Mechanistic analysis of a multiple product sterol methyltransferase implicated in ergosterol biosynthesis in Trypanosoma brucei

Abstract

Sterol methyltransferase (SMT) plays a key role in sterol biosynthesis in different pathogenic organisms by setting the pattern of the side chain structure of the final product. This catalyst, absent in humans, provides critical pathway-specific enzymatic steps in the production of ergosterol in fungi or phytosterols in plants. The new SMT gene was isolated from Trypanosoma brucei genomic DNA and cloned into an Escherichia coli expression system. The recombinant SMT was purified to homogeneity to give a band at 40.0 kDa upon SDS-PAGE and showed a tetrameric subunit organization by gel chromatography. It has a pH optimum of 7.5, an apparent kcat value of 0.01 s -1, and a Km of 47 ± 4 μM for zymosterol. The products of the reaction were a mixture of C24-monoalkylated sterols, ergosta-8,24 (25)-dienol, ergosta-8,25 (27)-dienol, and ergosta-8,24 (28)-dienol (fecosterol), and an unusual double C24-alkylated sterol, 24,24-dimethyl ergosta-8,25 (27)-dienol, typically found in plants. Inhibitory profile studies with 25-azalanosterol (Ki value of 39 nM) or 24(R,S),25- epiminolanosterol (Ki value of 49 nM), ergosterol (Ki value of 27 μM) and 26,27-dehydrozymosterol (Ki and k inact values of 29 μM and 0.26 min-1, respectively) and data showing zymosterol as the preferred acceptor strongly suggest that the protozoan SMT has an active site topography combining properties of the SMT1 from plants and yeast (37-47% identity). The enzymatic activation of this and other SMTs reveals that the catalytic requirements for the C-methyl reaction are remarkably versatile, whereas the inhibition studies provide a powerful approach to rational design of new anti-sleeping sickness chemotherapeutic drugs. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.