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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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