Site-Directed Mutagenesis of the Sterol Methyl Transferase Active Site from Saccharomyces cerevisiae Results in Formation of Novel 24-Ethyl Sterols.

Delta(24(28))-Sterols are end products of a mono C-methylation pathway catalyzed by the native Delta(24(25))- to Delta(24(28))-sterol methyl transferase (SMT) enzyme from Saccharomyces cerevisiae. Using a Tyr(81) to Phe mutant SMT enzyme of S. cerevisiae, generated by site-directed mutagenesis of a highly conserved residue in the sterol binding site, we found that several Delta(24(25))- and Delta(24(28))-sterols, which are not substrates for the native protein, were catalyzed to mono- and bis-C24-alkylated side chains.