Substrate Specificity and Unusual Reaction Mechanism of the Sterol 4‐Methyltransferase in Caenorhabdtis elegans

作者：Paxtyn M. Fisher, Wenxu Zhou, Matthew B. Miller, Yun Shen, Huazhong Shi, W. David Nes · 发表于：The FASEB Journal · 年份：2017 · DOI：10.1096/fasebj.31.1_supplement.629.22 · 被引用次数：1 · 研究领域：Cholesterol and Lipid Metabolism、Steroid Chemistry and Biochemistry

Studies on the biosynthesis of sterols in animals have been the subject of Nobel Prizes. In vertebrates, cholesterol biosynthesis proceeds from lanosterol to sterol intermediates methylated at the 4‐postion (Nes, W. D. (2011) Chem. Rev. 111 , 6423–6451). These compounds typically contain a double bond at the 8‐position and originate by the removal of C4‐methyl from lanosterol. In contrast, the invertebrate worm C. elegans converts cholesterol to C4‐methylsterol end products that possess a double ‐bond at the 7‐ or 8(14)‐position. The C4‐methyl addition is catalyzed by sterol C4‐methyltransferase (4‐SMT) (Hannich, J. T. et al. (2009) Dev. Cell 16 , 833–843). There is speculation that C4‐methyl sterols are produced through a route involving direct methylation of lathosterol (cholest‐7‐enol) to form lophenol (4‐methyl cholest‐7‐enol). There is support for this proposal as C3‐methyl hopanoids are considered to be formed by a radical mechanism of direct methylation. We report here that C. elegans , a sterol auxotroph that converts dietary cholesterol or cholestanol to 4‐methyl sterols, when fed cholest‐3‐one (I) or cholest‐7‐3‐one (II) convert these nutrients to 4‐methyl cholestanol and 4‐methyl cholest‐7‐enol, respectively, by a hypothetical 2‐step metabolism involving a 4‐SMT and 4‐reductase enzyme. Using a cloned 4‐SMT functionally expressed in E. coli and an assay system developed for the sterol C24‐methyltransferase (24‐SMT) that includes SAM or D3‐SAM, we observed that I con...