17.4: Lipid Metabolism
Introduction
There are many metabolic pathways, processes, or reactions that are involved in the synthesis or degradation of lipids and compounds derived from them. Please note that excess intake of energy-containing nutrients, including the consumption of excess carbohydrates and excess protein as well as eating too much fat in one's diet can lead to 'fat production' or lipogenesis, whereby the human body is designed to almost have unlimited fat store capabilities.
Quick Overview of Lipid Pathways, Cycles, Processes, or Reactions - names & definitions
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Lipolysis
- Triglyceride breakdown
- Lipolysis is the cleavage of triglycerides to glycerol and fatty acids. There are two primary lipolysis enzymes: Lipoprotein lipase (LPL) and Hormone-sensitive lipase (HSL).
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De novo Lipogenesis (Fatty Acid Synthesis)
- Thus, de novo lipogenesis is the synthesis of fatty acids, beginning with acetyl-CoA. Acetyl-CoA has to first move out of the mitochondria, where it is then converted to malonyl-CoA (3 carbons). Malonyl-CoA then is combined with another acetyl-CoA to form a 4 carbon fatty acid (1 carbon is given off as CO2). The addition of 2 carbons is repeated through a similar process 7 times to produce a 16 carbon fatty acid.
- The newly generated triglycerides can be stored in fat cells, also known as adipocytes, and packaged within cytoplasmic lipid droplets.
- Fatty Acid Oxidation (a.k.a beta-oxidation)
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- To generate energy from fatty acids, they must be oxidized. This process occurs in the mitochondria, but long chain fatty acids cannot diffuse across the mitochondrial membrane (similar to absorption into the enterocyte). Carnitine, an amino acid-derived compound, helps shuttle long-chain fatty acids into the mitochondria.
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Ketone Body Synthesis
- In cases where there is not enough glucose available for the brain (very low carbohydrate diets, starvation), the liver can use acetyl-CoA, primarily from fatty acids (but also certain amino acids), to synthesize ketone bodies (ketogenesis). The structures of the three ketone bodies; acetone, acetoacetic acid, and beta-hydroxybutyric acid.
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Cholesterol Synthesis
- Acetyl-CoA is also used to synthesize cholesterol. There are a large number of reactions and enzymes involved in cholesterol synthesis.
Let's pull it all together what you learned about 'fat and lipid metabolism'
Please watch this short video to help you remember: