Stereochemistry of hydrogen introduction at C-25 in ergosterol synthesized by the mevalonate-independent pathway

Feeding of [1-13C]glucose to Prototheca wickerhamii followed by 13C NMR analysis of the resulting 13C-labeled ergosterol demonstrated this yeast-like alga operates the mevalonate-independent pathway. Based on the 13C NMR signal assignment of [2,6,11,12,16,18,19, 21,23,27-13C10] ergosterol synthesized from [1-13C]glucose indicated the pro-Z methyl group of cycloartenol is derived from C-5 of IPP and that protonation at C-25 of the Δ25(27)-sterol intermediate takes place from the Si-face of Δ25 to form the isopropyl pro-R methyl group.