The acylglycerides or acylglycerols are the simple lipids like waxes (cerides). They are formed from an esterification reaction, being made up of a molecule of glycerin (propanetriol), to which one to three fatty acids (acyl groups) are attached.
Acylglycerides are included within the saponifiable lipids, as are other simple lipids such as cerides, and some more complex such as phosphoglycerides and sphingolipids.
Example of acylglyceride, a triglyceride. Source: Wolfgang Schaefer
Chemically, saponifiable lipids are esters of an alcohol and various fatty acids. Saponification would constitute the hydrolysis of an ester group, which results in the formation of a carboxylic acid and an alcohol.
characteristics
Acylglycerides are substances that are oily to the touch. They are insoluble in water and less dense. However, they are soluble in many organic solvents such as alcohol, acetone, ether, or chloroform.
These lipids can be liquid or solid with a moderate melting point. According to this criterion, the following can be recognized: oils (room temperature liquids), butters (solid that melt below 42 ° C) and tallow that melt above the temperature mentioned for butter.
Oils predominantly originate from plant tissues, having at least some unsaturated fatty acid in their structure. On the other hand, tallow and butter are of animal origin. Sebum is characterized by being exclusively made up of saturated fatty acids.
In contrast, butters are made up of a combination of saturated and unsaturated fatty acids. The latter are more complex than in oils, which gives them a solid state and a higher melting point.
Structure
Glycerin is an alcohol that has three -OH groups. In each of them an esterification reaction can occur. The -H of the carboxyl group of a fatty acid binds to a -OH group of glycerin giving rise to a water molecule (H2O) and an acylglyceride.
Fatty acids, as components of acylglycerols, have similar characteristics to each other. They are monocarboxylated, they consist of a non-branched, non-branched and non-branched alkyl chain (-COOH) and a polar ionizable carboxyl group (-COOH) (-COO - + H +).
For this reason, lipid molecules are amphipathic or hydrophobic, forming monolayers, bilayers or micelles in an aqueous medium. They normally contain an even number of C atoms, the most common being 14-24 pairs of carbon atoms, predominantly those from 16 C to 18 C. They can also be saturated or contain unsaturations (double bonds).
The fatty acids involved in the formation of acylglycerols are very diverse. However, the most important and abundant are butyric acid (with 4 carbon atoms), palmitic acid (with 16 carbon atoms), stearic acid (with 18 carbon atoms) and oleic acid (18 carbon and with a unsaturation).
Nomenclature
The nomenclature of glycerides requires listing the carbons of glycerol. In C-2 there is no ambiguity, but in C-1 and C-3. In effect, these carbons appear as equivalents, but it is enough that there is a substituent in one of them for the plane of symmetry to disappear and, consequently, the possibility of isomers exists.
It is for this reason that it has been agreed to list the C-2 of glycerol with the hydroxyl to the left (L-glycerol). The upper carbon receives the number 1 and the lower carbon number 3.
Types
According to the -OH number of the substituted glycerol, the monoacylglycerols, diacylglycerols and triacylglycerols are distinguished.
We then have the monoacylglycerols if a single fatty acid was involved in the esterification. Diacylglycerols, if two -OH groups of glycerol are esterified by fatty acids and triacylglycerols where 3 fatty acids bind to the glycerin structure, that is, to all its -OH groups.
The fatty acids attached to glycerol can be the same molecule, but most often they are different fatty acids. These molecules have different polarity, since this depends on the existence of free -OH groups in the glycerol. Only monoacylglycerides and diacylglycerides with 1 and 2 free -OH groups retain some polarity.
In contrast, in triacylglycerides there are no free -OH due to the union of three fatty acids and they do not have polarity, which is why they are also called neutral fats.
Monoacylglycerols and diacylglycerols basically function as precursors to triacylglycerols. In the food industry, they are used for the production of more homogeneous foods that are easier to process and treat.
Features
Natural oils and fats are fairly complex mixtures of triglycerides, including small amounts of other lipids, such as phospholipids and sphingolipids. They have several functions, among which are:
Energy storage
This type of lipid constitutes approximately 90% of the lipids that enter our diet and represent the main source of stored energy. Being made up of glycerol and fatty acids (for example palmitic and oleic acid), their oxidation, as in carbohydrates, leads to the production of CO2 and H2O as well as a lot of energy.
When stored in an anhydrous state, fats can generate two to six times energy than carbohydrates and proteins in the same amount of dry weight. For this reason, they constitute a long-term source of energy. In hibernating animals they are the main source of sustaining energy.
The storage of these molecules with great useable energy in metabolism occurs in adipocytes. Much of the cytoplasm of these cells contains large accumulations of triacylglycerols. Biosynthesis also occurs in them and they constitute the transport of said energy to the tissues that require it, using the circulatory system as a pathway.
In lipid metabolism, the oxidation of a fatty acid releases quite a bit of energy in each β-oxidation cycle providing huge amounts of ATP compared to glucose. For example, the complete oxidation of palmitic acid in the activated form (palmitoyl-CoA) generates almost 130 molecules of ATP.
Protection
Adipocytes provide mechanical protection or barrier in many areas of the body, including the contact surfaces of the palms of the hands and the soles of the feet.
They also function as thermal, physical and electrical insulators of the organs present in the abdominal region.
Soap formation
For example, if we consider the reaction of a triacylglyceride with a base (NaOH), the sodium atom binds to the -O of the carboxyl group of the fatty acid and the -OH group of the base binds to the C atoms of the molecule of glycerin. That way we would get a soap and a glycerin molecule.
References
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