A triglyceride (from tri- and glyceride; also TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and other vertebrates as well as vegetable fat. They are also present in the blood to enable the bidirectional transference of adipose fat and blood glucose from the liver and are a major component of human skin oils. Many types of triglycerides exist. One specific classification focuses on saturated and unsaturated types. Saturated fats have no C=C groups; unsaturated fats feature one or more C=C groups. Unsaturated fats tend to have a lower melting point than saturated analogues; as a result, they are often liquid at room temperature.
Chemical structure
The three fatty acids substituents can be the same, but they are usually different. The positions of the three fatty acids are specified using stereospecific numbering as sn-1, sn-2, and sn-3. The compositions of many fats and oils have been determined. Many triglycerides are known because many fatty acids are known. The chain lengths of the fatty acid groups vary in naturally occurring triglycerides. Those containing 16, 18, or 20 carbon atoms are defined as long-chain triglycerides, while medium-chain triglycerides contain shorter fatty acids. Animals synthesize even-numbered fatty acids, but bacteria possess the ability to synthesise odd- and branched-chain fatty acids. As a result, ruminant animal fat contains odd-numbered fatty acids, such as 15, due to the action of bacteria in the rumen. Many fatty acids are unsaturated; some are polyunsaturated (e.g., those derived from linoleic acid). Most natural fats contain a complex mixture of individual triglycerides. Because of their heterogeneity, they melt over a broad range of temperatures. Cocoa butter is unusual in that it is composed of only a few triglycerides, derived from palmitic, oleic, and stearic acids in the 1-, 2-, and 3-positions of glycerol, respectively. The simplest triglycerides are those where the three fatty acids are identical. Their names indicate the fatty acid: stearin derived from stearic acid, triolein derived from oleic acid, palmitin derived from palmitic acid, etc. These compounds can be obtained in three crystalline forms (polymorphs): α, β, and β′, the three forms differing in their melting points. A triglyceride containing different fatty acids is known as a mixed triglyceride. These are more common in nature. If all three fatty acids on the glycerol differ, then the mixed triglyceride is chiral.
Physical properties Triglycerides are colorless, although degraded samples can appear yellowish. Stearin, a simple, saturated, symmetrical triglyceride, is a solid near room temperature, but most examples are oils. Their densities range from around 0.89 with very long-chain fatty acids, through about 0.93 to 0.98 with medium-chain, and above 1.0 for very-short-chain acids.
Biosynthesis Triglycerides are tri-esters derived from the condensation reaction of glycerol with three fatty acids. Their formation can be summarised by the following overall equation:
CH(OH)(CH2OH)2 + RCOOH + R'COOH + R"COOH → RC(O)OCH2−CH(OC(O)R')−CH2C(O)OR" + 3H2O In nature, the formation of triglycerides is not random; rather, specific fatty acids are selectively condensed with the hydroxyl functional groups of glycerol. Animal fats typically have unsaturated fatty acid residues on carbon atoms 1 and 3. Extreme examples of non-random fats are cocoa butter (mentioned above) and lard, which contains about 20% triglyceride with palmitic acid on carbon 2 and oleic acid on carbons 1 and 3. An early step in the biosynthesis is the formation of the glycerol-1-phosphate:
CH(OH)(CH2OH)2 + H2PO−4 → HOCH2−CH(OH)−CH2−OPO3H− + H2O The three oxygen atoms in this phosphate ester are differentiated, setting the stage for regiospecific formation of triglycerides, as the diol reacts selectively with coenzyme-A derivatives of the fatty acids, RC(O)S–CoA:
HOCH2−CH(OH)−CH2−OPO3H− + RC(O)S−CoA + R'C(O)S−CoA → RC(O)O−CH2−CH(−OC(O)R')−CH2−OPO3H− + 2HS−CoA The phosphate ester linkage is then hydrolysed to make way for the introduction of a third fatty acid ester:
RC(O)O−CH2−CH(−OC(O)R')−CH2−OPO3H− + H2O → RC(O)O−CH2−CH(−OC(O)R')−CH2OH + H2PO−4 RC(O)O−CH2−CH(−OC(O)R')−CH2OH + R"C(O)S−CoA → RC(O)O−CH2−CH(−OC(O)R')−CH2−OC(O)R" + HS−CoA
Nomenclature
Common fat names Fats are often named after their source, e.g., olive oil, cod liver oil, shea butter, tail fat. Some have traditional names of their own, e.g., butter, lard, ghee, and margarine. The composition of these natural fats is somewhat variable. The oleic acid component in olive oil can vary from 64% to 86%.
Chemical fatty acid names Triglycerides are then commonly named as esters of those acids, as in glyceryl 1,2-dioleate 3-palmitate, the name for a brood pheromone of the honey bee. Where the fatty acid residues in a triglyceride are all the same, names like olein (for glyceryl trioleate) and palmitin (for glyceryl tripalmitate) are common.
IUPAC In the International Union of Pure and Applied Chemistry's (IUPAC's) general chemical nomenclature for organic compounds, any organic structure can be named by starting from its corresponding hydrocarbon and then specifying differences so as to describe its structure completely. For fatty acids, for example, the position and orientation of carbon-carbon double bonds is specified counting from the carboxyl functional group. Thus, oleic acid is formally named (9Z)-octadec-9-enoic acid, which describes that the compound has:
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