Triglycerides

Triglycerides

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Triglycerides are the most common storage form of fat in many organisms. They are neutral lipid molecules created via the esterification of three fatty acids to a single glycerol molecule. Triglycerides are an efficient storage medium because their highly hydrophobic nature allows them to be stored as part of droplets (in which they have little or no contact with water molecules).

Triglycerides vary in molecular composition according to the identities of the fatty acids used in their synthesis . Fatty acids that have been esterified to the glycerol moiety of the triglyceride may be unsaturated (containing double bonds) or saturated (containing no double bonds). The number of double bonds in the fatty acids affects the melting temperature of the triglyceride. Saturated fats have higher melting points and are often solids at room temperature. Unsaturated fats have lower melting points and are often liquids at room temperature. Although all triglycerides are correctly identified as fats, triglycerides with melting points below room temperature are also known as oils. Animal triglycerides generally have more saturated fatty acyl groups than plant triglycerides. Beef triglycerides (lard) have a saturated to unsaturated fatty acyl group ratio of 50:50, whereas the ratio for olive oil is about 20:80. Hydrogenation (removal of the double bonds in the fatty acids) will convert an oil, such as vegetable oil, from a liquid to a solid.

In animals, triglycerides are either ingested as part of the diet or synthesized in the liver. Triglycerides are transported in blood as part of lipoprotein particles. Dietary triglycerides are transported as part of lipoprotein particles called chylomicrons. Triglycerides synthesized in the liver are transported as part of lipoprotein particles called very low density lipoproteins or VLDLs. Triglycerides are then removed from lipoprotein particles as they move through the circulatory system. Tissues either utilize this transported triglyceride immediately, or it is stored in adipose tissue .

Triglyceride is stored within cells that make up adipose tissue (fat). Triglycerides are the most abundant form of stored potential fuel in the human body. A typical 70-kilogram (154-pound) man will have approximately 15 kilograms (33 pounds) of stored triglyceride, representing several months of stored fuel. When physiologic conditions necessitate the use of triglycerides stored in adipose tissue, a hormone or neurotransmitter signals their release. Exercise or stress triggers the release of the neurotransmitter norepinephrine from nerve terminals in the adipose tissue, thereby stimulating triglyceride release. Fasting also initiates the release of triglycerides. Insulin and glucagon, two hormones produced by the pancreas, control this release of triglycerides. During fasting, blood glucagon levels increase and insulin levels decrease. The combination of increased glucagon and decreased insulin levels in the blood is the hormonal signal that triggers the release of triglycerides from the adipose tissue. However, triglycerides do not exit adipose tissue intact. Hormonal signaling activates an enzyme called a lipase that hydrolyzes a triglyceride molecule into a glycerol molecule and three fatty acids. The glycerol and fatty acids are then transported within the circulatory system to tissues that will utilize them as fuel. Fatty acids are transported in the blood bound to the serum protein albumin, as their hydrophobic natures would otherwise make them insoluble in the blood.

see also Fats and Fatty Acids; Lipids.

Robert Noiva

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.