Arielle Cruz
Cholesterol is a type of sterol and a principal sterol of all animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. It is biosynthesized by all animal cells and is an essential structural and signaling component of animal cell membranes. Cholesterol is recycled in the body and is excreted by the liver into the gallbladder in a non-esterified form. Cholesterol esters are a type of dietary lipid and are ester derivatives of cholesterol. The ester bond is formed between the carboxylate group of a fatty acid and the hydroxyl group of cholesterol. Given the presence of the ester bond, cholesterol esters are formed from cholesterol and carboxylic acid. However, cholesterol itself is a type of alcohol.
| Characteristics | Values |
|---|---|
| Type of compound | Cholesterol is an unsaturated alcohol of the steroid family of compounds. |
| Solubility | Cholesterol and cholesteryl esters are lipids and are insoluble in aqueous solutions but soluble in organic solvents. |
| Presence in the body | Cholesterol is the principal sterol of all animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. |
| Function | Cholesterol is an essential structural and signaling component of animal cell membranes. It is also a precursor for the biosynthesis of steroid hormones, bile acid, and vitamin D. |
| Health impact | Elevated levels of cholesterol in the blood, especially when bound to low-density lipoprotein (LDL), may increase the risk of cardiovascular disease. |
| Synthesis | The liver synthesizes much of the cholesterol in the body, and it is essential for its removal in bile. Cholesterol is also biosynthesized by all animal cells. |
| Transport | Cholesterol is transported in the plasma associated with various lipoprotein particles such as chylomicrons, very-low-density lipoproteins (VLDL), and intermediate-density lipoproteins (IDL). |
| Esterification | Cholesterol can be esterified with a fatty acid to form a cholesteryl ester. |
| Hydrolysis | Cholesterol esters can undergo hydrolysis to liberate cholesterol and free fatty acids, which are important for membrane and lipoprotein formation and hormone synthesis. |
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What You'll Learn
Cholesterol is a sterol and a type of alcohol
Cholesterol is a sterol, a type of alcohol. It is a 27-carbon compound with a unique structure: a hydrocarbon tail, a central sterol nucleus made of four hydrocarbon rings, and a hydroxyl group. The name "cholesterol" comes from the Ancient Greek "chole" meaning bile and "stereos" meaning solid, with the -ol suffix indicating an alcohol.
Cholesterol is biosynthesized by all animal cells and is an essential structural and signaling component of animal cell membranes. It is distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. In vertebrates, the liver and hepatic cells produce the greatest amounts of cholesterol. In the brain, astrocytes produce cholesterol and transport it to neurons. While cholesterol is absent in prokaryotes, some exceptions exist, such as Mycoplasma, which requires cholesterol for growth.
Cholesterol plays a crucial role in the body's functions. It serves as a precursor for the biosynthesis of steroid hormones, bile acids, and vitamin D. However, elevated levels of cholesterol in the blood, particularly when bound to low-density lipoprotein (LDL) or "bad cholesterol," can increase the risk of cardiovascular disease. Therefore, maintaining healthy cholesterol levels is important for overall health.
The body regulates cholesterol levels through various mechanisms. The main regulatory mechanism involves the protein SREBP (sterol regulatory element-binding protein 1 and 2), which senses intracellular cholesterol levels in the endoplasmic reticulum. When cholesterol levels are high, SREBP inhibits the synthesis of new cholesterol. Conversely, when cholesterol levels are low, SREBP stimulates cholesterol production.
Additionally, cholesterol is recycled in the body. The liver excretes cholesterol into biliary fluids, which are stored in the gallbladder and then excreted in a non-esterified form into the digestive tract. Cholesterol is also converted into coprostanol, a non-absorbable sterol that is excreted in feces.
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Cholesterol esters are formed by esterification of cholesterol
Cholesterol is a sterol, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. It is biosynthesized by all animal cells and is an essential structural and signalling component of animal cell membranes. In plants, it is generally a minor component of a complex mixture of structurally related 'phytosterols'.
The process of cholesterol esterification is crucial in maintaining cholesterol homeostasis in the body. Cholesterol esters are relatively inert storage forms of cholesterol that prevent excess cholesterol from interfering with cell membranes. They are stored as lipid droplets within the cytosol, particularly in steroidogenic tissues, where they serve as precursors for the synthesis of steroid hormones. Additionally, cholesterol esters play a role in transporting cholesterol, as observed in cholesteryl ester lipid droplets found in the human brain.
The regulation of cholesterol esterification is essential for maintaining optimal cholesterol levels in the body. Elevated levels of cholesterol, especially when bound to low-density lipoprotein (LDL) or "bad cholesterol", are associated with an increased risk of cardiovascular disease. Inhibiting enzymes such as LCAT and ACAT involved in cholesterol esterification is one approach to lowering circulating lipid levels in plasma.
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Carboxylic acid is produced during cholesterol synthesis
Cholesterol is a 27-carbon compound with a unique structure: a hydrocarbon tail, a central sterol nucleus made of four hydrocarbon rings, and a hydroxyl group. It is a sterol, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. It is biosynthesized by all animal cells and is an essential structural and signalling component of animal cell membranes.
Cholesterol is recycled in the body. The liver excretes cholesterol into biliary fluids, which are then stored in the gallbladder and eventually excreted into the digestive tract. The liver also packages some of the esterified cholesterol into the hollow core of lipoproteins, primarily VLDL. VLDL is secreted from the hepatocyte into the blood and transports the cholesterol esters to the tissues that require greater amounts of cholesterol than they can synthesize de novo.
Cholesterol ester production in the liver is catalyzed by acyl-CoA‒cholesterol acyl transferase (ACAT). ACAT catalyzes the transfer of a fatty acid from coenzyme A to the hydroxyl group on carbon 3 of cholesterol. This reaction is similar to the one catalyzed by lecithin:cholesterol acyltransferase within the plasma associated with HDLs. The resulting cholesterol esters are more hydrophobic than free cholesterol.
The process of cholesterol synthesis involves four stages, with only the first stage being regulated. The first stage of cholesterol synthesis leads to the production of the intermediate mevalonate. In this stage, two molecules of acetyl-CoA condense, forming acetoacetyl-CoA, which then condenses with a third molecule of acetyl-CoA to yield the six-carbon compound β-hydroxy-β-methylglutaryl-CoA (HMG-CoA). The acetyl-CoA needed for this stage can be obtained from several sources, such as the β-oxidation of fatty acids, the oxidation of ketogenic amino acids, and the pyruvate dehydrogenase reaction.
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Cholesterol esters are transported to the liver
Cholesterol is a cyclic hydrocarbon that can be esterified with a fatty acid to form a cholesteryl ester. Both cholesterol and cholesteryl esters are lipids and are essentially insoluble in aqueous solutions but soluble in organic solvents.
There are two major routes by which cholesteryl esters formed within HDL are transported to the liver. The first route involves the transfer of a substantial part of HDL-cholesteryl esters by CETP to triglyceride-rich lipoproteins, which are metabolized into remnants and are then removed by the liver either directly or after conversion into LDL. The second route involves the uptake of HDL-cholesteryl esters by the liver through SR-BI-mediated selective uptake.
In addition to the two routes for HDL-cholesteryl esters transport to the liver, there is also an SR-BI-dependent pathway for the direct uptake of HDL unesterified cholesterol in the liver, with its subsequent excretion in bile.
Cholesterol is exported from the liver and transported to other tissues in the form of low-density lipoproteins (LDL) for uptake via their receptors, together with cholesterol obtained from the diet (mainly eggs and red meat). Peripheral tissues can obtain their cholesterol either from the circulation or by synthesis de novo.
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Cholesterol is recycled in the body
Cholesterol is a waxy, whitish-yellow fat that is biosynthesized by all animal cells and is an essential structural and signaling component of animal cell membranes. It is the principal sterol of all animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.
Cholesterol is also recycled in the body through the absorption of phytosterols, which are chemically similar substances that compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption. When intestinal lining cells absorb phytosterols, they usually excrete the phytosterol molecules back into the gastrointestinal tract, an important protective mechanism.
In addition, cholesterol is recycled in the body through the esterification process. Most ingested cholesterol is esterified, which causes it to be poorly absorbed by the gut. The body compensates for the absorption of ingested cholesterol by reducing its own cholesterol synthesis. The liver and enzymes such as lecithin:cholesterol acyltransferase (LCAT) and acyl-coenzyme A:cholesterol acyltransferase (ACAT) catalyze the esterification of cholesterol. Lyosomal acid lipase is an enzyme that is required for breaking down cholesteryl esters into free cholesterol.
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Frequently asked questions
Cholesterol is the principal sterol of all animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. It is biosynthesized by all animal cells and is an essential structural and signaling component of animal cell membranes.
A carboxylic acid has very different chemistry compared to an alcohol.
Alcohol esters are ester derivatives of cholesterol. The ester bond is formed between the carboxylate group of a fatty acid and the hydroxyl group of cholesterol.
Cholesterol is neither a carboxylic acid nor an alcohol ester. Cholesterol is a sterol that can be esterified with a fatty acid to form a cholesteryl ester.
