What is the Difference Between Cholesterol and Cholesteryl Ester

The main difference between cholesterol and cholesteryl ester is that cholesterol is a sterol, a type of lipid whereas cholesteryl ester is an ester of cholesterol, a type of dietary lipid. Furthermore, cholesterol is hydrophobic while cholesteryl ester is more hydrophobic. Moreover, cholesterol serves as an essential structural component of the animal cell membrane, while cholesteryl esters are the main form of cholesterol in lipoproteins. 

Free cholesterol and cholesteryl ester are the two forms of cholesterol that occur in the body. Therefore, both of them contribute to the total cholesterol level. 

Key Areas Covered

1. What is Cholesterol
     – Definition, Structure, Importance
2. What is Cholesteryl Ester
     – Definition, Structure, Importance
3. What are the Similarities Between Cholesterol and Cholesteryl Ester
     – Outline of Common Features
4. What is the Difference Between Cholesterol and Cholesteryl Ester
     – Comparison of Key Differences

Key Terms 

ACAT, Cholesterol, Cholesteryl Ester, Hydrophobicity, LCAT, Lipoproteins, Sterol

What is Cholesterol 

Cholesterol is a steroid, which is a modified sterol. It is a type of lipid that occurs in the body. Generally, cholesterol comes into the body through the diet. Moreover, the liver produces most of the cholesterol. Basically, the cholesterol molecule consists of four rings (A, B, C, D) with trans ring junctions, two methyl groups (C-18 and C-19), a hydroxyl group on C-3, and an iso-octyl side-chain at carbon 17. Therefore, it is a tetracyclic cyclopenta[a]phenanthrene.

Furthermore, both methyl and hydroxyl groups occur above the plane. Additionally, a double bond occurs between C-5 and C-6. Typically, cholesterol is a hydrophobic molecule with a rigid planar four-ring nucleus with a flexible tail.  

Figure 1: Cholesterol

Moreover, cholesterol performs vital functions in the body. Usually, it serves as an essential structural component of the cell membrane of animals. It also serves as a precursor for the synthesis of steroid hormones, bile acids, and vitamin D inside the body. Besides, it forms the myelin sheath, which electrically insulates the axon of the neurons to increase the speed of signal transmission.   

What is Cholesteryl Ester 

Cholesteryl ester is esterified cholesterol with long-chain fatty acids. Generally, the ester bond occurs between the carboxylate group of fatty acid and the hydroxyl group of cholesterol. Further, the esterification of cholesterol with these long-chain fatty acids increases the solubility of the molecule, increasing its hydrophobicity. This increased hydrophobicity makes cholesteryl esters more suitable as a transporting form of cholesterol. Therefore, cholesteryl esters are the main form of cholesterol that occur in lipoproteins. 

Figure 2: Cholesteryl Ester – Cholesterol Oleate

Moreover, the esterification of cholesterol mainly occurs in circulation. Typically, in plasma and HDL, the enzyme lecithin: cholesterol acyltransferase (LCAT), is responsible for the transfer of fatty acids to cholesterol from position sn-2 of phosphatidylcholine (‘lecithin’). On the other hand, in other animal tissues, the enzyme acyl-CoA: cholesterol acyltransferase (ACAT) is responsible for the synthesis of cholesterol esters from CoA esters of fatty acids and cholesterol. Cholesteryl esters come into the body through the diet as well. These dietary cholesteryl esters are hydrolyzed by pancreatic enzymes, cholesterol esterase, to produce cholesterol and free fatty acids. 

Similarities Between Cholesterol and Cholesteryl Ester 

• Cholesterol and cholesteryl ester are two forms of cholesterol in the body. 
• They are sterols.
• Both contribute to the total cholesterol level. 
• They have the four rings structure of cholesterol. 
• Moreover, they have distinct functions in the body. 
• Both can be obtained from the diet. Moreover, they are biosynthesized inside the body. 
• Furthermore, excessive amounts of them increase the risk of atherosclerosis. 

Difference Between Cholesterol and Cholesteryl Ester 

Definition 

Cholesterol refers to a compound of the sterol, which is an important constituent of cell membranes and precursors of other steroid compounds, while cholesterol ester refers to a dietary lipid, is an ester of cholesterol, serving as the major cholesterol form in lipoproteins. Hence, this is the main difference between cholesterol and cholesteryl ester.

Significance 

Moreover, cholesterol is the principal sterol synthesized by animals, while cholesteryl ester is an ester of cholesterol, which is a dietary lipid. 

Structure 

Also, cholesterol has a tetracyclic cyclopenta[a]phenanthrene structure with an iso-octyl side-chain at carbon 17 while cholesteryl ester has an ester bond formed between the carboxylate group of fatty acid and the hydroxyl group of cholesterol.  

Biosynthesis 

Besides, while cholesterol is synthesized in the liver, cholesteryl esters are synthesized in the plasma and cells in animal tissues. Thus, this is another difference between cholesterol and cholesteryl ester.

Solubility and Hydrophobicity 

Cholesterol is less soluble in water and is hydrophobic molecule while cholesteryl ester has a lesser solubility than cholesterol but, higher hydrophobicity. 

Function 

Furthermore, cholesterol serves as an essential structural component of the animal cell membrane and as a precursor for the biosynthesis of steroid hormones, bile acid, and vitamin D while cholesteryl ester serves as the main form of cholesterol in lipoproteins. 

Conclusion 

Cholesterol is a sterol, containing a four-ring structure. Moreover, most of the cholesterol is synthesized by the liver. It serves as an essential component of the animal cell membrane. Additionally, it serves as the precursor for the synthesize of steroid hormones, bile acids, and vitamin D. On the other hand, cholesteryl ester is an ester of cholesterol. It is made up of fatty acids and cholesterol. However, the main function of cholesteryl ester is to serve as the main form of cholesterol in lipoproteins. Generally, it has less solubility and high hydrophobicity when compared to cholesterol. Therefore, the main difference between cholesterol and cholesteryl ester is their structure and functional importance. 

References:

1. Christie, William W. “Sterols: 1. Cholesterol and Cholesterol Esters.” The LipidWeb, William W. Christie, 3 July 2019. Available Here.

Image Courtesy:

1. “Cholesterol with numbering” By Calvero. – Selfmade with ChemDraw. (Public Domain) via Commons Wikimedia  
2. “Cholesterol oleate” By User:Edgar181 – Own work (Public Domain) via Commons Wikimedia