Obtusifoliol demethylation

The next question is when in the biosynthetic pathway do the two methylations take place. As already indicated, experiments with enzyme preparations demonstrate that cycloartenol is the best substrate for the first methylation. However 24-methylene lophenol (5-F) and not 24-methylene cycloartanol (2-F), is the preferred substrate for the second methylation. This is formed by the pathway indicated in Fig. 8. 24-Methylene cycloartanol (2-F) is demethylated to cycloeucalenol (8-F) which is the substrate for cycloeucalenol obtusifoliol isomerase which opens the 9,19-cyclopropane ring to form obtusifoliol (13-F) demethylation at C-14 and isomerization... 

Fig. 6. 25 The major CYP51 substrates lanosterol, 24,25-dihydrolanosterol, and obtusifoliol. The upper panel shows the structures of CYP51 substrates lanosterol and 24,25-di-hydrolanosterol (substrates for mammalian and yeast CYPSls), and of the plant (and trypanosome) sterol substrate obtusifoliol [21, 447], The central panel shows the CYP51 sterol demethylation reaction that occurs by three consecutive oxidative reactions on the sterol 14a-methyl group to generate alcohol, then aldehyde, and then demethylation with release of formate. The lanosterol and 24,25-dihydrolanosterol demethylation products ultimately go on to form ergosterol and/or cholesterol. The obtusifoliol-demethylated product can ultimately be converted to sitosterol, or to other sterols including ergosterol. Cholesterol has an important role in Mtb infection and can be oxidized by Mtb P450s [356-358, 851], while ergosterol is the major membrane sterol in yeast [446, 852]...

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