The Biosynthesis of Centella Triterpenes and Triterpenoids
Terpene biosynthesis can be divided into four stages. Firstly, there is the formation of the isoprene
unit isopentenyl diphosphate. There are two known major pathways for the biosynthesis of the
isoprene unit, one based on mevalonic acid and the other one on 1-deoxyxylulose [10]. Secondly, there
is the association of these units to form the (C5)n isoprenoid backbone of the terpene families; thirdly
there is the cyclization of these to generate the carbon skeletons. Finally, there are the
interrelationships, hydroxylations and oxidations that lead to the individual terpenoids. The general
biosynthesis of terpenes leading up to sterols has been reviewed extensively by Benveniste and others
[10,20-24]. Triterpenes consist of six isoprene units and have the molecular formula C30H48. The linear
triterpene squalene, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate
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by squalene synthase (SQS). Squalene is then oxidized biosynthetically by squalene epoxidase (SQE)
to generate 2,3-oxidosqualene. Oxidosqualene cyclases (OSCs) cyclize 2,3-oxidosqualene through
cationic intermediates to triterpene alcohols or aldehydes including - and -amyrin and lupeol