TAG

• Triacylglycerols (TAG) are the major lipids in fat deposits and food
• TAGs are a form of storage of fatty acids and are mainly deposited in adipose tissue
• Fatty acids stored in triacylglycerols serve as fuels and provide the body with its major source of energy
• Triacylglycerols are formed from glycerol-3-phosphate and share steps with the synthesis of glycophospholipids
• Substrates for TAG synthesis are glycerol-3-phosphate and fatty acids
• The fatty acids need to be activated to Acyl-CoA by Acyl-CoA synthetases, requiring free energy (ATP)
• Sources of glycerol-3-phosphate
• In the liver, glycerol kinase catalyzes the phosphorylation of glycerol to glycerol-3-P
• In other tissues, including adipose, glycerol-3-P is generated by the reduction of DHAP to glycerol-3-P by glycerol-3-P dehydrogenase
• DHAP is formed in glycolysis
• In the epithelial cells of the intestine, absorbed dietary TAG products are reconverted to TAG by the enzyme 2-monoacylglycerol acyltransferase
• Utilization of fructose in the liver enhances TAG synthesis
• Glyceraldehyde is easily converted to glycerol-3-P by triose kinase, controlling the lipogenic potential of fructose
• Triose kinase limits fructose oxidation and favors lipogenic metabolism
• Catabolism of TAG (lipolysis)
• TAGs are hydrolyzed by lipases to their constituent fatty acids and glycerol
• Dietary TAGs are hydrolyzed in the gastrointestinal tract by tongue lipase, gastric lipase, and pancreatic lipase
• TAGs in chylomicrons and VLDL are hydrolyzed by lipoprotein lipase (LPL)
• A significant part of TAG hydrolysis occurs in adipose tissue, releasing glycerol and free fatty acids into the plasma
• Main points of lipolysis
• Lipolysis occurs in three stages with different enzymes acting at each step
• TAG is hydrolyzed to form diacylglycerol (DAG) by Adipose triglyceride lipase (ATGL)/desnutrin
• DAG is then hydrolyzed by hormone-sensitive lipase (HSL) to monoacylglycerol
• Monoacylglycerol is hydrolyzed to glycerol by MAG lipase (MGL)
• Activation of lipolysis in adipose tissue
• The mobilization of fat from adipose tissue is controlled by a complex series of interrelated cascades
• The activation of hormone-sensitive lipase can be stimulated by glucagon, epinephrine, or ACTH
• Insulin inhibits the mobilization of fat from adipose tissue
• Utilization of glycerol
• The utilization of glycerol depends on the presence of the enzyme glycerol kinase, mainly found in the liver
• Glycerol-3-P can enter gluconeogenesis, glycolysis for energy, or re-esterification
• Fatty acids are used by cells to store energy in the form of triglycerides
• Triglycerides consist of three fatty acids attached to a glycerol molecule
• Three types of macromolecules store potential energy in the body: glycogen, proteins, and triglycerides
• Triglycerides store much more energy than glycogen or proteins
• Triglycerides can sustain the body's energy needs for several weeks
• Triglycerides are highly reduced molecules, meaning lots of electrons have been removed from them
• Triglycerides are anhydrous, meaning they are free of water
• Glycogen is less concentrated for energy storage compared to triglycerides due to the presence of hydroxyl groups that attract water
• Complete oxidation of fatty acids yields about 38 kilojoules per gram