Table of Contents
Fatty Acid Meaning and Definition
Even and Odd Chain Fatty Acids
Saturated and Unsaturated Fatty Acids
Fatty Acid – Meaning and Definition
In biochemistry, fatty acids can be defined as carboxylic acids with long aliphatic chains that can either be branched or unbranched. The fatty acids that occur naturally, possess carbon atoms in even numbers and are usually unbranched. Fatty acids are major components of lipids; they exist in three main forms of esters: phospholipids, triglycerides and cholesteryl esters.
Types of Fatty Acids
Fatty acids are classified based on their number of carbons, length and saturation.
Even and Odd Chain Fatty Acids
Most naturally occuring fatty acids have an even number of carbons in their aliphatic chain. Example: oleic acid (18), stearic acid (18).
However, some fatty acids also have an odd number of carbons in their chain. They are known as odd-chain fatty acids (OCFA). Example: heptadecanoic and pentadecanoic acid that are found in dairy products.
The biosynthesis of odd chain fatty acids is a little more complex than the even chain fatty acids.
Saturated and Unsaturated Fatty Acids
The acids that have no double bond (C=C) in their aliphatic chain are known as saturated fatty acids. The chemical formula of saturated fatty acids can be written as CH3(CH2)nCOOH. Below is a table of common saturated fatty acids with their formula.
| Common Name | Chemical Formula |
|---|---|
| Caprylic Acid | CH3(CH2)6COOH |
| Capric Acid | CH3(CH2)8COOH |
| Lauric Acid | CH3(CH2)10COOH |
| Myristic Acid | CH3(CH2)12COOH |
| Palmitic Acid | CH3(CH2)14COOH |
| Stearic Acid | CH3(CH2)16COOH |
The unsaturated fatty acids possess at least one double bond in their aliphatic chain. The double bond in the molecule can generate two isomers for unsaturated fatty acids: cis and trans configurations.
The cis isomers have the two hydrogen atoms placed adjacently to the double bond on the same side of the aliphatic chain. The double bond gives rigidity to the molecule and the cis conformation limits the conformational freedom of the fatty acid. The more number of cis bonds found in a fatty acid, the less flexible and curved it becomes in conformation. Example: oleic and linoleic acid.
The trans isomers, conversely, have the two hydrogen atoms placed on opposite sides of the aliphatic chain. The trans configuration does not bend the structure of the molecule as seen in cis isomers, but remains straight like saturated fatty acids.
Most naturally occuring unsaturated fatty acids possess a cis configuration whereas most trans fats are a result of human processing, they do not occur naturally.
Length of Fatty Acids
- Fatty acids with aliphatic chains of five or lesser carbons are called short-chain fatty acids (SCFA). Example: butyric acid
- Fatty acids with aliphatic chains of 6 to 12 carbons are called medium-chain fatty acids (MCFA). Example: capric acid
- Fatty acids with aliphatic chains of 13 to 21 carbons are called long-chain fatty acids (LCFA). Example: oleic acid
- Fatty acids with aliphatic chains of 22 or more carbons are called very long chain fatty acids (VLCFA). Example: lignoceric acid
Properties of Fatty Acids
- Acidity: Fatty acids have similar acidities. As the chain length of a fatty acid increases, their solubility in water decreases, posing no or little effect on the pH of the aqueous solution. Example: Nonanoic acid (C9) has a pKa of 4.96 whereas acetic acid (C2) has a pKa of 4.76.
- Hydrogenation: Unsaturated fatty acids are prone to get rancid (autoxidation or hydrolysis of fats when exposed to air). Therefore the unsaturated fatty acids undergo hydrogenation to minimise this problem.
- Autoxidation: Unsaturated fatty acids undergo a chemical change in the presence of air and trace metals called autoxidation. Treatment with chelating agents can prevent this action as they remove the metal catalysts.
- Ozonolysis: Unsaturated fatty acids have high chances to get degraded by ozone.
Circulation of Fatty Acids
- Digestion and Intake: SCFA and MCFA are directly absorbed in our blood by the intestinal capillaries and travel via the hepatic portal vein, similar to other absorbed nutrients. LCFA, however, is not absorbed directly into the blood. They are absorbed in the villi of the intestine to form triglycerides. The triglycerides get coated with cholesterol and proteins to form chylomicrons. The chylomicrons are transported to a location near the heart via the lymphatic duct where they are either stored or broken down for energy.
- Metabolism: Fatty acids are broken down via beta-oxidation and citric acid cycle in mitochondria into CO2 and water. After oxidative phosphorylation they release energy in the form of ATP.
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Also Read:
- Fatty acid synthesis
- Importance of Fats – Types of Fats, Sources and Its Benefits
- Difference between Fats and Oils
- Difference between Lipids and Fats
