Synthesis of sterol biosynthesis inhibitors and metabolism of isotopically labeled sterols by Saccharomyces cerevisiae



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Texas Tech University


The dissertation focuses on the chemistry and mechanism of sterol biomethylation catalyzed by (S)-adenosyl-L-methionine:sterol methyl transferase (SMT) enzyme. The research is composed of two parts: synthesis of sterol methylation inhibitors and isotopically labeled substrates and mechanism studies of the SMT enzyme.

Part 1 describes preparation of ^H and ^^C-isotopically labeled sterols, and the design, synthesis and enzymatic evaluation of three types of sterol methylation inhibitors: (1) substrate analogs which act as product inhibitors of the reaction; (2) substrate analogs which act as mechanism-based inactivators; and (3) transition state analogs. The synthetic work focused on modification of the side chains of cholesterol, zymosterol, lanosterol, and cycloartenol which are natural substrates in C-methylation reactions. The sterol side chain was modified at C-20, C-21, and C-22 positions to study the effect of configuration and conformation of the sterol side chain on biomethylation. A series of novel sterol methylation inhibitors containing aza, aziridine, and ammonium groups at positions C-22 to C-25 were synthesized to serve as transition state analogs. Substrate analogs with the 26, 27-cyclopropylidene functional group were also synthesised and discovered to be potent irreversible mechanism-based inhibitors of the SMT enzyme. Several new 24, 25-ethano and 24, 28-methano sterols, 24- inyl and 29', 29"-cyclopropylidene lanosterols, and 24(25), 26(26')-diene and 24(25)-en-25-thylnyl sterols have been synthesized. All the inhibitors were characterized by gas chromatography-mass spectrometry, high pressure liquid chromatography, and 'H and ^^C nuclear magnetic resonance spectroscopy.

Part 2 describes research on the coupled methylenation-deprotonation of C-24 of the sterol side chain in plant and fungal sterol C-methylation reactions. These studies involved determination of the stereochemistry of hydrogen migration from C-24 to C-25 during biomethylation and of C-28 deprotonation. To accomplish our aims, [27-'^C], [24-^H] and [28-^H2] labeled sterols were prepared and assayed with SMT enzymes from a fungus {Saccharomyces cerevisiae) and a plant (Arabidopsis thaliana). As a result, migration of the hydrogen from C-24 to C-25 was found to be introduced from Re-facQ of the 24, 25-double bond of the sterol side chain to generate the similar 25R stereochemistry in S. cerevisiae and A. thaliana, suggesting a similar topography of the SMT active site.