LEARNING OBJECTIVES
In this article, author has explained introduction, occurrence, structure, classification, and functions of fatty acids.
Definition
Table of Contents
The simplest lipids with carboxylic acid and hydrocarbon side chains are called fatty acids. They are composed of carbon, hydrogen, and oxygen.
Fatty acids are building blocks of fats in our body and the food we eat. The fats are broken into fatty acids as a result of digestion. The fatty acids are then easily absorbed in the blood. Fatty acids are joined in a group of three called triglyceride.
Occurrence
Fatty acids occur in two forms; esterified and unesterified form. In the esterified form, fatty acids are not free while in the unesterified form they are free. Fatty acids can be found in plants and animals. Animals containing fatty acids are simpler in structure as compared to plants containing fatty acids. Fatty acids present in plants have epoxy, keto, hydroxy, and cyclo, pentane groups.
Structure
Fatty acids are composed of a carbon chain. The Methyl group is present at one end and carboxylic acid is present at the other end. The Methyl group is named omega (ω). Fatty acids are composed of two regions; a long hydrophobic chain and a hydrophilic carboxyl group. The hydrophobic tail is not much highly reactive whereas the hydrophilic COOH group is highly reactive. Fatty acids form covalent bonds via these carboxylic groups.
If fatty acids are unsaturated, bends and kinks are formed in them which stack the chains together.
Classification of fatty acids
Fatty acids are classified based on various factors.
1. On basis of the presence or absence of saturated fatty acids are classified into the following classes:
Saturated fatty acids
Those fatty acids which have no double bond are called saturated fatty acids.
Palmitic acid, stearic acid, and lauric acid are saturated fatty acids
Unsaturated fatty acids
Those fatty acids which contain double bonds are called unsaturated fatty acids.
Oleic acid, arachidonic acid, etc are examples of unsaturated fatty acids.
2. On basis of the degree of unsaturation fatty acids are further classified:
Monounsaturated fatty acids
The fatty acids which have only one double bond are called monounsaturated fatty acids.
Oleic and erucic are monounsaturated fatty acids
Polyunsaturated fatty acids
The fatty acids containing two or more two double bonds are called polyunsaturated fatty acids.
Linoleic acid is a polyunsaturated fatty acid containing two double bonds.
3. Fatty acids are classified into two classes based on their synthesis.
Essential fatty acids
The fatty acids which cannot be synthesized by the human body are called essential fatty acids. They are essential for good health.
Linoleic acid and α-linoleic acid are examples of essential fatty acids.
Non-essential fatty acids
Non-essential fatty acids are those fatty acids that a human body can manufacture.
Stearic acid and palmitic acid are non-essential fatty acids.
4. Based on the number of carbon fatty acids is classified into two classes:
Even the number of fatty acids
They have an even number of carbon atoms (14C-20C). Palmitic acid (16C) and stearic acid (18C) belong to this class
Odd number of fatty acids
They have an odd number of carbon atoms. Propionic acid (3C) and valeric acid (5C) are examples of odd-number fatty acids.
5. The last classification is based on the length of the hydrocarbon chain.
Short-chain fatty acids
They contain less than 6 carbon atoms.
Medium chain fatty acids
They contain 8 to 14 carbon atoms.
Long-chain fatty acids
They contain 16 to 24 carbon atoms.
Functions of fatty acid
Fatty acids have the following functions:
1. Biological signaling
Fatty acids play important role in biological signaling. The products of lipid peroxidation are precursors of biological signaling. Fatty acids are involved in the formation of eicosanoids. These are signaling molecules involved in the immune response. Eicosanoids consist of 20 carbon atoms. They are polyunsaturated fatty acids. Eicosanoids from the precursors of molecules that are responsible for platelet aggregation, chemotaxis, and growth factors. Polyunsaturated fatty acids cause the peroxidation of LDL. When peroxidized LDL is engulfed by macrophages, it causes the activation of the immune system.
Mono and polyunsaturated fat are associated with the anti-inflammatory response. Moreover, polyunsaturated fatty acids are required for the development, growth, and cognition of the brain. Polyunsaturated fatty acids inhibit the proliferation of cancer cells and produce anti-tumor activity.
2. Fuel source
When fatty acids are metabolized, free fatty acids are taken by the fatty acids binding proteins. These proteins transport the fatty acids intracellularly. Acyl-CoA activates the free fatty acids and transports them to mitochondria or peroxisomes. In mitochondria, these are converted to ATP and heat as a form of energy. Then they are transported to the endoplasmic reticulum for esterification into lipids. The lipids are used as energy storage.
Fatty acids are released from, triacylglycerol and processed into two-carbon molecules. The two carbon molecule is used to produce energy.
3. Energy storage
Fatty acids are used to store energy in the form of fat droplets in adipose tissues. These fat droplets are composed of hydrophobic triacylglycerol. Triacylglycerol consists of three fatty acids attached to a glycerol molecule. In this form, fatty acids provide electrical insulation and protection against mechanical compression. Fatty acids stores six times more energy as compared to glycogen.
4. Cell membrane formation
One of the most important functions of fatty acid is the formation of the cell membrane. The cell membrane envelops all the cells. The cell membrane is made up of two fatty acid chains bound to glycerol and a phosphate group joined to choline. Each phospholipid has a hydrophobic tail that is composed of two fatty acid chains and a hydrophilic head which consists of a phosphate group.
5. Protein modification
Fatty acids interact with various proteins. Protein acylation is carried by polyunsaturated fatty acids. Protein acylation is critical for the anchoring, folding, and function of the protein. Fatty acids also interact with nuclear receptor proteins. It promotes gene expression. Fatty acid-protein complex also functions as transcription factors. Fatty acids regulate the genes related to modification, cellular proliferation, and apoptosis.