Elementary plant Biochemistry and Biotechnology (2+1)

Sterols

• The characteristic structure of sterol is their steroid nucleus consisting of four fused rings, three with six carbons (Phenanthrene) and one with five carbons (cyclopentane). This parent structure is known as perhydro cyclopentano phenanthrene . The steroid nucleus is almost planar and relatively rigid . Steroids with methyl groups attached to carbons 10 and 13 and 8-10 carbon atoms in the side chain at position 17, an alcoholic group at position 3 and a double bond between carbons 5 and 6 are classified as sterols . Cholesterol is the most abundant sterol in animals. Cholesterol is a major component of animal plasma membranes and occurs in lesser amounts in the membranes of their subcellular organelles . Its polar OH group gives it a weak amphiphilic character, whereas its fused ring system provides it with greater stability than other membrane lipids.


• Cholesterol is therefore an important determinant of membrane properties. It is also abundant in blood plasma lipoproteins where 70% of it is esterified to long chain fatty acids to form cholesteryl esters. Plants contain little cholesterol. Rather, the most common sterol components of their membranes are stigmasterol and ß-sitosterol which differ from cholesterol only in their aliphatic side chains. Yeast and fungi have another sterol named ergosterol which has a double bond between C7 and C8 . In animal system, cholesterol functions as a precursor of various physiologically important compounds such as vitamin D, bile acids, female sex hormones and corticosteroids. In plants, cholesterol functions as an intermediate compound in the synthesis of various phytosteroids such as saponins, cardiac glycosides, phytoecdysteroids and brassinosteroids