Lipolysis and the Oxidation of Fatty Acids

http://themedicalbiochemistrypage.org/fatty-acid-oxidation.php

Utilization of dietary lipids requires that they first be absorbed through the intestine. As these molecules are oils they would be essentially insoluble in the aqueous intestinal environment. Solubilization (emulsification) of dietary lipid is accomplished initially via the agitation action as food passes through the stomach and then continues within the intestine viabile salts that are synthesized in the liver and secreted from the gallbladder.

The emulsified fats can then be degraded by salivary, gastric and pancreatic lipases. The lipases found in the gastrointestinal tract include lingual lipase (secreted by the serous glands of the tongue), gastric lipase (secreted by chief cells of the stomach), pancreatic lipase, and pancreatic lipase-related proteins 1 and 2. These enzymes generate free fatty acids and a mixture of mono- and diacylglycerols from dietary triacylglycerols. Pancreatic lipase degrades triacylglycerols at the 1 and 3 positions sequentially to generate 1,2-diacylglycerols and 2-acylglycerols. Phospholipids are degraded at the 2 position by pancreatic phospholipase A₂releasing a free fatty acid and the lysophospholipid.

Following absorption of the products of pancreatic lipase by the intestinal mucosal cells, the resynthesis of triacylglycerides occurs. The triacylglycerides are then solubilized inlipoprotein complexes (complexes of lipid and protein) called chylomicrons. A chylomicron contains lipid droplets surrounded by the more polar lipids and finally a layer of proteins. Triacylglycerides synthesized in the liver are packaged into VLDLs and released into the blood directly. Chylomicrons from the intestine are then released into the blood via the lymph system for delivery to the various tissues for storage or production of energy through oxidation.

The triacylglyceride components of VLDLs and chylomicrons are hydrolyzed to free fatty acids and glycerol in the capillaries of tissues such as liver, adipose tissue and skeletal muscle by the actions of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL). The free fatty acids are then absorbed by the cells and the glycerol is returned via the blood to the liver (principal site) and kidneys. The glycerol can then converted to the glycolytic intermediate dihydroxyacetone phosphate DHAP or phosphorylated by glycerol kinase to glycerol-3-phosphate for reuse in triglyceride synthesis.

The classification of blood lipids is distinguished based upon the density of the different lipoproteins. As lipid is less dense than protein, the lower the density of lipoprotein the less protein there is.