many protists and bacteria.

•  Generates acetyl-CoA in a four-step process. •  The acetyl-CoA often drives the Krebs cycle. •  In the liver it may be converted in ketone bodies. •  In plants it is mostly a biosynthetic precursor. •  It happens mostly in the mitochondria, but also in

peroxisomes and glyoxisomes.

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Fat as food

•  Per weight the energy stores in fat is more than double than that of sugar.

•  Per hydrated weight the differences is even bigger.

•  Thus fat is the ideal chemical for energy storage.

•  Emulsification is needed however.

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Fat sources

•  Food. •  Lipid droplets in cells / adipose tissue. •  Fat synthesized in one organ for export to

another. •  We can convert excess sugar into fat. •  We cannot do the opposite, although plants

can.

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7 Epinephrine Glucagon

Serum albumin

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Acyl-carnitine transport

•  Linkage of two pools of coenzyme A: •  Cytosol

– Biosynthesis of fatty acids •  Mitochondria:

– Degradations of pyruvate – Fatty acids – Some amino acids.

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Trans

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Unsaturated fatty acids

•  Recall that the C=C bond is cis. •  The hydratase though is highly stereo-

specific for a trans bond.

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1 C=C bond

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Polyunsaturated fatty acids

•  “Another” double bond, 3 carbons removed requires additional help.

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>1 C=C bond

Energy Input!

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Odd number fatty acids

•  Common in plants and some marine organisms.

•  Propionate is a common food additive (mold inhibition) and is also found naturally (ruminating animals).

•  The last cleavage will yield Acetyl-CoA and propionyl-CoA.

•  The propionyl-CoA is used differently.

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Regulation

•  Once the fatty acid enters the mitochondria it is committed to oxidation.

•  Thus the key step is the carnitine aided transfer.

•  Malonyl-CoA inhibits the carnitine acyltransferase I.

•  Two other enzymes are inhibited as well.

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Heat production

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Glyoxysomes

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Omega oxidation

•  Takes place in the ER. •  10-12 carbon fatty acids are the preferred

substrates. •  The oxygen is from O2 and the enzymes

involves cytochrome p450 and NADPH.

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• Ketone bodies • Can be used by the brain. • Produced in the liver and���exported to the rest of the body. • The production and export of���ketone bodies frees up Co-A ���for continued fatty acid oxidation.

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