Cholesterol is an amphipathic compound. That means it has both polar and non-polar parts. The polar part is represented by a hydroxyl group, which makes the molecule soluble in water. The non-polar part is formed by the steroid nucleus and hydrocarbon chain. These parts are fat soluble. However, cholesterol is not soluble enough to exist alone in blood, therefore it is bound to lipoproteins.
Source of Cholesterol[edit | edit source]
We accept cholesterol exogenously from food (mainly animal - meat, dairy products, liver, cereals...) and it is also produced endogenously.
Cholesterol Synthesis[edit | edit source]
Endogenous synthesis occurs mainly in the liver (10%), as well as in the intestines (15%), adrenal and genital organs.
- acetyl-CoA reacts with acetoacetyl-CoA, with HMG-CoA-synthase as the catalyzer, to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA),
- HMG-CoA is reduced to mevalonate by HMG-CoA-reductase (This reaction is irreversible),
- In the presence of ATP and after loss of 1 CO2 molecule mevalonate is converted into isopentenyl pyrophosphate,
- Isopentenyl pyrophosphate is converted into squalene,
- Squalene is converted to cholesterol via lanosterol
Functions in cells[edit | edit source]
Cholesterol forms part of cell membranes (approximately every two phospholipid molecules account for one cholesterol molecule). It stabilizes their structure by binding to hydroxyl groups with their polar parts of phospholipids and sphingolipids, and binding the steroid part of cholesterol with chains of fatty acids. It ensures the permeability of membranes (especially for small molecules) by increasing the packing of phospholipids, and also prevent membrane crystallization. It is also part of the membranes of intracellular organelles (mitochondria, endoplasmic reticulum). Participates in intercellular communication (intracellular transport, transmission of nerve impulses, cellular signals).
Transformation of cholesterol[edit | edit source]
Used for the synthesis of steroid hormones (glucocorticoids, mineralocorticoids, sex hormones), vitamin D and bile acids.
Bile acids[edit | edit source]
They are synthesized from cholesterol in the liver. Course of synthesis: hydroxylation in several places of the molecule, loss of double bond in the steroid bulk, shortening of 3 hydrocarbons and replacing it with the carboxyl group. This produces primary bile acids: cholic acid and chenodexycholic acid. The synthesis of bile acids is subject to the regulation of cholesterol and cholic acid. Bile acids are conjugated in hepatocytes with glycine or taurine to form bile salts.
Vitamin D[edit | edit source]
Cholesterol is a precursor for the synthesis of vitamin D. Vitamin D is also present directly in the diet. Inhibition of cholesterol synthesis can cause vitamin D deficiency. Cholesterol is often found in the diet together with vitamin D, therefore a low cholesterol diet can lead to hypovitaminosis.