Catabolism of Lipids

8/3/2019 Catabolism of Lipids

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Catabolism of lipidsCatabolism of lipids

Lipolysis


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Introduction In order for muscle, liver, and adipose tissue to oxidize the fatty acids

derived from triglycerides to produce ATP, the triglycerides must first be

split into glycerol and fatty acids,a process called Lipolysis

Lipases Epinephrine and norepinephrine enhance triglyceride breakdown into fatty

acids and glycerol.

Hormones- Sympathetic tone increases(exercise)


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Keyconcept Glycerol may be converted to glyceraldehyde 3-phosphate,which can then

converted to glucose or enter the krebs cycle for oxidation.

Fatty acids undergo beta oxidation and enters the krebs cycle via acetylcoenzymes A.


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Glycerol Glycerol is converted by many cells of body to glyceraldehyde 3-

phosphate,one of the compounds also formed during the catabolism of

glycose

If ATP supply in a cell is high, glyceraldehyde 3-phosphate is converted

into glycose(glyconeogenesis)

If ATP supply in a cell is low, glyceraldehyde 3-phosphate enters the

catabolic pathway to pyruvic acid


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Fatty acids Yeilds more ATP than glycerol

Beta oxidation (matric of mitochondria)


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Dehydrogenation by FAD: The first step is the oxidation ofthefatty acid by Acyl-CoA-Dehydrogenase. The enzyme catalyzesthe formation ofa double bond between the C-2 and C-3.


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Hydration: The next step is the hydration ofthe bond betweenC-2 and C-3. The reaction is stereospecific, forming only the

L isomer.


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Oxidation by NAD+: The third step is the oxidation of L--hydroxyacyl CoAby NAD+. This converts the hydroxyl group into

a keto group.


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Thiolysis The final step is the cleavage of-ketoacyl CoAby

the thiol group ofanother molecule of CoA. The thiol isinserted between C-2 and C-3.


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Ketogenesis

Ketogenesis is the process by which ketone bodies are produced as a

result of fatty acid breakdown.

Ketogenesis may or may not occur, depending on levels of availablecarbohydrates in the cell or body. This is closely related to the paths of

acetyl-CoA:

When the body has ample carbohydrates available as energy

source, glucose is completely oxidized to CO2; acetyl-CoA is formed as anintermediate in this process, first entering the citric acid cycle followed by

complete conversion of its chemical energy to ATP in oxidative

phosporylation.


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When the body has excess carbohydrates available, some glucose is fully

metabolized, and some of it is stored by using acetyl-CoA to create fatty

acids. (CoA is also recycled here.)

When the body has no free carbohydrates available, fat must be broken

down into acetyl-CoA in order to get energy. Acetyl-CoA is not being

recycled through the citric acid cycle because the citric acid cycle

intermediates (mainly oxaloacetate) have been depleted to feed

the gluconeogenesis pathway, and the resulting accumulation of acetyl-

CoA activates ketogenesis.


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Catabolism of Lipids