Sterol methyl transferase. Evidence for successive C-methyl transfer reactions generating Δ24(28)- and Δ25(27)-olefins by a single plant enzyme

The native SMT from Glycine max (soybean) expressed in Escherichia coli cells catalyzes the step-wise conversion of cycloartenol to 24(28)-methylene cycloartanol to a mixture of stereochemically related Δ24(28)Z-ethylidene-Δ24(28)E-ethylidene- and Δ25(27)-24β-ethylcyclosterols. [27-13C]Cycloartenol was tested as a substrate to determine the C-25 configuration of the Δ24(28)-olefin formed during the first C1-transfer reaction. This substrate yielded (25-R)-[27-13C]24(28)-methylenecycloartanol. These results are entirely consistent with the observed stereochemistries of pathway specific 24-alkyl sterols found in plants and additionally revealed for the first time that a SMT can sequentially C-methylate the Δ24-bond to generate multiple olefins.