40 



Bile acids 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



CH3 CH3 



I 



— CH— (CH2) 2— COOH 



Androgenic substances CH3 OH or O 



Luteal substances CH3 Acetyl 



Oestrogenic substances (benzene ring) OH or O 



1. Sterols 



In the biosphere, sterols generally have a single hydroxyl group together 

 with a side chain. 



Their general structure 



CH3 



HO 



causes them to be classified, from the chemical point of view, as derived 

 from cholestane C27H48 or from coprostane (allocholestane). 



CH 



21CH3 26CH3 



I ^ 



CH— CH2— CH2— CH2— CH 



17\ 20 22 23 24 2S\ 



Cholestane 



The molecule of cholestane contains eight asymmetric carbon atoms 

 (5, 8, 9, 10, 13, 14, 17, 20) : there are theoretically 256 stereoisomers. If a 

 hydroxyl group is attached to C-3, the number of stereoisomers is raised to 

 512. Most of the natural isomers differ only in the arrangement of groups 

 in the region of C-3 or C-5. In general, the methyl group on C-10 is 

 situated above the plane of the paper. If, as is generally the case, the 

 hydroxyl group on C-3 is also above the plane of the paper, the isomer is 

 designated cis or ^. In the inverse case, the compound is called trans or a. 

 When a single bond in cholestane is replaced by a double bond, it 



