How Are Fatty Acids Formed Into Lipids?
Lipids are formed in the body through a process known as esterification; a reaction where an acid and an alcohol combine to form an ester. In this reaction, the alcohol is always glycerol. Fatty acids combine with glycerol to form simple lipids.
The acid component of these esters may be any number of acids, which have certain things in common. They are straight-chain carboxylic acids with virtually no branching. They range in size from about 10-20 carbons and have an even number of carbon atoms. They usually have no functional groups other than the carboxyl functional group (–COOH) and double bonds in some.
The acids are even-numbered acids because the body constructs them exclusively from acetic acid units (CH3COOH, vinegar) and therefore the carbon atoms are placed two at a time.
The glycerol molecule has three hydroxyl groups (-OH), which allows the attachment of three fatty acid molecules.
CH2—OH <- Linoleic acid, 18 carbons (Fatty acid)
CH—OH <- Lauric acid, 12 carbons (Fatty acid)
CH2—OH <- Arachidonic acid, 20 carbons (Fatty acid)
These compounds are the most common types of lipids and are commonly referred to as triglycerides. They may have three identical fatty acids or a combination of two or three different fatty acids. Mono- and diglycerides are also relatively common and result when only one or two of the –OH groups of the glycerol molecule are esterified.
Triglycerides are classified as simple lipids. More complex lipids such as phospholipids and glycolipids are formed in a similar fashion. Phospholipids are further divided into phosphoglycerides (where the alcohol is glycerol) and sphingolipids (where the alcohol is sphingosine).
In phosphoglycerides, two of the hydroxyl groups are esterified by fatty acids, while the third is esterified by a phosphate group, which is connect to an additional alcohol.
CH2—OH <- [Fatty acid]
CH—OH <- [Fatty acid]
CH2—OH <- [PO4] [alcohol*]
*In phosphoglycerides, the additional alcohol can be ethanolamine, serine, choline or certain others.
In sphingolipids, the –OH group at the end is esterified by phosphorycholine while a long-chain fatty acids connects by amide linkage to the NH2 group.
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OH NH2 OH—PO4—choline
Glycolipids are even more complicated and detailing those complexities would be beyond the scope of this discussion. It would appear that the fatty acid connects to the molecule by a similar amide linkage as seen in sphingolipids. In addition, a beta glycosidic bond is formed with a glucose or galactose carbohydrate unit.
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OH NH2 OH—glucose or galactose