Triglyceride: Difference between revisions

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{{Short description|Any ester of glycerol having all three hydroxyl groups esterified with fatty acids}}

{{Fats}}

[[Image:Fat triglyceride shorthand formula.PNG|thumb|upright=1.35|Example of an unsaturated fat triglyceride (C₅₅H₉₈O₆). Left part: [[glycerol]]; right part, from top to bottom: [[palmitic acid]], [[oleic acid]], [[alpha-linolenic acid]].]]

A '''triglyceride''' ('''TG''', '''triacylglycerol''', '''TAG''', or '''triacylglyceride''') is an [[ester]] derived from [[glycerol]] and three [[fatty acids]] (from ''[[wikt:tri-#Prefix|tri-]]'' and ''[[glyceride]]'').

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 tissue|adipose]] fat and blood glucose from the liver, and are a major component of [[sebum|human skin oils]].

Many types of triglycerides exist.  One specific classification focuses on [[Saturated and unsaturated compounds|saturated and unsaturated]] types. [[Saturated fat]]s have ''no'' C=C groups; [[unsaturated fat]]s 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.

[[File:Unsaturated Triglyceride Structural Formula V1.svg|thumb|upright=1.35|Example of a natural mixed triglyceride with residues of three different fatty acids. The first fatty acid residue is saturated (<span style="color:blue;">'''blue'''</span> highlighted), the second fatty acid residue contains one [[double bond]] within the carbon chain (<span style="color:green;">'''green'''</span> highlighted). The third fatty acid residue (a polyunsaturated fatty acid residue, highlighted in <span style="color:red;">'''red'''</span>) contains three [[double bond]]s within the carbon chain. All carbon-carbon double bonds shown are ''cis'' isomers.]]

The three fatty acids [[substituents]] can be the same, but they are usually different.  Many triglycerides are known because many [[fatty acid]]s are known and their combinations are even more numerous. The chain lengths of the fatty acids in naturally occurring triglycerides vary, but most contain 16, 18, or 20 [[carbon]] atoms, defined as '''long-chain triglycerides''', while [[medium-chain triglycerides]] contain shorter fatty acids. Animals [[fatty acid synthesis|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 acid|palmitic]], [[Oleic acid|oleic]], and [[stearic acid]]s 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 ([[Polymorphism (materials science)|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''. If the first and third fatty acids on the glycerol differ, then the mixed triglyceride is [[Chirality (chemistry)|chiral]].

Triglycerides are tri-[[ester]]s derived from the [[condensation reaction]] of [[glycerol]] with three [[fatty acid]]s. Their formation can be summarised by the following overall equation:

:{{chem2| CH(OH)(CH2OH)2  +  RCOOH  +  R'COOH  +  R"COOH  →  RC(O)OCH2\sCH(OC(O)R')\sCH2C(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]]:

:{{chem2| CH(OH)(CH2OH)2  +  H2PO4-  ->  HOCH2\sCH(OH)\sCH2\sOPO3H-  +  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&ndash;CoA:

:{{chem2| HOCH2\sCH(OH)\sCH2\sOPO3H-  +  RC(O)S\sCoA  +  R'C(O)S\sCoA  ->  RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2\sOPO3H-  +  2HS\sCoA}}

The phosphate ester linkage is then hydrolysed to make way for the introduction of a third fatty acid ester:

:{{chem2| RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2\sOPO3H-  +  H2O  ->  RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2OH  +  H2PO4-}}

:{{chem2| RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2OH  +  R"C(O)S\sCoA  ->  RC(O)O\sCH2\sCH(\sOC(O)R')\sCH2\sOC(O)R"  +  HS\sCoA}}

===Common fat names===

Fats are usually named after their source (like [[olive oil]], [[cod liver oil]], [[shea butter]], [[tail fat]]) or have traditional names of their own (like butter, lard, [[ghee]], and [[margarine]]). Some of these names refer to products that contain substantial amounts of other components besides fats proper.

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.

In the [[International Union of Pure and Applied Chemistry]]'s (IUPAC's) general [[IUPAC nomenclature of organic chemistry|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 (9''Z'')-octadec-9-enoic acid, which describes that the compound has:

*an 18 carbon chain ("octadec-") with the carbon of the carboxyl ("-oic acid") given the number 1

IUPAC nomenclature can also handle branched chains and derivatives where hydrogen atoms are replaced by other chemical groups.  Triglycerides take formal IUPAC names according to the rule governing naming of esters.  For example, the formal name propane-1,2,3-tryl 1,2-bis((9''Z'')-octadec-9-enoate) 3-(hexadecanoate) applies to the pheromone informally named as glyceryl 1,2-dioleate-3-palmitate, and also known by other common names including 1,2-dioleoyl-3-palmitoylglycerol, glycerol dioleate palmitate, and 3-palmito-1,2-diolein.

A notation specific for fatty acids with unbranched chain, that is as precise as the IUPAC one but easier to parse, is a code of the form "{N}:{D} ''cis''-{CCC} ''trans''-{TTT}", where {N} is the number of carbons (including the carboxyl one), {D} is the number of double bonds, {CCC} is a list of the positions of the ''cis'' double bonds, and {TTT} is a list of the positions of the ''trans'' bonds.  Either or both ''cis'' and ''trans'' lists and their labels are omitted if there are no multiple bonds with that geometry.  For example, the codes for stearic, oleic, elaidic, and vaccenic acids are "18:0", "18:1 ''cis''-9", "18:1 ''trans''-9", and "18:1 ''trans''-11", respectively. [[Catalpic acid]], (9''E'',11''E'',13''Z'')-octadeca-9,11,13-trienoic acid according to IUPAC nomenclature, has the code "18:3 ''cis''-13 ''trans''-9,11".

For human nutrition, an important classification of fats is based on the number and position of [[double bond]]s in the constituent fatty acids.  '''Saturated fat''' has a predominance of [[saturated fatty acid]]s, without any double bonds, while '''unsaturated fat''' has predominantly [[unsaturated fatty acid|unsaturated acids]] with double bonds.  (The names refer to the fact that each double bond means two fewer hydrogen atoms in the chemical formula. Thus, a saturated fatty acid, having no double bonds, has the maximum number of hydrogen atoms for a given number of carbon atoms{{snd}}that is, it is "saturated" with hydrogen atoms.)

Unsaturated fatty acids are further classified into '''[[monounsaturated fatty acid|monounsaturated]]''' (MUFAs), with a single double bond, and '''[[polyunsaturated fatty acid|polyunsaturated]]''' (PUFAs), with two or more.  Natural fats usually contain several different saturated and unsaturated acids, even on the same molecule. For example, in most vegetable oils, the saturated [[palmitic acid|palmitic]] (C16:0) and [[stearic acid|stearic]] (C18:0) [[acyl group|acid residues]] are usually attached to positions 1 and 3 (sn1 and sn3) of the glycerol hub, whereas the middle position (sn2) is usually occupied by an unsaturated one, such as [[oleic acid|oleic]] (C18:1, ω–9) or [[linoleic acid|linoleic]] (C18:2, ω–6).)

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While it is the ''nutritional'' aspects of polyunsaturated fatty acids that are generally of greatest interest, these materials also have non-food applications. They include the [[drying oil]]s, such as [[linseed oil|linseed (flax seed)]], [[tung oil|tung]], [[poppyseed oil|poppyseed]], [[perilla oil|perilla]], and [[walnut oil]], which [[polymerize]] on exposure to [[oxygen]] to form solid films, and are used to make [[paint]]s and [[varnish]]es.

Saturated fats generally have a higher melting point than unsaturated ones with the same molecular weight, and thus are more likely to be solid at room temperature.  For example, the animal fats [[tallow]] and [[lard]] are high in saturated fatty acid content and are solids.  Olive and linseed oils on the other hand are unsaturated and liquid.  Unsaturated fats are prone to [[oxidation]] by air, which causes them to become rancid and inedible.

The double bonds in unsaturated fats can be converted into single bonds by reaction with hydrogen effected by a catalyst.  This process, called [[fat hydrogenation|hydrogenation]], is used to turn vegetable oils into solid or semisolid [[vegetable fat]]s like [[margarine]], which can substitute for tallow and butter and (unlike unsaturated fats) can be stored indefinitely without becoming rancid. However, partial hydrogenation also creates some unwanted ''trans'' acids from ''cis'' acids.

In cellular [[metabolism]], unsaturated fat molecules yield slightly less energy (i.e., fewer [[calories]]) than an equivalent amount of saturated fat. The heats of combustion of saturated, mono-, di-, and tri-unsaturated 18-carbon fatty acid esters have been measured as 2859, 2828, 2794, and 2750 kcal/mol, respectively; or, on a weight basis, 10.75, 10.71, 10.66, and 10.58 kcal/g{{snd}}a decrease of about 0.6% for each additional double bond.

The greater the degree of unsaturation in a fatty acid (i.e., the more double bonds in the fatty acid) the more vulnerable it is to [[lipid peroxidation]] ([[rancidification|rancidity]]). [[Antioxidant]]s can protect unsaturated fat from lipid peroxidation.

Linseed oil and related oils are important components of useful products used in [[oil paint]]s and related coatings. Linseed oil is rich in di- and tri-unsaturated fatty acid components, which tend to harden in the presence of oxygen. This heat-producing hardening process is peculiar to these so-called ''drying oils''. It is caused by a [[polymerization]] process that begins with oxygen molecules attacking the carbon backbone.

Triglycerides are also split into their components via [[transesterification]] during the manufacture of [[biodiesel]]. The resulting fatty acid esters can be used as fuel in [[diesel engine]]s. The [[glycerin]] has many uses, such as in the manufacture of food and in the production of pharmaceuticals.

[[Staining (biology)|Staining]] for fatty acids, triglycerides, lipoproteins, and other lipids is done through the use of [[lysochrome]]s (fat-soluble dyes).  These dyes can allow the qualification of a certain fat of interest by staining the material a specific color. Some examples: [[Sudan IV]], [[Oil Red O]], and [[Sudan Black B]].

{{StatinPathway WP430|highlight=Triglyceride}}

* [[Diglyceride acyltransferase]], an enzyme that produces triglycerides

* [[Glycerol-3-phosphate acyltransferase]]s, enzymes involved in early step in biosynthesis of triglycerides

* [[Hypertriglyceridemia]], the presence of high amounts of triglycerides in the blood.

{{Portal|Chemistry}}

* [https://www.emedicinehealth.com/how_can_i_lower_my_triglycerides_quickly/article_em.htm Lowering Triglycerides] (EMedicineHealth.com; October 2020)

{{Lipids}}

{{Lipoprotein metabolism}}

{{Clinical biochemistry blood tests}}

{{二次利用|date=4 December 2023}}

[[Category:Triglycerides| ]]