Lanosterol, cholesterol from structure

Fig. 34. 3. Structures of selected sterols. Sources animal - lanosterol, cholesterol and ergosterol (also microbial) plant - all others. (From Warner, K., Su, C, and White, P.J. "Role of Antioxidants and Polymerization Inhibitors in Protecting Frying Oils" in Frying Technology and Practices, M.K. Gupta, K. Warner, and P.J. White (Eds.), pp. 37-49, AOCS Press, Champaign, IL 2004. With permission.)...

Within 2 years of the publication of the Whitmore paper, Robinson proposed the formation of the steroids (including cholesterol) from squalene (a C30 polyunsaturated polyisoprene molecule) via an incredible series of intermediates and rearrangements. Later, following the elucidation of the structure of lanosterol, R. B. Woodward and K. Bloch made a brilliant proposal that at once rationalized the biosynthetic origin of both lanosterol and cholesterol and implicated lanosterol as an intermediate in cholesterol bios)mthesis. Their mechanism involved the concerted (bonds made and broken simultaneously) cydization of four rings, as well as four rearrangements... 

Cholesterol is the most common steroid of mammalian membranes. It is formed biologically from lanosterol, as shown. Ergosterol is the most common steroid of fungal membranes. It differs from cholesterol by the presence of two additional double bonds that affect its three dimensional structure. Also shown are three so-called steroid hormones, andros-terone, estradiol, and testosterone. Note the presence of an aromatic A-ring in estradiol. 

Some T. have major physiological significance. Thus, lanosterol is converted biosynthetically to cholesterol, the precursor of all steroid hormones, bile acids, and vitamin D3. In fungi, lanosterol is converted to er-gosterol (see sterols), an essential component of the fungal cell membrane. Plant cell membranes also incorporate steroids (phytosterols). In prokaryotes, the hopanoids take over the functions of steroids in the cell membranes. As a component of animal and plant waxes T. strengthen the structures. They protect the plant surface from desiccation and attack by microorganisms (e.g., betulin, lupeol, oleanolic acid, and ursolic acid). 

The steroids are biosynthetically derived from a triterpenoid nucleus through degradation. Compared with the triterpenoids, these compounds lack the dimethyl moiety at C-4 and a methyl group at the C-14 position. To show the difference between these chemical structures, lanosterol, a representative triterpene, and cholesterol, a representative steroid, are shown [1]. 

Some steroids, such as cholic acid, progesterone and testosterone were already mentioned in the chapters discussing aldehydes, ketones and carboxylic acids. The most common steroid in humans is cholesterol. Although the compound has been discovered in the eighteenth century its complete molecular structure was determined only in the middle of the last century. Cholesterol appears in most of tissues and it has a special role in the regulation of blood circulation. An imbalance of cholesterol in the organism can cause serious health problems similar to arthero-sclerosis. The cholesterol molecule, like other steroids, is formed by a particular biosynthetic pathway from the terpene precursors, squalene and lanosterol. Since cholesterol has 27 carbon atoms, 3 atoms less than the triterpene squalene (which has 30 C-atoms), 3 C-atoms are eliminated during the biosynthetic process. 

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