Overview of Fatty Acid Catabolism
Fatty acid catabolism is the mechanism by which the body accesses energy stored as triglycerides. There are three steps in fatty acid catabolism. First the body must mobilize the lipid stores by breaking down triglycerides into free fatty acids and glycerols.
Lipid Mobilization in Fat Cells
During fat mobilization, triglycerides are broken down into free fatty acids and glycerols in a process called lipolysis. Lipolysis is stimulated by the hormones epinephrin, glycagon, or adrenalcorticaltropic hormone (ACTH). These hormones bind to receptors on the plasma membrane of the cell and initiate a signal cascade. The first step of the signal cascade is the activation of adenylyl cyclase, which is the enzyme required to synthesize cyclic AMP from ATP. High levels of cyclic AMP activate protein kinase A. The protein kinase then uses ATP to activate the triaclyglycerol lipase. The phosphorylated lipase can then catalyze the hydrolysis of triglycerides to free fatty acids. Multiple lipases work to hydrolyze the fatty acids off of the glycerol.
Lipid Mobilization in the Duodenum
Dietary fats are mobilized in the duodenum, by lipases secreted by the pancreas.
The Fate of Glycerol and Fatty Acids
Once a triglyceride is completely broken down, the free fatty acids bind to serum albumin in the blood stream and carries the free fatty acids to the tissues that need energy. Glycerol is absorbed by the liver.
Now that we have glycerols and free fatty acids, what do we do with them?
The glycerol represents 5% of the energy available from triglycerides. In order to use the glycerol we need to activate it by phosphorylating it. Glycerol 3-phosphate is oxidized into Dihydroxyacetone phosphate (DHAP), which is then isomerized into Glyceraldehyde 3- phosphate (G3P). You should recognize G3P as an intermediate in the glycolytic and gluconeogenic pathways and can be converted into pyruvate or glucose.
Transport of Activated of Fatty Acids into the Mitochondria:
β-Oxidation of Fatty Acids Produces Acetyl CoA, NADH, and FADH2: