Introduction
Lipids are naturally occurring molecules comprising a large group of compounds that are soluble in an organic solvent like chloroform, ether, or benzene, but poorly soluble in water. These biomolecules consist of carbon, hydrogen, and oxygen. Lipids have been classified into simple lipids, compound lipids, and steroids. Sphingomyelins are a type of compound lipids (1).
- Sphingolipids are a class of lipids having a central compound of sphingolipids known as sphingosine.
- It has a polar head and two nonpolar tails but they contain no glycerol.
- These lipids are composed of one molecule of the long chain of amino alcohol sphingosine.
- Most sphingolipids are also phosphodiesters. But sometimes phosphoric acid in diester linkage at the polar head group (3).
There are three types of sphingolipids. Sphingomyelins is one of them. They have a phosphocholine molecule with an ester linkage to the 1-hydroxy group of a ceramide. These are the lipids in which glycerol is replaced by either sphingosine or phytosphingosine. They are very important lipids for all organisms. Below is a description of what sphingomyelin is, its structure, and its function (5).
Sphingomyelin
Sphingomyelin is the most common sphingolipid that contains phosphate. It is the primary phospholipid in the myelin sheath of nerve tissue.
It is also known as phosphosphingosides. The amino group of the sphingosine portion is engaged in an amide bond with a long-chain fatty acid. As in lecithin, sphingomyelin exists as a zwitterion over a wide range of pHs. It is mainly found in animal cells (4).
This lipid molecule was first discovered by German chemist Johann L. W. Thudicum in 1880. They have a long chain. But sometimes the lengths of the two hydrophobic chains are different. The hydrophobic chains are much more saturated than other types of phospholipids. These lipids are obtained from eggs and bovine brains (6).
Location
These types of lipids are commonly found in nerve tissue especially, in the myelin sheath of the nerve. So they are known as Sphingomyelin. In addition to the myelin sheath, it is also found in large quantities in the brain and very small amounts in other tissues. These lipids are apparently lacking in plants and microorganisms. Sometimes these lipids occur in the blood (2) & (8).
Interesting features of sphingomyelin
1. This type of phospholipid has phosphocholine head groups.
2. It is derived from sphingosine and choline.
3. This type of lipid is mainly found in nerve tissue.
4. It does not contain glycerol.
5. They are electrically charged molecules.
6. This lipid contains an 18-carbon unsaturated amino alcohol, which is known as sphingosine.
7. It is a type of sphingolipid found in the animal cell membrane, endoplasmic reticulum, and mitochondria.
8. The phosphocholine group in this molecule is shifted to the terminal hydroxyl group of the sphingosine by a reaction with this phosphocholine.
9. It has long-chain fatty acids.
10. They are more prone to intermolecular hydrogen bonding than other phospholipids.
11. The molecules of sphingomyelin are shaped like a cylinder (5) & (7).
Structure of Sphingomyelin
Sphingomyelin is an essential sphingolipid in the animal cell membrane and is a type of phospholipid. Its chemical formula is C₄₇H₉₃N₂O₆P. These lipids include 5- 20% of the total phospholipids in most cell types (1) & (2).
It is an amphipathic molecule composed of a polar head and two nonpolar tails. The polar head group is a phosphocholine molecule, so it looks like a glycerophospholipid phosphatidylcholine. However, there are considerable differences between these two molecules regarding the interfacial and hydrophobic regions (9).
This type of sphingolipid produces one molecule of fatty acid, phosphoric acid, nitrogenous base choline, and one molecule of unsaturated amino alcohol sphingosine.
In the sphingosine molecule where a fatty acyl group is replaced by the -NH₂ group, it is called ceramide. When the phosphate group is attached to the ceramide, it is called ceramide phosphate. Ceramide phosphate is left when choline is isolated from sphingomyelin. The enzyme sphingomyelinase is hydrolyzed to form ceramide and phosphorylcholine (4).
Ceramide is the most common basis for the sphingomyelin molecule in mammals. It is composed of sphingosine which has a dual bond between the carbon at the 4th and 5th positions of the Trans hydrocarbon. The length of the hydrophobic tails of sphingomyelin ranges from 16 to 24 carbon atoms and the composition of fatty acids varies depending on the tissue (3).
Function of Sphingomyelin
Sphingomyelin is an important phospholipid in animal cells. It is an essential element in the plasma membrane. The function of this lipid is described below.
- It plays a significant structural and functional role in animal cells.
- They form a plasma membrane component and participate in many signaling pathways.
- These lipids can create ceramide degradation that is involved in the apoptotic signaling pathways.
- It increases the ability of high-density lipoproteins. Lipoprotein acts as an extracellular acceptor for cholesterol fluxing from cells.
- They can form more compacted membrane domains, which have important functional effects from a protein. This is because it can establish specific domains for some integral membrane proteins.
- Its metabolic processes produce many products that play a vital role in cell membranes.
- They are the building blocks of the myelin sheath.
- It forms lipid rafts when it combines with cholesterol.
- They decrease blood clot formation.
- It has a role in signal transduction. The synthesis of sphingomyelin in the plasma membrane by sphingomyelin synthase 2 produces diacylglycerol (3) & (8).
Q&A
1. In which tissue are sphingomyelins found?
Sphingomyelins are mainly found in nerve tissue, especially myelin sheaths.
2. How do we recognize sphingomyelins in chemistry?
Sphingomyelin is an essential sphingolipid in the cell membrane. It is a phospholipid that is not derived from glycerol. Sphingomyelin is also found in large quantities in the brain and very small amounts in other tissues. It has a polar head group and two lipophilic tails (3).
3. Sphingomyelins have what function in the body?
These lipids play a significant structural and functional role in animal cells.
They form a plasma membrane component and participate in many signaling pathways.
It can form more compacted membrane domains, which have important functional effects from a protein. This is because it can establish specific domains for some integral membrane proteins.
They are the building blocks of the myelin sheath.
4. Sphingomyelins serve which of the following functions?
- It increases the ability of high-density lipoproteins. Lipoprotein acts as an extracellular acceptor for cholesterol fluxing from cells.
- Its metabolic processes produce many products that play a vital role in cell membranes.
- They are the building blocks of the myelin sheath.
- It forms lipid rafts when it combines with cholesterol.
- Sphingomyelin decreases blood clot formation.
5. How do sphingomyelins and cerebrosides differ structurally?
- Sphingomyelin is produced by the reaction of phosphatidylcholine with ceramide. On the other hand, cerebrosides are formed by the addition of amino sugars to ceramide.
- Cerebrosides consist of ceramide and a monosaccharide. Sphingomyelins consist of a phosphocholine head group and a fatty acid.
Written By: Manisha Bharati
References
1. Ajoy Paul. Zoology Honours, Volume- 1, Books & Allied (P) Ltd. Chapter: Lipids. Page no- 761 to 770.
2. B. Powar and G. R. Chatwal. Biochemistry, B. SC (general & honors course) and M. Sc. Himalaya publishing house, Chapter: Chemistry of lipids. Page no: 303 to 321.
