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Cholesterol is a tetracyclic alcohol and a type of sterol. Added to the sterol frame with the alcohol group at position 3 are 2 methyl groups at carbon positions 10 and 13 and a 2-isooctyl group at position 17. The molecule is unsaturated at position 5,6 with an alkene group. The total number of stereocenters is 8.
Research is being done for other compounds which block different steps in the biosynthesis of cholesterol, including the reaction performed by oxidosqualene cyclase which cyclizes squalene to form lanosterol. [7] Oxidosqualene cyclase, which is downstream of squalene in the pathway, is an attractive target for inhibition.
Cholesterol is the principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils. [3] [4]Cholesterol is biosynthesized by all animal cells [citation needed] and is an essential structural and signaling component of animal cell membranes.
Lipid metabolism also occurs in plants, though the processes differ in some ways when compared to animals. [8] The second step after the hydrolysis is the absorption of the fatty acids into the epithelial cells of the intestinal wall. [6] In the epithelial cells, fatty acids are packaged and transported to the rest of the body. [9]
The cytosolic acetyl-CoA can also condense with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA which is the rate-limiting step controlling the synthesis of cholesterol. [16] Cholesterol can be used as is, as a structural component of cellular membranes, or it can be used to synthesize steroid hormones, bile salts, and vitamin D.
These involve 2 hydroxylations of the cholesterol side-chain, which generate, first, 22R-hydroxycholesterol and then 20alpha,22R-dihydroxycholesterol. The final step cleaves the bond between carbons 20 and 22, resulting in the production of pregnenolone and isocaproic aldehyde. Each monooxygenase step requires 2 electrons (reducing equivalents).
Cholesterol also serves as a precursor for the biosynthesis of steroid hormones, bile acid [2] and vitamin D. In mammals cholesterol is either absorbed from dietary sources or is synthesized de novo. Up to 70-80% of de novo cholesterol synthesis occurs in the liver, and about 10% of de novo cholesterol synthesis occurs in the small intestine. [3]
Lanosterol is a key four-ringed intermediate in cholesterol biosynthesis. [6] [7] In humans, lanosterol synthase is encoded by the LSS gene. [8] [9] In eukaryotes, lanosterol synthase is an integral monotopic protein associated with the cytosolic side of the endoplasmic reticulum. [10]