Cycloeucalenol

This enzyme [EC 5.5.1.9], also called cycloeucalenol cycloisomerase, catalyzes the interconversion of cycloeucalenol to obtusifoliol. 

Cycloartenol ferulic acid ester Bran° ° Cycloeucalenol Sd oiP ° ... 

The full details of the conversion of cycloeucalenol into [19- H]obtusifoliol by the microsomes of Zea mays in DjO have appeared (see Vol. 7, p. 135). The formation of a A intermediate in the enzymatic cleavage of the cyclopropane ring is excluded by the incorporation of only one deuterium atom. Both cyclo-artenol and lanosterol are converted into normal sterols by Saprolegnia ferax and Chlorella ellipsoidea. ... 

Obtusifoliol Gramisterol Cycloeucalenol Citrostadienol p-Amyrin a-Amyrin Cycloartenol 24-methylene cycloartanol Cyclobranol... 

Already in 1974, Heintz and Benveniste (1974) reported the enz)matically catalysed opening of the cyclopropane ring of cycloeucalenol in bramble... 

Lovato, M.A., Hart, E.A., Segura, M.J., Giner, J.L. and Matsuda, S.P (2000) Functional cloning of an Arabidopsis thaliana cDNA encoding cycloeucalenol cycloisomerase. /. Biol. Chem., 275, 13394-7. 

Rahier, A., Taton, M. and Benveniste, P. (1989) Cycloeucalenol-obtusifoliol isomerase. Structural requirements for transformation or binding of substrates and inhibitors. Eur. J. Biochem., 181,615-26. 

The confirmation of this structure and proof of the stereochemistry of the skeleton was provided by the following correlation of dihydrocyclo-buxine-D with cycloeucalenol (183, 191). 

In the presence of D2O microsomes of maize embryos convert cycloeucalenol (44) into [19- H]obtusifoliol (45). This reaction is the biochemical equivalent of an acid-catalysed opening of the cyclopropane ring. The resonances of some actein and cimigoside derivatives have been assigned. ... 

Cycloeucalenol (86), which is considered to be one of the intermediates of sterol biosynthesis in the photosynthetic eukaryotes, on incubation in D2O with micro-somes from Zea mays gave [19- H]]obtusifoliol (87), " a result that was to be expected since it has previously (1975) been shown that peas germinated in D2O produced cycloartenol deuteriated specifically at C-19, presumably by a similar route. 

Further new natural products are (38) from the toxic micro-organism Fusarium sporotrichioides 921,30 (39) and (40) from olive oil,31 triphyllol (41) from Adeno-phora triphylla (Campanulaceae),32 and cyclonervilol (42) and cyclohomonervilol (43) from Nervilia purpurea (Orchidaceae). The structure of (43) was supported by its conversion into cycloeucalenol.33 Two new xylosides from Cimicifuga japonica (Ranunculaceae) are based on the genuine sapogenins (44) and (45). Treatment of... 

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. 9. Mechanism for the opening of the cyclopropane ring of cycloeucalenol B and.N are active groups at the active site of enzyme the hydrogen which is transferred from B to C-19 can exchange with in the aqueous environment.

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